@article{shelton_mctaggart_allen_mercer_crenshaw_franz_2024, title={Does the effect of walking balance perturbations generalize across contexts?}, volume={93}, ISSN={["1872-7646"]}, DOI={10.1016/j.humov.2023.103158}, abstractNote={Balance perturbations are used to study locomotor instability. However, these perturbations are designed to provoke a specific context of instability that may or may not generalize to a broader understanding of falls risk. The purpose of this study was to determine if the effect of balance perturbations on instability generalizes across contexts. 29 younger adults and 28 older adults completed four experimental trials, including unperturbed walking and walking while responding to three perturbation contexts: mediolateral optical flow, treadmill-induced slips, and lateral waist-pulls. We quantified the effect of perturbations as an absolute change in margin of stability from unperturbed walking. We found significant changes in mediolateral and anteroposterior margin of stability for all perturbations compared to unperturbed walking in both cohorts (p-values ≤ 0.042). In older adults, the mediolateral effects of lateral waist-pulls significantly correlated with those of optical flow perturbations and treadmill-induced slips (r ≥ 0.398, p-values ≤ 0.036). In younger adults but not in older adults, we found positive and significant correlations between the anteroposterior effect of waist-pull perturbations and optical flow perturbations, and the anteroposterior and mediolateral effect of treadmill-induced slips (r ≥ 0.428, p-values ≤ 0.021). We found no "goldilocks" perturbation paradigm to endorse that would support universal interpretations about locomotor instability. Building the most accurate patient profiles of instability likely requires a series of perturbation paradigms designed to emulate the variety of environmental contexts in which falls may occur.}, journal={HUMAN MOVEMENT SCIENCE}, author={Shelton, Andrew D. and McTaggart, Ellora M. and Allen, Jessica L. and Mercer, Vicki S. and Crenshaw, Jeremy R. and Franz, Jason R.}, year={2024}, month={Feb} }
 @article{armitano-lago_evans-pickett_davis-wilson_munsch_longobardi_willcockson_schwartz_franz_pietrosimone_2024, title={Modifying loading during gait leads to biochemical changes in serum cartilage oligomeric matrix protein concentrations in a subgroup of individuals with anterior cruciate ligament reconstruction}, ISSN={["1434-9949"]}, DOI={10.1007/s10067-024-06898-4}, journal={CLINICAL RHEUMATOLOGY}, author={Armitano-Lago, Cortney and Evans-Pickett, Alyssa and Davis-Wilson, Hope and Munsch, Amanda and Longobardi, Lara and Willcockson, Helen and Schwartz, Todd A. and Franz, Jason R. and Pietrosimone, Brian}, year={2024}, month={Feb} }
 @article{jang_franz_pietrosimone_wikstrom_2024, title={Muscle contributions to reduced ankle joint contact force during drop vertical jumps in patients with chronic ankle instability}, volume={163}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2024.111926}, abstractNote={Chronic ankle instability is a condition linked to progressive early ankle joint degeneration. Patients with chronic ankle instability exhibit altered biomechanics during gait and jump landings and these alterations are believed to contribute to aberrant joint loading and subsequent joint degeneration. Musculoskeletal modeling has the capacity to estimate joint loads from individual muscle forces. However, the influence of chronic ankle instability on joint contact forces remains largely unknown. The objective of this study was to compare tri-axial (i.e., compressive, anterior-posterior, and medial–lateral) ankle joint contact forces between those with and without chronic ankle instability during the ground contact phase of a drop vertical jump. Fifteen individuals with and 15 individuals without chronic ankle instability completed drop vertical jump maneuvers in a research laboratory. We used those data to drive three-dimensional musculoskeletal simulations and estimate muscle forces and tri-axial joint contact force variables (i.e., peak and impulse). Compared to those without chronic ankle instability, the ankles of patients with chronic ankle instability underwent lower compressive ankle joint contact forces as well as lower anterior-posterior and medial–lateral shearing forces during the weight acceptance phase of landing (p <.05). These findings suggest that patients with chronic ankle instability exhibit lower ankle joint loading patterns than uninjured individuals during a drop vertical jump, which may be considered in rehabilitation to potentially reduce the risk of early onset of ankle joint degeneration.}, journal={JOURNAL OF BIOMECHANICS}, author={Jang, Jaeho and Franz, Jason R. and Pietrosimone, Brian G. and Wikstrom, Erik A.}, year={2024}, month={Jan} }
 @article{davis-wilson_thoma_franz_blackburn_longobardi_schwartz_hackney_pietrosimone_2024, title={Physical Activity Associates with T1rho MRI of Femoral Cartilage After Anterior Cruciate Ligament Reconstruction}, volume={56}, ISSN={["1530-0315"]}, DOI={10.1249/MSS.0000000000003318}, abstractNote={ABSTRACT}, number={3}, journal={MEDICINE & SCIENCE IN SPORTS & EXERCISE}, author={Davis-Wilson, Hope C. and Thoma, Louise M. and Franz, Jason R. and Blackburn, J. Troy and Longobardi, Lara and Schwartz, Todd A. and Hackney, Anthony C. and Pietrosimone, Brian}, year={2024}, month={Mar}, pages={411–417} }
 @article{bjornsen_berkoff_blackburn_davis-wilson_evans-pickett_franz_harkey_horton_lisee_luc-harkey_et al._2024, title={Sustained Limb-Level Loading: A Ground Reaction Force Phenotype Common to Individuals at High Risk for and Those With Knee Osteoarthritis}, ISSN={["2326-5205"]}, DOI={10.1002/art.42744}, abstractNote={ObjectiveThe objective of this study was to compare the vertical (vGRF), anterior‐posterior (apGRF), and medial‐lateral (mlGRF) ground reaction force (GRF) profiles throughout the stance phase of gait (1) between individuals 6 to 12 months post–anterior cruciate ligament reconstruction (ACLR) and uninjured matched controls and (2) between ACLR and individuals with differing radiographic severities of knee osteoarthritis (KOA), defined as Kellgren and Lawrence (KL) grades KL2, KL3, and KL4.}, journal={ARTHRITIS & RHEUMATOLOGY}, author={Bjornsen, Elizabeth and Berkoff, David and Blackburn, J. Troy and Davis-Wilson, Hope and Evans-Pickett, Alyssa and Franz, Jason R. and Harkey, Matthew S. and Horton, W. Zachary and Lisee, Caroline and Luc-Harkey, Brittney and et al.}, year={2024}, month={Jan} }
 @article{cone_kim_thelen_franz_2023, title={3D characterization of the triple-bundle Achilles tendon from in vivo high-field MRI}, ISSN={["1554-527X"]}, DOI={10.1002/jor.25654}, abstractNote={Abstract}, journal={JOURNAL OF ORTHOPAEDIC RESEARCH}, author={Cone, Stephanie G. and Kim, Hoon and Thelen, Darryl G. and Franz, Jason R.}, year={2023}, month={Jul} }
 @article{dieffenderfer_brewer_noonan_smith_eichenlaub_haley_jacks_lobaton_neupert_hess_et al._2023, title={A Wearable System for Continuous Monitoring and Assessment of Speech, Gait, and Cognitive Decline for Early Diagnosis of ADRD}, ISSN={["1558-4615"]}, DOI={10.1109/EMBC40787.2023.10339986}, abstractNote={Early detection of cognitive decline is essential to study mild cognitive impairment and Alzheimer’s Disease in order to develop targeted interventions and prevent or stop the progression of dementia. This requires continuous and longitudinal assessment and tracking of the related physiological and behavioral changes during daily life. In this paper, we present a low cost and low power wearable system custom designed to track the trends in speech, gait, and cognitive stress while also considering the important human factor needs such as privacy and compliance. In the form factors of a wristband and waist-patch, this multimodal, multi-sensor system measures inertial signals, sound, heart rate, electrodermal activity and pulse transit time. A total power consumption of 2.6 mW without any duty cycling allows for more than 3 weeks of run time between charges when 1500 mAh batteries are used.Clinical Relevance— Much earlier detection of Alzheimer’s disease and related dementias may be possible by continuous monitoring of physiological and behavioral state using application specific wearable sensors during the activities of daily life.}, journal={2023 45TH ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE & BIOLOGY SOCIETY, EMBC}, author={Dieffenderfer, James and Brewer, Alec and Noonan, Maxwell A. and Smith, Madeline and Eichenlaub, Emily and Haley, Katarina L. and Jacks, Adam and Lobaton, Edgar and Neupert, Shevaun D. and Hess, Thomas M. and et al.}, year={2023} }
 @misc{boyer_hayes_umberger_adamczyk_bean_brach_clark_clark_ferrucci_finley_et al._2023, title={Age-related changes in gait biomechanics and their impact on the metabolic cost of walking: Report from a National Institute on Aging workshop}, volume={173}, ISSN={["1873-6815"]}, DOI={10.1016/j.exger.2023.112102}, abstractNote={Changes in old age that contribute to the complex issue of an increased metabolic cost of walking (mass-specific energy cost per unit distance traveled) in older adults appear to center at least in part on changes in gait biomechanics. However, age-related changes in energy metabolism, neuromuscular function and connective tissue properties also likely contribute to this problem, of which the consequences are poor mobility and increased risk of inactivity-related disease and disability. The U.S. National Institute on Aging convened a workshop in September 2021 with an interdisciplinary group of scientists to address the gaps in research related to the mechanisms and consequences of changes in mobility in old age. The goal of the workshop was to identify promising ways to move the field forward toward improving gait performance, decreasing energy cost, and enhancing mobility for older adults. This report summarizes the workshop and brings multidisciplinary insight into the known and potential causes and consequences of age-related changes in gait biomechanics. We highlight how gait mechanics and energy cost change with aging, the potential neuromuscular mechanisms and role of connective tissue in these changes, and cutting-edge interventions and technologies that may be used to measure and improve gait and mobility in older adults. Key gaps in the literature that warrant targeted research in the future are identified and discussed.}, journal={EXPERIMENTAL GERONTOLOGY}, author={Boyer, Katherine A. and Hayes, Kate L. and Umberger, Brian R. and Adamczyk, Peter Gabriel and Bean, Jonathan F. and Brach, Jennifer S. and Clark, Brian C. and Clark, David J. and Ferrucci, Luigi and Finley, James and et al.}, year={2023}, month={Mar} }
 @article{lisee_evans-pickett_davis-wilson_munsch_longobardi_schwartz_lalush_franz_pietrosimone_2023, title={Delayed cartilage oligomeric matrix protein response to loading is associated with femoral cartilage composition post-ACLR}, ISSN={["1439-6327"]}, DOI={10.1007/s00421-023-05253-w}, abstractNote={{"Label"=>"PURPOSE", "NlmCategory"=>"OBJECTIVE"} To determine associations between immediate and delayed response of serum cartilage oligomeric matrix protein (sCOMP) to loading (i.e., 3000 walking steps) and femoral cartilage interlimb T1ρ relaxation times in individual's post-anterior cruciate ligament reconstruction (ACLR). {"Label"=>"METHODS", "NlmCategory"=>"METHODS"} This cross-sectional study included 20 individuals 6-12 months following primary ACLR (65% female, 20.5 ± 4.0 years old, 24.9 ± 3.0 kg/m {"sup"=>"2"} , 7.3 ± 1.5 months post-ACLR). Serum samples were collected prior to, immediately following, and 3.5 h following walking 3000 steps on a treadmill at habitual walking speed. sCOMP concentrations were processed using enzyme-linked immunosorbent assays. Immediate and delayed absolute sCOMP responses to loading were evaluated immediately and 3.5 h post-walking, respectively. Participants underwent bilateral magnetic resonance imaging with T1ρ sequences to calculate resting femoral cartilage interlimb T1ρ relaxation time ratios between limbs (i.e., ACLR/Uninjured limb). Linear regression models were fitted to determine associations between sCOMP response to loading and femoral cartilage T1ρ outcomes controlling for pre-loading sCOMP concentrations. {"Label"=>"RESULTS", "NlmCategory"=>"RESULTS"} Greater increases in delayed sCOMP response to loading were associated with greater lateral (∆R {"sup"=>"2"}  = 0.29, p = 0.02) but not medial (∆R {"sup"=>"2"}  < 0.01, p = 0.99) femoral cartilage interlimb T1ρ ratios. Associations between immediate sCOMP response to loading with femoral cartilage interlimb T1ρ ratios were weak and non-significant (∆R {"sup"=>"2"}  range = 0.02-0.09, p range = 0.21-0.58). {"Label"=>"CONCLUSION", "NlmCategory"=>"CONCLUSIONS"} Greater delayed sCOMP response to loading, a biomarker of cartilage breakdown, is associated with worse lateral femoral cartilage composition in the ACLR limb compared to the uninjured limb. Delayed sCOMP response to loading may be a more indicative metabolic indicator linked to deleterious changes in composition than immediate sCOMP response.}, journal={EUROPEAN JOURNAL OF APPLIED PHYSIOLOGY}, author={Lisee, Caroline and Evans-Pickett, Alyssa and Davis-Wilson, Hope and Munsch, Amanda E. and Longobardi, Lara and Schwartz, Todd A. and Lalush, David and Franz, Jason R. and Pietrosimone, Brian}, year={2023}, month={Jun} }
 @article{bjornsen_davis-wilson_evans-picket_horton_lisee_munsch_nissman_blackburn_franz_pietrosimone_2023, title={Knee kinetics and the medial femoral cartilage cross-sectional area response to loading in indviduals with anterior cruciate ligament reconstruction}, volume={105}, ISSN={["1879-1271"]}, DOI={10.1016/j.clinbiomech.2023.105979}, abstractNote={Ultrasonography is capable of detecting morphological changes in femoral articular cartilage cross-sectional area in response to an acute bout of walking; yet, the response of femoral cartilage cross-sectional area varies between individuals. It is hypothesized that differences in joint kinetics may influence the response of cartilage to a standardized walking protocol. Therefore, the study purpose was to compare internal knee abduction and extension moments between individuals with anterior cruciate ligament reconstruction who demonstrate an acute increase, decrease, or unchanged medial femoral cross-sectional area response following 3000 steps.The medial femoral cartilage in the anterior cruciate ligament reconstructed limb was assessed with ultrasonography before and immediately following 3000 steps of treadmill walking. Knee joint moments were calculated in the anterior cruciate ligament reconstructed limb and compared between groups throughout the stance phase of gait using linear regression and functional, mixed effects waveform analyses.No associations between peak knee joint moments and the cross-sectional area response were observed. The group that demonstrated an acute cross-sectional area increase exhibited 1) lower knee abduction moments in early stance in comparison to the group that exhibited a decreased cross-sectional area response; and 2) greater knee extension moments in early stance in comparison to the group with an unchanged cross-sectional area response.The propensity of femoral cartilage to acutely increase cross-sectional area in response to walking is consistent with less-dynamic knee abduction and knee extension moment profiles.}, journal={CLINICAL BIOMECHANICS}, author={Bjornsen, Elizabeth and Davis-Wilson, Hope and Evans-Picket, Alyssa and Horton, W. Zachary and Lisee, Caroline and Munsch, Amanda E. and Nissman, Daniel and Blackburn, J. Troy and Franz, Jason R. and Pietrosimone, Brian}, year={2023}, month={May} }
 @article{twiddy_peterson_maddocks_macpherson_pimentel_yates_armitano-lago_kiefer_pietrosimone_franz_et al._2022, title={A Low-Cost, Open Source Wireless Body Area Network for Clinical Gait Rehabilitation}, ISSN={["1930-0395"]}, DOI={10.1109/SENSORS52175.2022.9967362}, abstractNote={Wearable inertial sensors represent an opportunity to enable gait monitoring and feedback-based rehabilitation in real-world environments. Here, we describe the development of an inexpensive I MU-based wireless body area network capable of recording 9-axis motion data from 8 sites on the body simultaneously. This system can generate data comparable to existing commercial sensor networks and can distinguish varying loading conditions observed during real-time biofeedback-based human subject testing.}, journal={2022 IEEE SENSORS}, author={Twiddy, Jack and Peterson, Kaila and Maddocks, Grace and MacPherson, Ryan and Pimentel, Ricky and Yates, Max and Armitano-Lago, Cortney and Kiefer, Adam and Pietrosimone, Brian and Franz, Jason and et al.}, year={2022} }
 @article{davis-wilson_thoma_longobardi_franz_blackburn_hackney_pietrosimone_2022, title={Association of Quality of Life With Moderate-to-Vigorous Physical Activity After Anterior Cruciate Ligament Reconstruction}, volume={57}, ISSN={["1938-162X"]}, DOI={10.4085/1062-6050-0670.20}, abstractNote={
                  Context
                  Better knee function is linked to psychological readiness to return to sport after anterior cruciate ligament reconstruction (ACLR). Individuals with ACLR participate in less physical activity than matched uninjured control individuals, yet the association between knee function and physical activity post–ACLR remains unclear.
               }, number={6}, journal={JOURNAL OF ATHLETIC TRAINING}, author={Davis-Wilson, Hope C. and Thoma, Louise M. and Longobardi, Lara and Franz, Jason R. and Blackburn, J. Troy and Hackney, A. C. and Pietrosimone, Brian}, year={2022}, month={Jun}, pages={532–539} }
 @article{lisee_davis-wilson_evans-pickett_horton_blackburn_franz_thoma_spang_pietrosimone_2022, title={Linking Gait Biomechanics and Daily Steps After ACL Reconstruction}, volume={54}, ISSN={["1530-0315"]}, DOI={10.1249/MSS.0000000000002860}, abstractNote={ABSTRACT}, number={5}, journal={MEDICINE & SCIENCE IN SPORTS & EXERCISE}, author={Lisee, Caroline and Davis-Wilson, Hope C. and Evans-Pickett, Alyssa and Horton, W. Zachary and Blackburn, J. Troy and Franz, Jason R. and Thoma, Louise M. and Spang, Jeffrey T. and Pietrosimone, Brian G.}, year={2022}, month={May}, pages={709–716} }
 @misc{lynch_spangler_franz_krupenevich_kim_nissman_zhang_li_sumner_batsis_2022, title={Multimodal Diagnostic Approaches to Advance Precision Medicine in Sarcopenia and Frailty}, volume={14}, ISSN={["2072-6643"]}, DOI={10.3390/nu14071384}, abstractNote={Sarcopenia, defined as the loss of muscle mass, strength, and function with aging, is a geriatric syndrome with important implications for patients and healthcare systems. Sarcopenia increases the risk of clinical decompensation when faced with physiological stressors and increases vulnerability, termed frailty. Sarcopenia develops due to inflammatory, hormonal, and myocellular changes in response to physiological and pathological aging, which promote progressive gains in fat mass and loss of lean mass and muscle strength. Progression of these pathophysiological changes can lead to sarcopenic obesity and physical frailty. These syndromes independently increase the risk of adverse patient outcomes including hospitalizations, long-term care placement, mortality, and decreased quality of life. This risk increases substantially when these syndromes co-exist. While there is evidence suggesting that the progression of sarcopenia, sarcopenic obesity, and frailty can be slowed or reversed, the adoption of broad-based screening or interventions has been slow to implement. Factors contributing to slow implementation include the lack of cost-effective, timely bedside diagnostics and interventions that target fundamental biological processes. This paper describes how clinical, radiographic, and biological data can be used to evaluate older adults with sarcopenia and sarcopenic obesity and to further the understanding of the mechanisms leading to declines in physical function and frailty.}, number={7}, journal={NUTRIENTS}, author={Lynch, David H. and Spangler, Hillary B. and Franz, Jason R. and Krupenevich, Rebecca L. and Kim, Hoon and Nissman, Daniel and Zhang, Janet and Li, Yuan-Yuan and Sumner, Susan and Batsis, John A.}, year={2022}, month={Apr} }
 @article{zhang_clark_franz_sharma_2022, title={Personalized fusion of ultrasound and electromyography-derived neuromuscular features increases prediction accuracy of ankle moment during plantarflexion}, volume={71}, ISSN={["1746-8108"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85114124714&partnerID=MN8TOARS}, DOI={10.1016/j.bspc.2021.103100}, abstractNote={Compared to mechanical signals that are used for estimating human limb motion intention, non-invasive surface electromyography (sEMG) is a preferred signal in human-robotic systems. However, noise interference, crosstalk from adjacent muscle groups, and an inability to measure deeper muscle tissues are disadvantageous to sEMG's reliable use. In this work, we hypothesize that a fusion between sEMG and in vivo ultrasound (US) imaging will result in more accurate detection of ankle movement intention. Nine young able-bodied participants were included to volitionally perform isometric plantarflexion tasks with different fixed-end ankle postures, while the sEMG and US imaging data of plantarflexors were synchronously collected. We created three dominant feature sets, sole sEMG feature set, sole US feature set, and sEMG-US feature fusion set, to calibrate and validate a support vector machine regression model (SVR) and a feedforward neural network model (FFNN) with labeled net moment measurements. The results showed that, compared to the sole sEMG feature set, the sEMG-US fusion set reduced the average net moment prediction error by 35.7% (p < 0.05), when using SVR, and by 21.5% (p < 0.05), when using FFNN. In SVR, the sole US feature set reduced the prediction error by 24.9% (p < 0.05) when compared to the sole sEMG feature set. In FFNN, the sEMG-US fusion set reduced the prediction error by 28.2% (p < 0.05) when compared to the sole US feature set. These findings indicate that the combination of sEMG signals and US imaging is a superior sensing modality for predicting human plantarflexion intention and can enable future clinical rehabilitation devices.}, journal={BIOMEDICAL SIGNAL PROCESSING AND CONTROL}, author={Zhang, Qiang and Clark, William H. and Franz, Jason R. and Sharma, Nitin}, year={2022}, month={Jan} }
 @article{munsch_evans-pickett_davis-wilson_pietrosimone_franz_2022, title={Quadriceps Muscle Action and Association With Knee Joint Biomechanics in Individuals with Anterior Cruciate Ligament Reconstruction}, volume={38}, ISSN={["1543-2688"]}, DOI={10.1123/jab.2021-0381Inc}, number={5}, journal={JOURNAL OF APPLIED BIOMECHANICS}, author={Munsch, Amanda E. and Evans-Pickett, Alyssa and Davis-Wilson, Hope and Pietrosimone, Brian and Franz, Jason R.}, year={2022}, month={Oct}, pages={328–335} }
 @article{pimentel_feldman_lewek_franz_2022, title={Quantifying mechanical and metabolic interdependence between speed and propulsive force during walking}, volume={4}, ISSN={["2624-9367"]}, DOI={10.3389/fspor.2022.942498}, abstractNote={Walking speed is a useful surrogate for health status across the population. Walking speed appears to be governed in part by interlimb coordination between propulsive (FP) and braking (FB) forces generated during step-to-step transitions and is simultaneously optimized to minimize metabolic cost. Of those forces, FP generated during push-off has received significantly more attention as a contributor to walking performance. Our goal was to first establish empirical relations between FP and walking speed and then to quantify their effects on metabolic cost in young adults. To specifically address any link between FP and walking speed, we used a self-paced treadmill controller and real-time biofeedback to independently prescribe walking speed or FP across a range of condition intensities. Walking with larger and smaller FP led to instinctively faster and slower walking speeds, respectively, with ~80% of variance in walking speed explained by FP. We also found that comparable changes in either FP or walking speed elicited predictable and relatively uniform changes in metabolic cost, together explaining ~53% of the variance in net metabolic power and ~14% of the variance in cost of transport. These results provide empirical data in support of an interdependent relation between FP and walking speed, building confidence that interventions designed to increase FP will translate to improved walking speed. Repeating this protocol in other populations may identify other relations that could inform the time course of gait decline due to age and disease.}, journal={FRONTIERS IN SPORTS AND ACTIVE LIVING}, author={Pimentel, Richard E. and Feldman, Jordan N. and Lewek, Michael D. and Franz, Jason R.}, year={2022}, month={Sep} }
 @article{beck_trejo_schroeder_franz_sawicki_2022, title={Shorter muscle fascicle operating lengths increase the metabolic cost of cyclic force production}, volume={133}, ISSN={["1522-1601"]}, DOI={10.1152/japplphysiol.00720.2021}, abstractNote={ During locomotion, active muscles undergo cyclic length-changing contractions. In this study, we isolated confounding variables and revealed that cyclically producing force at relatively shorter fascicle lengths increases metabolic energy expenditure. Therefore, muscle fascicle operating lengths likely have a measurable effect on the metabolic energy expenditure during locomotion. }, number={3}, journal={JOURNAL OF APPLIED PHYSIOLOGY}, author={Beck, Owen N. and Trejo, Lindsey H. and Schroeder, Jordyn N. and Franz, Jason R. and Sawicki, Gregory S.}, year={2022}, month={Sep}, pages={524–533} }
 @article{shelton_mctaggart_allen_mercer_franz_2022, title={Slowing down to preserve balance in the presence of optical flow perturbations}, volume={96}, ISSN={["1879-2219"]}, DOI={10.1016/j.gaitpost.2022.07.002}, abstractNote={The use of sensory and mechanical perturbations applied during walking has grown in popularity due to their ability to elicit instability relevant to falls. However, the vast majority of perturbation studies on walking balance are performed on a treadmill at a fixed speed.The aim of this study was to quantify the effects of mediolateral optical flow perturbations on walking speed and balance outcomes in young adults walking with fixed-speed and self-paced treadmill controllers.Fifteen healthy young adults (8 female, age: 23.1 ± 4.6 yrs) completed four five-minute randomized walking trials in a speed-matched virtual reality hallway. In two of the trials, we added continuous mediolateral optical flow perturbations to the virtual hallway. Trials with and without optical flow perturbations were performed with either a fixed-speed or self-paced treadmill controller. We measured walking speed, balance outcomes (step width, margin of stability, local dynamic instability) and gait variability (step width variability and margin of stability variability).We found significant increases in step width (+20%, p = 0.004) and local dynamic instability (+11%, p = 0.008) of participants while responding to optical flow perturbations at a fixed treadmill speed. We found no significant differences in these outcome measures when perturbations were applied on a self-paced treadmill. Instead, participants walked 5.7% slower between the self-paced treadmill controller conditions when responding to optical flow perturbations (1.48 ± 0.13 m/s vs. 1.57 ± 0.16 m/s, p = 0.005).Our findings suggest that during walking, when presented with a balance challenge, an individual will instinctively reduce their walking speed in order to better preserve stability. However, comparisons to prior literature suggest that this response may depend on environmental and/or perturbation context. Cumulatively, our results point to opportunities for leveraging self-paced treadmill controllers as a more ecologically-relevant option in balance research with potential clinical applications in diagnostics and rehabilitation.}, journal={GAIT & POSTURE}, author={Shelton, Andrew D. and McTaggart, Ellora M. and Allen, Jessica L. and Mercer, Vicki S. and Franz, Jason R.}, year={2022}, month={Jul}, pages={365–370} }
 @article{ahuja_franz_2022, title={The metabolic cost of walking balance control and adaptation in young adults}, volume={96}, ISSN={["1879-2219"]}, DOI={10.1016/j.gaitpost.2022.05.031}, abstractNote={Our aim was to quantify the role of metabolic energy cost in governing neuromuscular adaptation to prolonged exposure to optical flow walking balance perturbations in young adults.We hypothesized that metabolic cost would increase at the onset of balance perturbations in a manner consistent with wider and shorter steps and increased step-to-step variability. We also hypothesized that metabolic cost would decrease with prolonged exposure in a manner consistent with a return of step width and step length to values seen during normal, unperturbed walking.Healthy young adults (n = 18) walked on a treadmill while viewing a virtual hallway. Optical flow balance perturbations were introduced over a 10-minute interval during a 20-minute walking bout while measuring step kinematics and metabolic energy cost. For all outcome measures, we computed average values during the following four time periods of interest: Pre (minutes 3-5), Early Perturbation (minutes 5-7), Late Perturbation (minutes 13-15), and Post (minutes 18-20). A repeated-measures ANOVA tested for main effects of time, following by post-hoc pairwise comparisons.With the onset of perturbations, participants walked with 3% shorter, 17% wider, and 53-73% more variable steps. These changes were accompanied by a significant 12% increase in net metabolic power compared to walking normally. With prolonged exposure to perturbations, step width and step length tended toward values seen during normal, unperturbed walking - changes accompanied by a 5% reduction in metabolic power (p-values≤0.05).Our study reveals that the adoption of generalized anticipatory control at the onset of optical flow balance perturbations and the subsequent shift to task-specific reactive control following prolonged exposure have meaningful metabolic consequences. Moreover, our findings suggest that metabolic energy cost may shape the strategies we use to adapt walking balance in response to perturbations.}, journal={GAIT & POSTURE}, author={Ahuja, Shawn and Franz, Jason R.}, year={2022}, month={Jul}, pages={190–194} }
 @article{franz_2021, title={A sound approach to improving exoskeletons and exosuits}, volume={6}, ISSN={["2470-9476"]}, DOI={10.1126/scirobotics.abm6369}, abstractNote={Integrating onboard ultrasound imaging to an exosuit enhances muscle-machine coordination to improve economy of locomotion.}, number={60}, journal={SCIENCE ROBOTICS}, author={Franz, Jason R.}, year={2021}, month={Nov} }
 @article{waanders_murgia_devita_franz_hortobagyi_2021, title={Age does not affect the relationship between muscle activation and joint work during incline and decline walking}, volume={124}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2021.110555}, abstractNote={Older compared with younger adults walk with different configurations of mechanical joint work and greater muscle activation but it is unclear if age, walking speed, and slope would each affect the relationship between muscle activation and net joint work. We hypothesized that a unit increase in positive but not negative net joint work requires greater muscle activation in older compared with younger adults. Healthy younger (age: 22.1 yrs, n = 19) and older adults (age: 69.8 yrs, n = 16) ascended and descended a 7° ramp at slow (~1.20 m/s) and moderate (~1.50 m/s) walking speeds while lower-extremity marker positions, electromyography, and ground reaction force data were collected. Compared to younger adults, older adults took 11% (incline) and 8% (decline) shorter strides, and performed 21% less positive ankle plantarflexor work (incline) and 19% less negative knee extensor work (decline) (all p < .05). However, age did not affect (all p > .05) the regression coefficients between the muscle activation integral and positive hip extensor or ankle plantarflexor work during ascent, nor between that and negative knee extensor or ankle dorsiflexor work during descent. With increased walking speed, muscle activation tended to increase in younger but changed little in older adults across ascent (10 ± 12% vs. −1.0 ± 10%) and descent (3.6 ± 10.2% vs. −2.6 ± 7.7%) (p = .006, r = 0.47). Age does not affect the relationship between muscle activation and net joint work during incline and decline walking at freely-chosen step lengths. The electromechanical cost of joint work production does not underlie the age-related reconfiguration of joint work during walking.}, journal={JOURNAL OF BIOMECHANICS}, author={Waanders, Jeroen B. and Murgia, Alessio and DeVita, Paul and Franz, Jason R. and Hortobagyi, Tibor}, year={2021}, month={Jul} }
 @article{clark_franz_2021, title={Age-related changes to triceps surae muscle-subtendon interaction dynamics during walking}, volume={11}, ISSN={["2045-2322"]}, DOI={10.1038/s41598-021-00451-y}, abstractNote={Abstract}, number={1}, journal={SCIENTIFIC REPORTS}, author={Clark, William H. and Franz, Jason R.}, year={2021}, month={Oct} }
 @article{kim_franz_2021, title={Age-related differences in calf muscle recruitment strategies in the time-frequency domain during walking as a function of task demand}, volume={131}, ISSN={["1522-1601"]}, DOI={10.1152/japplphysiol.00262.2021}, abstractNote={ We use time-frequency analyses of calf muscle activation to quantify age-related differences in motor recruitment in walking. Gastrocnemius activation in older versus young adults was lower across all frequencies during midstance and in slow-to-middle frequencies during push-off, independent of speed, and shifted to slower frequencies with earlier timing as speed increased. Our results implicate gastrocnemius time-frequency content as a potential determinant of hallmark ankle push-off deficits due to aging, particularly at faster speeds. }, number={4}, journal={JOURNAL OF APPLIED PHYSIOLOGY}, author={Kim, Hoon and Franz, Jason R.}, year={2021}, month={Oct}, pages={1348–1360} }
 @article{armitano-lago_pietrosimone_evans-pickett_davis-wilson_franz_blackburn_kiefer_2021, title={Cueing Changes in Peak Vertical Ground Reaction Force to Improve Coordination Dynamics in Walking}, ISSN={["1940-1027"]}, DOI={10.1080/00222895.2021.1929810}, abstractNote={Abstracts Biofeedback has been effectively implemented to improve the mediation and distribution of joint loads during gait, however, the inability to effectively coordinate lower limb movement by altering loading patterns may increase pathological stress and risk of injury and deleterious joint changes. This study examined the influence cueing an increase or decrease in lower extremity loading has on inter- and intralimb joint coordination during gait, applied herein for 12 persons following anterior cruciate ligament reconstruction across three loading conditions (control, high, and low). Visual biofeedback was presented on a screen via a force-measuring treadmill with targeted changes prescribed based on stride-to-stride peak vertical ground reaction forces bilaterally. The pattern and stability of coordination dynamics among each of the ankle, hip and knee joint pairs were assessed via discrete relative phase and cross-recurrence quantification analyses for each condition. High and low loading altered the pattern and stability of intralimb coordination; low loading led to decreased coordination stability (20° greater than control condition) and high loading resulted in a more tightly coupled coordination pattern (higher %CDET). With thoughtful consideration for movement control, biofeedback can be used to target mechanisms leading to long-term deleterious joint adaptations.}, journal={JOURNAL OF MOTOR BEHAVIOR}, author={Armitano-Lago, Cortney and Pietrosimone, Brian and Evans-Pickett, Alyssa and Davis-Wilson, Hope and Franz, Jason R. and Blackburn, Troy and Kiefer, Adam W.}, year={2021}, month={May} }
 @article{maas_arndt_franz_2021, title={Editorial: Tendon Structure-Function Relationship in Health, Ageing, and Injury}, volume={3}, ISSN={["2624-9367"]}, DOI={10.3389/fspor.2021.701815}, abstractNote={EDITORIAL article Front. Sports Act. Living, 18 June 2021Sec. Biomechanics and Control of Human Movement https://doi.org/10.3389/fspor.2021.701815}, journal={FRONTIERS IN SPORTS AND ACTIVE LIVING}, author={Maas, Huub and Arndt, Toni and Franz, Jason R.}, year={2021}, month={Jun} }
 @article{conway_crudup_lewek_franz_2021, title={Effects of Horizontal Impeding Force Gait Training on Older Adult Push-Off Intensity}, volume={53}, ISSN={["1530-0315"]}, DOI={10.1249/MSS.0000000000002500}, abstractNote={ABSTRACT}, number={3}, journal={MEDICINE AND SCIENCE IN SPORTS AND EXERCISE}, author={Conway, Katie A. and Crudup, Keyaira L. and Lewek, Michael D. and Franz, Jason R.}, year={2021}, month={Mar}, pages={574–580} }
 @article{herrero_franz_lewek_2021, title={Gradually learning to increase gait propulsion in young unimpaired adults}, volume={75}, ISSN={["1872-7646"]}, DOI={10.1016/j.humov.2020.102745}, abstractNote={Distorted visual feedback (DVF) may employ both implicit and explicit approaches to enhance motor learning. Our purpose was to test the effect of DVF of gait propulsion on the capacity to alter propulsive forces, and to determine the biomechanical determinants of propulsion. Seventeen young unimpaired individuals walked for three minutes of baseline (no feedback), then completed three randomly ordered, 10-minute Learning conditions: Real, 10DVF, and 20DVF. During the DVF conditions, we gradually decreased the feedback value without the participants' knowledge. For all Learning conditions, participants were instructed to maintain the propulsive force between two targets representing ±1 standard deviation as obtained from baseline. A one-minute retention trial without any feedback was performed immediately after Learning. Participants increased propulsive forces and trailing limb angle in both DVF conditions that persisted through retention; however, no change in ankle plantarflexion moment was noted. These findings offer promise of translation to clinical populations with propulsion deficits and require combined implicit and explicit learning components.}, journal={HUMAN MOVEMENT SCIENCE}, author={Herrero, Luciana and Franz, Jason R. and Lewek, Michael D.}, year={2021}, month={Feb} }
 @article{clark_pimentel_franz_2021, title={Imaging and Simulation of Inter-muscular Differences in Triceps Surae Contributions to Forward Propulsion During Walking}, volume={49}, ISSN={["1573-9686"]}, DOI={10.1007/s10439-020-02594-x}, abstractNote={Forward propulsion during the push-off phase of walking is largely governed at the ankle by differential neuromechanical contributions from the biarticular medial (MG) and lateral gastrocnemii (LG) and the uniarticular soleus (SOL). However, the relative contribution of these individual muscles to forward propulsion is equivocal, with important implications for the design and control of wearable assistive devices and for targeted therapeutics. The aim of this study was to evaluate the agreement between empirical and model-predicted triceps surae (i.e., MG, LG, and SOL) contributions to forward propulsion during walking using conditions that systematically manipulated both walking speed and the mechanical demand for forward propulsion at a fixed speed-through the use of aiding and impeding forces. Ten young adults (age: 24.1 ± 3.6 years, 6M/4F) participated. We found that muscle-specific responses derived from experimental measurements (i.e., activation and fascicle behavior) were consistent with those derived from musculoskeletal simulations (i.e., muscle force and positive mechanical work) within the same subjects. In vivo, compared to walking normally, only LG muscle activation was affected by both aiding and impeding forces. Similarly, increased propulsive demand elicited greater relative fascicle shortening in the MG but not the SOL. In silico, only MG and LG force and positive mechanical work increased significantly to meet the increased demands for forward propulsion. By combining electromyography, ultrasound imaging, and musculoskeletal modeling in the same subjects, our cumulative findings suggest that the biarticular gastrocnemius muscles play a more significant role than the uniarticular soleus in governing changes in forward propulsion during the mid to late stance phase of walking.}, number={2}, journal={ANNALS OF BIOMEDICAL ENGINEERING}, author={Clark, William H. and Pimentel, Richard E. and Franz, Jason R.}, year={2021}, month={Feb}, pages={703–715} }
 @article{krupenevich_beck_sawicki_franz_2021, title={Reduced Achilles Tendon Stiffness Disrupts Calf Muscle Neuromechanics in Elderly Gait}, ISSN={["1423-0003"]}, DOI={10.1159/000516910}, abstractNote={Older adults walk slower and with a higher metabolic energy expenditure than younger adults. In this review, we explore the hypothesis that age-related declines in Achilles tendon stiffness increase the metabolic cost of walking due to less economical calf muscle contractions and increased proximal joint work. This viewpoint may motivate interventions to restore ankle muscle-tendon stiffness, improve walking mechanics, and reduce metabolic cost in older adults. }, journal={GERONTOLOGY}, author={Krupenevich, Rebecca L. and Beck, Owen N. and Sawicki, Gregory S. and Franz, Jason R.}, year={2021}, month={Jul} }
 @article{pieper_baudendistel_hass_diaz_krupenevich_franz_2021, title={The metabolic and mechanical consequences of altered propulsive force generation in walking}, volume={122}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2021.110447}, abstractNote={Older adults walk with greater metabolic energy consumption than younger for reasons that are not well understood. We suspect that a distal-to-proximal redistribution of leg muscle demand, from muscles spanning the ankle to those spanning the hip, contributes to greater metabolic energy costs. Recently, we found that when younger adults using biofeedback target smaller than normal peak propulsive forces (FP), they do so via a similar redistribution of leg muscle demand during walking. This alludes to an experimental paradigm that emulates characteristics of elderly gait independent of other age-related changes relevant to metabolic energy cost. Thus, our purpose was to quantify the metabolic and limb- and joint-level mechanical energy costs associated with modulating propulsive forces during walking in younger adults. Walking with larger FP increased net metabolic power by 47% (main effect, p = 0.001), which was accompanied by small by relatively uniform increases in hip, knee, and ankle joint power and which correlated with total joint power (R2 = 0.151, p = 0.019). Walking with smaller FP increased net metabolic power by 58% (main effect, p < 0.001), which was accompanied by higher step frequencies and increased total joint power due to disproportionate increases in hip joint power. Increases in hip joint power when targeting smaller than normal FP accounted for more than 65% of the variance in the measured changes in net metabolic power. Our findings suggest that walking with a diminished push-off exacts a metabolic penalty because of higher step frequencies and more total limb work due to an increased demand on proximal leg muscles.}, journal={JOURNAL OF BIOMECHANICS}, author={Pieper, Noah L. and Baudendistel, Sidney T. and Hass, Chris J. and Diaz, Gabriela B. and Krupenevich, Rebecca L. and Franz, Jason R.}, year={2021}, month={Jun} }
 @article{armitano-lago_pietrosimone_davis-wilson_evans-pickett_franz_blackburn_kiefer_2020, title={Biofeedback augmenting lower limb loading alters the underlying temporal structure of gait following anterior cruciate ligament reconstruction}, volume={73}, ISSN={["1872-7646"]}, DOI={10.1016/j.humov.2020.102685}, abstractNote={Biofeedback has recently been explored to target deviant lower extremity loading mechanics following anterior cruciate ligament reconstruction (ACLR) to mitigate the development of post traumatic osteoarthritis. The impact this feedback has on the structure of the stride interval dynamics-a barometer of gait system health-however, have yet to be examined. This study was designed to assess how feedback, used to alter lower-extremity loading during gait, affects the structure of stride interval variability by examining long-range stride-to-stride correlations during gait in those with unilateral ACLR. Twelve participants walked under three separate loading conditions: (1) control (i.e., no cue) (2) high loading, and (3) low loading. Baseline vertical ground reaction force (vGRF) data was used to calculate a target 5% change in vGRF for the appropriate loading condition (i.e., high loading was +5% vGRF, low loading was -5% vGRF). The target for the load condition was displayed on a screen along with real-time vGRF values, prescribing changes in stride-to-stride peak vertical ground reaction forces of each limb. From time-series of stride intervals (i.e., duration), we analyzed the mean and standard deviation of stride-to-stride variability and, via detrended fluctuation analysis (i.e., DFA α), temporal persistence for each feedback condition. Both the high and low loading conditions exhibited a change toward more temporally persistent stride intervals (high loading: α =0.92, low loading: α = 0.98) than walking under the control condition (α = 0.78; high vs. control: p = .026, low vs. control: p = .001). Overall, these results indicate that altering lower extremity load changes the temporal persistence of the stride internal dynamics in ACLR individuals, demonstrating the implications of the design of gait training interventions and the influence feedback has on movement strategies.}, journal={HUMAN MOVEMENT SCIENCE}, author={Armitano-Lago, Cortney and Pietrosimone, Brian and Davis-Wilson, Hope C. and Evans-Pickett, Alyssa and Franz, Jason R. and Blackburn, Troy and Kiefer, Adam W.}, year={2020}, month={Oct} }
 @article{evans-pickett_davis-wilson_luc-harkey_blackburn_franz_padua_seeley_pietrosimone_2020, title={Biomechanical effects of manipulating peak vertical ground reaction force throughout gait in individuals 6-12 months after anterior cruciate ligament reconstruction}, volume={76}, ISSN={["1879-1271"]}, DOI={10.1016/j.clinbiomech.2020.105014}, abstractNote={We aimed to determine the effect of cueing an increase or decrease in the vertical ground reaction force impact peak (peak in the first 50% of stance) on vertical ground reaction force, knee flexion angle, internal knee extension moment, and internal knee abduction moment waveforms throughout stance in individuals 6-12 months after an anterior cruciate ligament reconstruction.Twelve individuals completed 3 conditions (High, Low, and Control) where High and Low Conditions cue a 5% body weight increase or decrease, respectively, in the vertical ground reaction force impact peak compared to usual walking. Biomechanics during High and Low Conditions were compared to the Control Condition throughout stance.The High Condition resulted in: (a) increased vertical ground reaction forces at each peak and decreased during mid-stance, (b) greater knee excursion (i.e., greater knee flexion angle in early stance and a more extended knee in late stance), (c) greater internal extension moment for the majority of stance, and (d) lesser second internal knee abduction moment peak. The Low Condition resulted in: (a) vertical ground reaction forces decreased during early stance and increased during mid-stance, (b) decreased knee excursion, (c) increased internal extension moment throughout stance, and (d) decreased internal knee abduction moment peaks.Cueing a 5% body weight increase in vertical ground reaction force impact peak resulted in a more dynamic vertical ground reaction force loading pattern, increased knee excursion, and a greater internal extension moment during stance which may be useful in restoring gait patterns following anterior cruciate ligament reconstruction.}, journal={CLINICAL BIOMECHANICS}, author={Evans-Pickett, Alyssa and Davis-Wilson, Hope C. and Luc-Harkey, Brittney A. and Blackburn, J. Troy and Franz, Jason R. and Padua, Darin A. and Seeley, Matthew K. and Pietrosimone, Brian}, year={2020}, month={Jun} }
 @article{selgrade_meyer_sosnoff_franz_2020, title={Can optical flow perturbations detect walking balance impairment in people with multiple sclerosis?}, volume={15}, ISBN={1932-6203}, DOI={10.1371/journal.pone.0230202}, abstractNote={People with multiple sclerosis (PwMS) who exhibit minimal to no disability are still over twice as likely to fall as the general population and many of these falls occur during walking. There is a need for more effective ways to detect preclinical walking balance deficits in PwMS. Therefore, the purpose of this study was to investigate the effects of optical flow perturbations applied using virtual reality on walking balance in PwMS compared to age-matched controls. We hypothesized that susceptibility to perturbations–especially those in the mediolateral direction–would be larger in PwMS compared to controls. Fourteen PwMS and fourteen age-matched controls walked on a treadmill while viewing a virtual hallway with and without optical flow perturbations in the mediolateral or anterior-posterior directions. We quantified foot placement kinematics, gait variability, lateral margin of stability and, in a separate session, performance on the standing sensory organization test (SOT). We found only modest differences between groups during normal, unperturbed walking. These differences were larger and more pervasive in the presence of mediolateral perturbations, evidenced by higher variability in step width, sacrum position, and margin of stability at heel-strike in PwMS than controls. PwMS also performed worse than controls on the SOT, and there was a modest correlation between step width variability during perturbed gait and SOT visual score. In conclusion, mediolateral optical flow perturbations revealed differences in walking balance in PwMS that went undetected during normal, unperturbed walking. Targeting this difference may be a promising approach to more effectively detect preclinical walking balance deficits in PwMS.}, number={3}, journal={PLOS ONE}, author={Selgrade, Brian P. and Meyer, Diane and Sosnoff, Jacob J. and Franz, Jason R.}, year={2020} }
 @article{pieper_lewek_franz_2020, title={Can shank acceleration provide a clinically feasible surrogate for individual limb propulsion during walking?}, volume={98}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2019.109449}, abstractNote={Aging and many pathologies that affect gait are associated with reduced ankle power output and thus trailing limb propulsion during walking. However, quantifying trailing limb propulsion requires sophisticated measurement equipment at significant expense that fundamentally limits clinical translation for diagnostics or gait rehabilitation. As a component of joint power, our purpose was to determine if shank acceleration estimated via accelerometers during push-off can serve as a clinically feasible surrogate for ankle power output and peak anterior ground reaction forces (GRF) during walking. As hypothesized, we found that young adults modulated walking speed via changes in peak anterior GRF and peak ankle power output that correlated with proportional changes in shank acceleration during push-off, both at the individual subject (R2 ≥ 0.80, p < 0.01) and group average (R2 ≥ 0.74, p < 0.01) levels. In addition, we found that unilateral deficits in trailing limb propulsion induced via a leg bracing elicited unilateral and relatively proportional reductions in peak anterior GRF, peak ankle power, and peak shank acceleration. These unilateral leg bracing effects on peak shank acceleration correlated with those in peak ankle power (braced leg: R2 = 0.43, p = 0.028) but those effects in both peak shank acceleration and peak ankle power were disassociated from those in peak anterior GRF. In conclusion, our findings in young adults provide an early benchmark for the development of affordable and clinically feasible alternatives for assessing and monitoring trailing limb propulsion during walking.}, journal={JOURNAL OF BIOMECHANICS}, author={Pieper, Noah L. and Lewek, Michael D. and Franz, Jason R.}, year={2020}, month={Jan} }
 @article{selgrade_childs_franz_2020, title={Effects of aging and target location on reaction time and accuracy of lateral precision stepping during walking}, volume={104}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2020.109710}, abstractNote={Older adults have poorer lateral balance and deficits in precision stepping accuracy, but the way these deficits manifest with lateral step distance is unclear. The purpose of this study was to investigate aging effects on lateral precision stepping performance in reaction to near and distant foot placement targets during treadmill walking. We hypothesized that older adults would step to targets later and less accurately than young adults, and that these difference would be more pronounced for distant targets. During the study, young and older adults stepped on lateral targets projected onto the surface of a treadmill one stride prior to their targeting step. We measured stepping accuracy to the target, the time when the swing foot diverged from its normal swing trajectory, and swing phase gluteus medius activity. Both groups had similar performance stepping to near targets, suggesting that giving older subjects a full stride to react to target location mitigates visuomotor processing delays that have contributed to deficits in stepping performance in prior studies. However, when stepping to distant targets, older adults had larger errors and later divergence times than young adults. This suggests that age-related deficits other than those in visuomotor processing contribute to poorer performance for more difficult stepping tasks. Furthermore, while young adults increased early swing gluteus medius activity with lateral target distance, older adults did not. This is the first study to show a potential neuromuscular basis for precision stepping deficits in older adults.}, journal={JOURNAL OF BIOMECHANICS}, author={Selgrade, Brian P. and Childs, Marcus E. and Franz, Jason R.}, year={2020}, month={May} }
 @article{waanders_murgia_hortobagyi_devita_franz_2020, title={How age and surface inclination affect joint moment strategies to accelerate and decelerate individual leg joints during walking}, volume={98}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2019.109440}, abstractNote={A joint moment also causes motion at other joints of the body. This joint coupling-perspective allows more insight into two age-related phenomena during gait. First, whether increased hip kinetic output compensates for decreased ankle kinetic output during positive joint work. Second, whether preserved joint kinetic patterns during negative joint work in older age have any functional implication. Therefore, we examined how age and surface inclination affect joint moment strategies to accelerate and/or decelerate individual leg joints during walking. Healthy young (age: 22.5 ± 4.1 years, n = 18) and older (age: 76.0 ± 5.7 years, n = 22) adults walked at 1.4 m/s on a split-belt instrumented treadmill at three grades (0%, 10%, −10%). Lower-extremity moment-induced angular accelerations were calculated for the hip (0% and 10%) and knee (0% and −10%) joints. During level and uphill walking, both age groups showed comparable ankle moment-induced ipsilateral (p = 0.774) and contralateral (p = 0.047) hip accelerations, although older adults generated lower ankle moments in late stance. However, ankle moment-induced contralateral hip accelerations were smaller (p = 0.001) in an older adult subgroup (n = 13) who showed larger hip extension moments in early stance than young adults. During level and downhill walking, leg joint moment-induced knee accelerations were unaffected by age (all p > 0.05). These findings suggest that during level and uphill walking increased hip flexor mechanical output in older adults does not arise from reduced ankle moments, contrary to increased hip extensor mechanical output. Additionally, results during level and downhill walking imply that preserved eccentric knee extensor function is important in maintaining knee stabilization in older age.}, journal={JOURNAL OF BIOMECHANICS}, author={Waanders, Jeroen B. and Murgia, Alessio and Hortobagyi, Tibor and DeVita, Paul and Franz, Jason R.}, year={2020}, month={Jan} }
 @article{luc-harkey_franz_hackney_blackburn_padua_schwartz_davis-wilson_spang_pietrosimone_2020, title={Immediate Biochemical Changes After Gait Biofeedback in Individuals With Anterior Cruciate Ligament Reconstruction}, volume={55}, ISSN={["1938-162X"]}, DOI={10.4085/1062-6050-0372.19}, abstractNote={
                  Context
                  Gait biomechanics are linked to biochemical changes that contribute to the development of posttraumatic knee osteoarthritis in individuals with anterior cruciate ligament reconstruction (ACLR). It remains unknown if modifying peak loading during gait using real-time biofeedback will result in acute biochemical changes related to cartilage metabolism.
               }, number={10}, journal={JOURNAL OF ATHLETIC TRAINING}, author={Luc-Harkey, Brittney A. and Franz, Jason and Hackney, Anthony C. and Blackburn, J. Troy and Padua, Darin A. and Schwartz, Todd and Davis-Wilson, Hope and Spang, Jeffrey and Pietrosimone, Brian}, year={2020}, month={Oct}, pages={1106–1115} }
 @article{conway_franz_2020, title={Increasing the Propulsive Demands of Walking to Their Maximum Elucidates Functionally Limiting Impairments in Older Adult Gait}, volume={28}, ISSN={["1543-267X"]}, DOI={10.1123/japa.2018-0327}, abstractNote={The authors elucidated functional limitations in older adult gait by increasing horizontal impeding forces and walking speed to their maximums compared with dynamometry and with data from their young counterparts. Specifically, the authors investigated which determinants of push-off intensity represent genuine functionally limiting impairments in older adult gait versus biomechanical changes that do not directly limit walking performance. They found that older adults walked at their preferred speed with hallmark deficits in push-off intensity. These subjects were fully capable of overcoming deficits in propulsive ground reaction force, trailing limb positive work, trailing leg and hip extension, and ankle power generation when the propulsive demands of walking were increased to maximum. Of the outcomes tested, age-related deficits in ankle moment emerged as the lone genuine functionally limiting impairment in older adults. Distinguishing genuine functional limitations from age-related differences masquerading as limitations represents a critical step toward the development and prescription of effective interventions.}, number={1}, journal={JOURNAL OF AGING AND PHYSICAL ACTIVITY}, author={Conway, Katie A. and Franz, Jason R.}, year={2020}, month={Feb}, pages={1–8} }
 @article{krupenevich_clark_sawicki_franz_2020, title={Older Adults Overcome Reduced Triceps Surae Structural Stiffness to Preserve Ankle Joint Quasi-Stiffness During Walking}, volume={36}, ISSN={["1543-2688"]}, DOI={10.1123/jab.2019-0340}, abstractNote={Ankle joint quasi-stiffness is an aggregate measure of the interaction between triceps surae muscle stiffness and Achilles tendon stiffness. This interaction may be altered due to age-related changes in the structural properties and functional behavior of the Achilles tendon and triceps surae muscles. The authors hypothesized that, due to a more compliant of Achilles’ tendon, older adults would exhibit lower ankle joint quasi-stiffness than young adults during walking and during isolated contractions at matched triceps surae muscle activations. The authors also hypothesized that, independent of age, triceps surae muscle stiffness and ankle joint quasi-stiffness would increase with triceps surae muscle activation. The authors used conventional gait analysis in one experiment and, in another, electromyographic biofeedback and in vivo ultrasound imaging applied during isolated contractions. The authors found no difference in ankle joint quasi-stiffness between young and older adults during walking. Conversely, this study found that (1) young and older adults modulated ankle joint quasi-stiffness via activation-dependent changes in triceps surae muscle length–tension behavior and (2) at matched activation, older adults exhibited lower ankle joint quasi-stiffness than young adults. Despite age-related reductions during isolated contractions, ankle joint quasi-stiffness was maintained in older adults during walking, which may be governed via activation-mediated increases in muscle stiffness.}, number={4}, journal={JOURNAL OF APPLIED BIOMECHANICS}, author={Krupenevich, Rebecca L. and Clark, William H. and Sawicki, Gregory S. and Franz, Jason R.}, year={2020}, month={Aug}, pages={209–216} }
 @article{conway_franz_2020, title={Shorter gastrocnemius fascicle lengths in older adults associate with worse capacity to enhance push-off intensity in walking}, volume={77}, ISSN={["1879-2219"]}, DOI={10.1016/j.gaitpost.2020.01.018}, abstractNote={Reduced push-off intensity during walking is thought to play an important role in age-related mobility impairment. We posit that an age-related shift toward shorter plantarflexor operating lengths during walking functionally limits force generation, and thereby the ability of those muscles to respond to increased propulsive demands during walking. To determine whether gastrocnemius muscle fascicle lengths during normal walking: (1) are shorter in older than young adults, and (2) correlate with one's capacity to increase the propulsive demands of walking to their maximum. We used in vivo cine B-mode ultrasound to measure gastrocnemius fascicle lengths in 9 older and 9 young adults walking at their preferred speed, their maximum speed, and with horizontal impeding forces that increased in a ramped design at 1%BW/s to their maximum. A repeated measures ANOVA tested for effects of age and walking condition, and Pearson correlations assessed the relation between fascicle outcomes and condition performance. A tendency toward shorter medial gastrocnemius muscle fascicle lengths in older versus young adults was not statistically significant. However, older adults walked with reduced peak fascicle shortening during all conditions compared to young adults – an outcome not explained by reduced muscle-tendon unit shortening and exacerbated during tasks with greater than normal propulsive demand. As hypothesized, we found a strong and significant positive correlation in older subjects between gastrocnemius fascicle lengths during normal walking and performance on the ramped impeding force condition (p = 0.005, r² = 0.704), even after controlling for isometric strength (p = 0.011, r² = 0.792) and subject stature (p = 0.010, r² = 0.700). Our findings provide muscle-level insight to develop more effective rehabilitation techniques to improve push-off intensity in older adults and assistive technologies designed to steer plantarflexor muscle fascicle operating behavior during functional tasks.}, journal={GAIT & POSTURE}, author={Conway, Katie A. and Franz, Jason R.}, year={2020}, month={Mar}, pages={89–94} }
 @article{munsch_pietrosimone_franz_2020, title={The effects of knee extensor moment biofeedback on gait biomechanics and quadriceps contractile behavior}, volume={8}, ISSN={["2167-8359"]}, DOI={10.7717/peerj.9509}, abstractNote={Individuals with knee joint pathologies exhibit quadriceps dysfunction that, during walking, manifests as smaller peak knee extensor moment (pKEM) and reduced knee flexion excursion. These changes persist despite muscle strengthening and may alter stance phase knee joint loading considered relevant to osteoarthritis risk. Novel rehabilitation strategies that more directly augment quadriceps mechanical output during functional movements are needed to reduce this risk. As an important first step, we tested the efficacy of real-time biofeedback during walking to prescribe changes of ±20% and ±40% of normal walking pKEM values in 11 uninjured young adults. We simultaneously recorded knee joint kinematics, ground reaction forces, and, via ultrasound, vastus lateralis (VL) fascicle length change behavior. Participants successfully responded to real-time biofeedback and averaged up to 55% larger and 51% smaller than normal pKEM values with concomitant and potentially favorable changes in knee flexion excursion. While the VL muscle-tendon unit (MTU) lengthened, VL fascicles accommodated weight acceptance during walking largely through isometric, or even slight concentric, rather than eccentric action as is commonly presumed. Targeted pKEM biofeedback may be a useful rehabilitative and/or scientific tool to elicit desirable changes in knee joint biomechanics considered relevant to the development of osteoarthritis.}, journal={PEERJ}, author={Munsch, Amanda E. and Pietrosimone, Brian and Franz, Jason R.}, year={2020}, month={Jul} }
 @misc{awad_lewek_kesar_franz_bowden_2020, title={These legs were made for propulsion: advancing the diagnosis and treatment of post-stroke propulsion deficits}, volume={17}, ISSN={["1743-0003"]}, DOI={10.1186/s12984-020-00747-6}, abstractNote={Abstract}, number={1}, journal={JOURNAL OF NEUROENGINEERING AND REHABILITATION}, author={Awad, Louis N. and Lewek, Michael D. and Kesar, Trisha M. and Franz, Jason R. and Bowden, Mark G.}, year={2020}, month={Oct} }
 @article{clark_franz_2020, title={Triceps surae muscle-subtendon interaction differs between young and older adults}, volume={61}, ISSN={["1607-8438"]}, DOI={10.1080/03008207.2019.1612384}, abstractNote={ABSTRACT Background: Mechanical power generated via triceps surae muscle-tendon interaction during walking is important for walking performance. This interaction is made complex by distinct “subtendons” arising from the lateral and medial gastrocnemius (GAS) and soleus (SOL) muscles. Comparative data and our own in vivo evidence allude to a reduced capacity for sliding between adjacent subtendons compromising the Achilles tendon in old age. However, its unclear if and how these changes affect muscle contractile behavior.Objective: We investigated aging effects on triceps surae muscle-subtendon interaction using dual-probe ultrasound imaging during isolated muscle contractions. We hypothesized that, compared to young adults, older adults would have more uniform subtendon tissue displacements that are accompanied by anatomically consistent differences in GAS versus SOL muscle length change behavior.Materials and Methods: 9 younger subjects (age: 25.1 ± 5.6 years) and 10 older adult subjects (age: 74.3 ± 3.4 years) completed a series of ramped maximum isometric voluntary contractions at ankle angles spanning 0° (neutral) to 30° plantarflexion. Two linear array ultrasound transducers simultaneously recorded GAS and SOL fascicle kinematics and tissue displacements in their associated tendinous structures.Results: We revealed that older adults have more uniform subtendon tissue displacements that extend to anatomically consistent and potentially unfavorable changes in muscle contractile behavior – evidenced by smaller differences between gastrocnemius and soleus peak shortening during isometric force generation.Conclusions: These findings provide an important biomechanical basis for previously reported correlations between more uniform Achilles subtendon behavior and reduced ankle moment generation during waking in older adults.}, number={1}, journal={CONNECTIVE TISSUE RESEARCH}, author={Clark, William H. and Franz, Jason R.}, year={2020}, month={Jan}, pages={104–113} }
 @article{clark_franz_2019, title={Activation-Dependent Changes in Soleus Length-Tension Behavior Augment Ankle Joint Quasi-Stiffness}, volume={35}, ISSN={["1543-2688"]}, DOI={10.1123/jab.2018-0297}, abstractNote={The triceps surae muscle-tendon units are important in governing walking performance, acting to regulate mechanical behavior of the ankle through interaction between active muscle and passive elastic structures. Ankle joint quasi-stiffness (the slope of the relation between ankle moment and ankle rotation, kA) is a useful aggregate measure of this mechanical behavior. However, the role of muscle activation and length-tension behavior in augmenting kA remains unclear. In this study, 10 subjects completed eccentric isokinetic contractions at rest and at 2 soleus activation levels (25% and 75% isometric voluntary contraction) prescribed using electromyographic biofeedback. Ultrasound imaging quantified activation-dependent modulation of soleus muscle length-tension behavior and its role in augmenting kA. The authors found that soleus muscle stiffness (kM) and kA exhibit nonlinear relations with muscle activation and both were more sensitive to the onset of activation than to subsequent increases in activation. Our findings also suggest that kA can be modulated via activation through changes in soleus muscle length-tension behavior. However, this modulation is more complex than previously appreciated-reflecting interaction between active muscle and passive elastic tissues. Our findings may have implications for understanding normal and pathological ankle joint function and the design of impedance-based prostheses.}, number={3}, journal={JOURNAL OF APPLIED BIOMECHANICS}, author={Clark, William H. and Franz, Jason R.}, year={2019}, month={Jun}, pages={182–189} }
 @article{waanders_hortobagyi_murgia_devita_franz_2019, title={Advanced Age Redistributes Positive but Not Negative Leg Joint Work during Walking}, volume={51}, ISSN={["1530-0315"]}, DOI={10.1249/MSS.0000000000001828}, abstractNote={ABSTRACT}, number={4}, journal={MEDICINE AND SCIENCE IN SPORTS AND EXERCISE}, author={Waanders, Jeroen B. and Hortobagyi, Tibor and Murgia, Alessio and Devita, Paul and Franz, Jason R.}, year={2019}, month={Apr}, pages={615–623} }
 @article{franz_khanchandani_mckenny_clark_2019, title={Ankle Rotation and Muscle Loading Effects on the Calcaneal Tendon Moment Arm: An In Vivo Imaging and Modeling Study}, volume={47}, ISSN={["1573-9686"]}, DOI={10.1007/s10439-018-02162-4}, abstractNote={In this combined in vivo and computational modeling study, we tested the central hypothesis that ankle joint rotation and triceps surae muscle loading have independent and combinatory effects on the calcaneal (i.e., Achilles) tendon moment arm (CTma) that are not fully captured in contemporary musculoskeletal models of human movement. We used motion capture guided ultrasound imaging to estimate instantaneous variations in the CTma during a series of isometric and isotonic contractions compared to predictions from scaled, lower extremity computational models. As hypothesized, we found that muscle loading: (i) independently increased the CTma by up to 8% and (ii) attenuated the effects of ankle joint rotation, the latter likely through changes in tendon slack and tendon curvature. Neglecting the effects of triceps surae muscle loading in lower extremity models led to an underestimation of the CTma, on average, particularly in plantarflexion when those effects were most prominent. We also found little agreement between in vivo estimates and model predictions on an individual subject by subject basis, alluding to unaccounted for variation in anatomical morphology and thus fundamental limitations in model scaling. Together, these findings contribute to improving our understanding of the physiology of ankle moment and power generation and novel opportunities for model development.}, number={2}, journal={ANNALS OF BIOMEDICAL ENGINEERING}, author={Franz, Jason R. and Khanchandani, Ashish and McKenny, Hannah and Clark, William H.}, year={2019}, month={Feb}, pages={590–600} }
 @article{browne_franz_2019, title={Ankle power biofeedback attenuates the distal-to-proximal redistribution in older adults}, volume={71}, ISSN={["1879-2219"]}, DOI={10.1016/j.gaitpost.2019.04.011}, abstractNote={Compared to young adults, older adults walk slower, with shorter strides, and with a characteristic decrease in ankle power output. Seemingly in response, older adults rely more than young on hip power output, a phenomenon known as a distal-to-proximal redistribution. Nevertheless, older adults can increase ankle power to walk faster or uphill, revealing a translationally important gap in our understanding. Our purpose was to implement a novel ankle power biofeedback paradigm to encourage favorable biomechanical adaptations (i.e. reverse the distal-redistribution) during habitual speed walking in older adults. 10 healthy older adults walked at their preferred speeds while real-time visual biofeedback provided target increases and decreases of 10 and 20% different from preferred ankle power. We evaluated the effect of changes in ankle power on joint kinetics, kinematics, and propulsive ground reaction forces. Pre and post overground walking speed assessments evaluated the effect of increased ankle power recall on walking speed. Biofeedback systematically elicited changes in ankle power; increasing and decreasing ankle power by 14% and 17% when targeting ±20% different from preferred, respectively. We observed a significant negative correlation between ankle power and hip extensor work. Older adults relied more heavily on changes in ankle angular velocity than ankle moment to modulate ankle power. Lastly, older adults walked almost 11% faster when recalling increased ankle power overground. Older adults are capable of increasing ankle power through targeted ankle power biofeedback – effects that are accompanied by diminished hip power output and attenuation of the distal-to-proximal redistribution. The associated increase in preferred walking speed during recall suggests a functional benefit to increased ankle power output via transfer to overground walking. Further, our mechanistic insights allude to translational success using ankle angular velocity as a surrogate to modulate ankle power through biofeedback.}, journal={GAIT & POSTURE}, author={Browne, Michael G. and Franz, Jason R.}, year={2019}, month={Jun}, pages={44–49} }
 @article{acuna_francis_franz_thelen_2019, title={The effects of cognitive load and optical flow on antagonist leg muscle coactivation during walking for young and older adults}, volume={44}, ISSN={["1873-5711"]}, DOI={10.1016/j.jelekin.2018.11.003}, abstractNote={The purpose of this study was to compare how healthy aging interacts with environments that challenge cognitive load and optical flow to affect antagonist leg muscle coactivation during walking. We measured leg muscle activity in sixteen older adults (70.4 ± 4.2 years) and twelve young adults (23.6 ± 3.9 years) walking on a treadmill at their preferred speed while watching a speed-matched virtual hallway. Cognitive load was challenged using a dual-task to interfere with available attentional resources. Optical flow was challenged using perturbations designed to create a perception of lateral imbalance. We found antagonist coactivation increased with aging, independent of condition. We also found that, compared to unperturbed walking, only in the presence of optical flow perturbations did the older adults increase their antagonist coactivation. Antagonist coactivation in the young adults was not affected by either condition. Our findings provide evidence that antagonist leg muscle coactivation in healthy older adults is more sensitive to walking environments that challenge optical flow than environments that challenge cognitive load. As increased antagonist coactivation may indicate compromised balance, these findings may be relevant in the design of living environments to reduce falls risk.}, journal={JOURNAL OF ELECTROMYOGRAPHY AND KINESIOLOGY}, author={Acuna, Samuel A. and Francis, Carrie A. and Franz, Jason R. and Thelen, Darryl G.}, year={2019}, month={Feb}, pages={8–14} }
 @article{richards_selgrade_qiao_plummer_wikstrom_franz_2019, title={Time-dependent tuning of balance control and aftereffects following optical flow perturbation training in older adults}, volume={16}, ISBN={1743-0003}, DOI={10.1186/s12984-019-0555-3}, abstractNote={Walking balance in older adults is disproportionately susceptible to lateral instability provoked by optical flow perturbations. The prolonged exposure to these perturbations could promote reactive balance control and increased balance confidence in older adults, but this scientific premise has yet to be investigated. This proof of concept study was designed to investigate the propensity for time-dependent tuning of walking balance control and the presence of aftereffects in older adults following a single session of optical flow perturbation training. Thirteen older adults participated in a randomized, crossover design performed on different days that included 10 min of treadmill walking with (experimental session) and without (control session) optical flow perturbations. We used electromyographic recordings of leg muscle activity and 3D motion capture to quantify foot placement kinematics, lateral margin of stability, and antagonist coactivation during normal walking (baseline), early (min 1) and late (min 10) responses to perturbations, and aftereffects immediately following perturbation cessation (post). At their onset, perturbations elicited 17% wider and 7% shorter steps, higher step width and length variability (+171% and +132%, respectively), larger and more variable margins of stability (MoS), and roughly twice the antagonist leg muscle coactivation (p-values<0.05). Despite continued perturbations, most outcomes returned to values observed during normal, unperturbed walking by the end of prolonged exposure. After 10 min of perturbation training and their subsequent cessation, older adults walked with longer and more narrow steps, modest increases in foot placement variability, and roughly half the MoS variability and antagonist lower leg muscle coactivation as they did before training. Findings suggest that older adults: (i) respond to the onset of perturbations using generalized anticipatory balance control, (ii) deprioritize that strategy following prolonged exposure to perturbations, and (iii) upon removal of perturbations, exhibit short-term aftereffects that indicate a lessening of anticipatory control, an increase in reactive control, and/or increased balance confidence. We consider this an early, proof-of-concept study into the clinical utility of prolonged exposure to optical flow perturbations as a training tool for corrective motor adjustments relevant to walking balance integrity toward reinforcing task-specific, reactive control and/or improving balance confidence in older adults. clinicaltrials.gov ( NCT03341728 ). Registered 14 November 2017.}, journal={JOURNAL OF NEUROENGINEERING AND REHABILITATION}, author={Richards, Jackson T. and Selgrade, Brian P. and Qiao, Mu and Plummer, Prudence and Wikstrom, Erik A. and Franz, Jason R.}, year={2019} }
 @article{qiao_richards_franz_2019, title={Visuomotor error augmentation affects mediolateral head and trunk stabilization during walking}, volume={68}, ISSN={["1872-7646"]}, DOI={10.1016/j.humov.2019.102525}, abstractNote={Prior work demonstrates that humans spontaneously synchronize their head and trunk kinematics to a broad range of driving frequencies of perceived mediolateral motion prescribed using optical flow. Using a closed-loop visuomotor error augmentation task in an immersive virtual environment, we sought to understand whether unifying visual with vestibular and somatosensory feedback is a control goal during human walking, at least in the context of head and trunk stabilization. We hypothesized that humans would minimize visual errors during walking – i.e., those between the visual perception of movement and actual movement of the trunk. We found that subjects did not minimize errors between the visual perception of movement and actual movement of the head and trunk. Rather, subjects increased mediolateral trunk range of motion in response to error-augmented optical flow with positive feedback gains. Our results are more consistent with our alternative hypothesis – that visual feedback can override other sensory modalities and independently compel adjustments in head and trunk position. Also, aftereffects following exposure to error-augmented optical flow included longer, narrower steps and reduced mediolateral postural sway, particularly in response to larger amplitude positive feedback gains. Our results allude to a recalibration of head and trunk stabilization toward more tightly regulated postural control following exposure to error-augmented visual feedback. Lasting reductions in mediolateral postural sway may have implications for using error-augmented optical flow to enhance the integrity of walking balance control through training, for example in older adults.}, journal={HUMAN MOVEMENT SCIENCE}, author={Qiao, Mu and Richards, Jackson T. and Franz, Jason R.}, year={2019}, month={Dec} }
 @article{qiao_feld_franz_2018, title={Aging effects on leg joint variability during walking with balance perturbations}, volume={62}, ISSN={["1879-2219"]}, DOI={10.1016/j.gaitpost.2018.02.020}, abstractNote={Older adults are more susceptible to balance perturbations during walking than young adults. However, we lack an individual joint-level understanding of how aging affects the neuromechanical strategies used to accommodate balance perturbations. We investigated gait phase-dependence in and aging effects on leg joint kinematic variability during walking with balance perturbations. We hypothesized that leg joint variability would: 1) vary across the gait cycle and 2) increase with balance perturbations. We also hypothesized that perturbation effects on leg joint kinematic variability would be larger and more pervasive in older versus young adults. We collected leg joint kinematics in young and older adults walking with and without mediolateral optical flow perturbations of different amplitudes. We first found that leg joint variability during walking is gait phase-dependent, with step-to-step adjustments occurring predominantly during push-off and early swing. Second, young adults accommodated perturbations almost exclusively by increasing coronal plane hip joint variability, likely to adjust step width. Third, perturbations elicited larger and more pervasive increases in all joint kinematic outcome measures in older adults. Finally, we also provide insight into which joints contribute more to foot placement variability in walking, adding that variability in sagittal plane knee and coronal plane hip joint angles contributed most to that in step length and step width, respectively. Taken together, our findings may be highly relevant to identifying specific joint-level therapeutic targets to mitigate balance impairment in our aging population.}, journal={GAIT & POSTURE}, author={Qiao, Mu and Feld, Jody A. and Franz, Jason R.}, year={2018}, month={May}, pages={27–33} }
 @article{luc-harkey_franz_losina_pietrosimone_2018, title={Association between kinesiophobia and walking gait characteristics in physically active individuals with anterior cruciate ligament reconstruction}, volume={64}, ISSN={["1879-2219"]}, DOI={10.1016/j.gaitpost.2018.06.029}, abstractNote={Individuals with anterior cruciate ligament reconstruction (ACLR) demonstrate persistent alterations in walking gait characteristics that contribute to poor long-term outcomes. Higher kinesiophobia, or fear of movement/re-injury, may result in the avoidance of movements that increase loading on the ACLR limb. Determine the association between kinesiophobia and walking gait characteristics in physically active individuals with ACLR. We enrolled thirty participants with a history of unilateral ACLR (49.35 ± 27.29 months following ACLR) into this cross-sectional study. We used the Tampa Scale for Kinesiophobia (TSK-11) to measure kinesiophobia. We collected walking gait characteristics during a 60-s walking trial, which included gait speed, peak vertical ground reaction force (vGRF), instantaneous vGRF loading rate, peak internal knee extension moment (KEM), and knee flexion excursion. We calculated lower extremity kinetic and kinematic measures on the ACLR limb, and limb symmetry indices between ACLR and contralateral limbs (LSI= [ACLR/contralateral]*100). We used linear regression models to determine the association between TSK-11 score and each walking gait characteristic. We determined the change in R2 (ΔR2) when adding TSK-11 scores into the linear regression model after accounting for demographic covariates (sex, Tegner activity score, graft type, time since reconstruction, history of concomitant meniscal procedure). We did not find a significant association between kinesiophobia and self-selected gait speed (ΔR2 0.038, P = 0.319). Kinesiophobia demonstrated weak, non-significant associations with kinetic and kinematic outcomes on the ACLR limb and all LSI outcomes (ΔR2 range = 0.001–0.098). These data do not support that kinesiophobia is a critical factor contributing to walking gait characteristics in physically active individuals with ACLR.}, journal={GAIT & POSTURE}, author={Luc-Harkey, Brittney A. and Franz, Jason R. and Losina, Elena and Pietrosimone, Brian}, year={2018}, month={Jul}, pages={220–225} }
 @article{clark_franz_2018, title={Do triceps surae muscle dynamics govern non-uniform Achilles tendon deformations?}, volume={6}, journal={PeerJ}, author={Clark, W. H. and Franz, J. R.}, year={2018} }
 @article{qiao_truong_franz_2018, title={Does local dynamic stability during unperturbed walking predict the response to balance perturbations? An examination across age and falls history}, volume={62}, ISSN={["1879-2219"]}, DOI={10.1016/j.gaitpost.2018.03.011}, abstractNote={Older adults are at an exceptionally high risk of falls, and most falls occur during locomotor activities such as walking. Reduced local dynamic stability in old age is often interpreted to suggest a lessened capacity to respond to more significant balance challenges encountered during walking and future falls risk. However, it remains unclear whether local dynamic stability during normal, unperturbed walking predicts the response to larger external balance disturbances. We tested the hypothesis that larger values of local dynamic instability during unperturbed walking would positively correlate with larger changes thereof due to optical flow balance perturbations. We used trunk kinematics collected in subjects across a spectrum of walking balance integrity – young adults, older non-fallers, and older fallers – during walking with and without mediolateral optical flow perturbations of four different amplitudes. We first found evidence that optical flow perturbations of sufficient amplitude appear capable of revealing independent effects of aging and falls history that are not otherwise apparent during normal, unperturbed walking. We also reject our primary hypothesis; a significant negative correlation only in young adults indicated that individuals with more local dynamic instability during normal, unperturbed walking exhibited smaller responses to optical flow perturbations. In contrast, most prominently in older fallers, the response to optical flow perturbations appeared independent of their baseline level of dynamic instability. We propose that predicting the response to balance perturbations in older fallers, at least that measured using local dynamic stability, likely requires measuring that response directly.}, journal={GAIT & POSTURE}, author={Qiao, Mu and Truong, Kinh N. and Franz, Jason R.}, year={2018}, month={May}, pages={80–85} }
 @article{conway_bissette_franz_2018, title={The Functional Utilization of Propulsive Capacity During Human Walking}, volume={34}, ISSN={["1543-2688"]}, DOI={10.1123/jab.2017-0389}, abstractNote={Aging and many gait pathologies are characterized by reduced propulsive forces and ankle moment and power generation during trailing leg push-off in walking. Despite those changes, we posit that many individuals retain an underutilized reserve for enhancing push-off intensity during walking that may be missed using conventional dynamometry. By using a maximum ramped impeding force protocol and maximum speed walking, we gained mechanistic insight into the factors that govern push-off intensity and the available capacity thereof during walking in young subjects. We discovered in part that young subjects walking at their preferred speed retain a reserve capacity for exerting larger propulsive forces of 49%, peak ankle power of 43%, and peak ankle moment of 22% during push-off—the latter overlooked by maximum isometric dynamometry. We also provide evidence that these reserve capacities are governed at least in part by the neuromechanical behavior of the plantarflexor muscles, at least with regard to ankle moment generation. We envision that a similar paradigm used to quantify propulsive reserves in older adults or people with gait pathology would empower the more discriminate and personalized prescription of gait interventions seeking to improve push-off intensity and thus walking performance.}, number={6}, journal={JOURNAL OF APPLIED BIOMECHANICS}, author={Conway, Katie A. and Bissette, Randall G. and Franz, Jason R.}, year={2018}, month={Dec}, pages={474–482} }
 @article{thompson_franz_2017, title={Do kinematic metrics of walking balance adapt to perturbed optical flow?}, volume={54}, ISSN={["1872-7646"]}, DOI={10.1016/j.humov.2017.03.004}, abstractNote={Visual (i.e., optical flow) perturbations can be used to study balance control and balance deficits. However, it remains unclear whether walking balance control adapts to such perturbations over time. Our purpose was to investigate the propensity for visuomotor adaptation in walking balance control using prolonged exposure to optical flow perturbations. Ten subjects (age: 25.4 ± 3.8 years) walked on a treadmill while watching a speed-matched virtual hallway with and without continuous mediolateral optical flow perturbations of three different amplitudes. Each of three perturbation trials consisted of 8 min of prolonged exposure followed by 1 min of unperturbed walking. Using 3D motion capture, we analyzed changes in foot placement kinematics and mediolateral sacrum motion. At their onset, perturbations elicited wider and shorter steps, alluding to a more cautious, general anticipatory balance control strategy. As perturbations continued, foot placement tended toward values seen during unperturbed walking while step width variability and mediolateral sacrum motion concurrently increased. Our findings suggest that subjects progressively shifted from a general anticipatory balance control strategy to a reactive, task-specific strategy using step-to-step adjustments. Prolonged exposure to optical flow perturbations may have clinical utility to reinforce reactive, task-specific balance control through training.}, journal={HUMAN MOVEMENT SCIENCE}, author={Thompson, Jessica D. and Franz, Jason R.}, year={2017}, month={Aug}, pages={34–40} }
 @article{browne_franz_2017, title={Does dynamic stability govern propulsive force generation in human walking?}, volume={4}, ISSN={["2054-5703"]}, DOI={10.1098/rsos.171673}, abstractNote={
            Before succumbing to slower speeds, older adults may walk with a diminished push-off to prioritize stability over mobility. However, direct evidence for trade-offs between push-off intensity and balance control in human walking, independent of changes in speed, has remained elusive. As a critical first step, we conducted two experiments to investigate: (i) the independent effects of walking speed and propulsive force (
            F
            P
            ) generation on dynamic stability in young adults, and (ii) the extent to which young adults prioritize dynamic stability in selecting their preferred combination of walking speed and
            F
            P
            generation. Subjects walked on a force-measuring treadmill across a range of speeds as well as at constant speeds while modulating their
            F
            P
            according to a visual biofeedback paradigm based on real-time force measurements. In contrast to improvements when walking slower, walking with a diminished push-off worsened dynamic stability by up to 32%. Rather, we find that young adults adopt an
            F
            P
            at their preferred walking speed that maximizes dynamic stability. One implication of these findings is that the onset of a diminished push-off in old age may independently contribute to poorer balance control and precipitate slower walking speeds.
          }, number={11}, journal={ROYAL SOCIETY OPEN SCIENCE}, author={Browne, Michael G. and Franz, Jason R.}, year={2017}, month={Nov} }
 @article{zelik_franz_2017, title={It's positive to be negative: Achilles tendon work loops during human locomotion}, volume={12}, ISSN={["1932-6203"]}, DOI={10.1371/journal.pone.0179976}, abstractNote={Ultrasound imaging is increasingly used with motion and force data to quantify tendon dynamics during human movement. Frequently, tendon dynamics are estimated indirectly from muscle fascicle kinematics (by subtracting muscle from muscle-tendon unit length), but there is mounting evidence that this Indirect approach yields implausible tendon work loops. Since tendons are passive viscoelastic structures, when they undergo a loading-unloading cycle they must exhibit a negative work loop (i.e., perform net negative work). However, prior studies using this Indirect approach report large positive work loops, often estimating that tendons return 2–5 J of elastic energy for every 1 J of energy stored. More direct ultrasound estimates of tendon kinematics have emerged that quantify tendon elongations by tracking either the muscle-tendon junction or localized tendon tissue. However, it is unclear if these yield more plausible estimates of tendon dynamics. Our objective was to compute tendon work loops and hysteresis losses using these two Direct tendon kinematics estimates during human walking. We found that Direct estimates generally resulted in negative work loops, with average tendon hysteresis losses of 2–11% at 1.25 m/s and 33–49% at 0.75 m/s (N = 8), alluding to 0.51–0.98 J of tendon energy returned for every 1 J stored. We interpret this finding to suggest that Direct approaches provide more plausible estimates than the Indirect approach, and may be preferable for understanding tendon energy storage and return. However, the Direct approaches did exhibit speed-dependent trends that are not consistent with isolated, in vitro tendon hysteresis losses of about 5–10%. These trends suggest that Direct estimates also contain some level of error, albeit much smaller than Indirect estimates. Overall, this study serves to highlight the complexity and difficulty of estimating tendon dynamics non-invasively, and the care that must be taken to interpret biological function from current ultrasound-based estimates.}, number={7}, journal={PLOS ONE}, author={Zelik, Karl E. and Franz, Jason R.}, year={2017}, month={Jul} }
 @article{wittenberg_thompson_nam_franz_2017, title={Neuroimaging of Human Balance Control: A Systematic Review}, volume={11}, ISSN={1662-5161}, url={http://dx.doi.org/10.3389/fnhum.2017.00170}, DOI={10.3389/fnhum.2017.00170}, abstractNote={This review examined 83 articles using neuroimaging modalities to investigate the neural correlates underlying static and dynamic human balance control, with aims to support future mobile neuroimaging research in the balance control domain. Furthermore, this review analyzed the mobility of the neuroimaging hardware and research paradigms as well as the analytical methodology to identify and remove movement artifact in the acquired brain signal. We found that the majority of static balance control tasks utilized mechanical perturbations to invoke feet-in-place responses (27 out of 38 studies), while cognitive dual-task conditions were commonly used to challenge balance in dynamic balance control tasks (20 out of 32 studies). While frequency analysis and event related potential characteristics supported enhanced brain activation during static balance control, that in dynamic balance control studies was supported by spatial and frequency analysis. Twenty-three of the 50 studies utilizing EEG utilized independent component analysis to remove movement artifacts from the acquired brain signals. Lastly, only eight studies used truly mobile neuroimaging hardware systems. This review provides evidence to support an increase in brain activation in balance control tasks, regardless of mechanical, cognitive, or sensory challenges. Furthermore, the current body of literature demonstrates the use of advanced signal processing methodologies to analyze brain activity during movement. However, the static nature of neuroimaging hardware and conventional balance control paradigms prevent full mobility and limit our knowledge of neural mechanisms underlying balance control.}, journal={Frontiers in Human Neuroscience}, publisher={Frontiers Media SA}, author={Wittenberg, Ellen and Thompson, Jessica and Nam, Chang S. and Franz, Jason R.}, year={2017}, month={Apr} }
 @article{stokes_thompson_franz_2017, title={The Neuromuscular Origins of Kinematic Variability during Perturbed Walking}, volume={7}, ISSN={["2045-2322"]}, DOI={10.1038/s41598-017-00942-x}, abstractNote={Abstract}, journal={SCIENTIFIC REPORTS}, author={Stokes, Heather E. and Thompson, Jessica D. and Franz, Jason R.}, year={2017}, month={Apr} }
 @article{orselli_franz_thelen_2017, title={The effects of Achilles tendon compliance on triceps surae mechanics and energetics in walking}, volume={60}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2017.06.022}, abstractNote={Achilles tendon (AT) compliance can affect the generation and transmission of triceps surae muscle forces, and thus has important biomechanical consequences for walking performance. However, the uniarticular soleus (SOL) and the biarticular (GAS) function differently during walking, with in vivo evidence suggesting that their associated fascicles and tendinous structures exhibit unique kinematics during walking. Given the strong association between muscle fiber length, velocity and force production, we conjectured that SOL and GAS mechanics and energetic behavior would respond differently to altered AT compliance. To test this, we characterized GAS and SOL muscle and tendon mechanics and energetics due to systematic changes in tendon compliance using musculoskeletal simulations of walking. Increased tendon compliance enlarged GAS and SOL tendon excursions, shortened fiber operation lengths and affected muscle excitation patterns. For both muscles, an optimal tendon compliance (tendon strains of approximately 5% with maximum isometric force) existed that minimized metabolic energy consumption. However, GAS muscle-tendon mechanics and energetics were significantly more sensitive to changes in tendon compliance than were those for SOL. In addition, GAS was not able to return stored tendon energy during push-off as effectively as SOL, particularly for larger values of tendon compliance. These fundamental differences between GAS and SOL sensitivity to altered tendon compliance seem to arise from the biarticular nature of GAS. These insights are potentially important for understanding the functional consequences of altered Achilles tendon compliance due to aging, injury, or disease.}, journal={JOURNAL OF BIOMECHANICS}, author={Orselli, Maria Isabel V. and Franz, Jason R. and Thelen, Darryl G.}, year={2017}, month={Jul}, pages={227–231} }
 @article{browne_franz_2017, title={The independent effects of speed and propulsive force on joint power generation in walking}, volume={55}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2017.02.011}, abstractNote={Walking speed is modulated using propulsive forces (FP) during push-off and both preferred speed and FP decrease with aging. However, even prior to walking slower, reduced FP may be accompanied by potentially unfavorable changes in joint power generation. For example, compared to young adults, older adults exhibit a redistribution of mechanical power generation from the propulsive plantarflexor muscles to more proximal muscles acting across the knee and hip. Here, we used visual biofeedback based on real-time FP measurements to decouple and investigate the interaction between joint-level coordination, whole-body FP, and walking speed. 12 healthy young subjects walked on a dual-belt instrumented treadmill at a range of speeds (0.9–1.3 m/s). We immediately calculated the average FP from each speed. Subjects then walked at 1.3 m/s while completing a series of biofeedback trials with instructions to match their instantaneous FP to their averaged FP from slower speeds. Walking slower decreased FP and total positive joint work with little effect on relative joint-level contributions. Conversely, subjects walked at a constant speed with reduced FP, not by reducing total positive joint work, but by redistributing the mechanical demands of each step from the plantarflexor muscles during push-off to more proximal leg muscles during single support. Interestingly, these naturally emergent joint- and limb-level biomechanical changes, in the absence of neuromuscular constraints, resemble those due to aging. Our findings provide important reference data to understand the presumably complex interactions between joint power generation, whole-body FP, and walking speed in our aging population.}, journal={JOURNAL OF BIOMECHANICS}, author={Browne, Michael G. and Franz, Jason R.}, year={2017}, month={Apr}, pages={48–55} }
 @article{rasske_thelen_franz_2017, title={Variation in the human Achilles tendon moment arm during walking}, volume={20}, ISSN={["1476-8259"]}, DOI={10.1080/10255842.2016.1213818}, abstractNote={Abstract The Achilles tendon (AT) moment arm is an important determinant of ankle moment and power generation during locomotion. Load and depth-dependent variations in the AT moment arm are generally not considered, but may be relevant given the complex triceps surae architecture. We coupled motion analysis and ultrasound imaging to characterize AT moment arms during walking in 10 subjects. Muscle loading during push-off amplified the AT moment arm by 10% relative to heel strike. AT moment arms also varied by 14% over the tendon thickness. In walking, AT moment arms are not strictly dependent on kinematics, but exhibit important load and spatial dependencies.}, number={2}, journal={COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING}, author={Rasske, Kristen and Thelen, Darryl G. and Franz, Jason R.}, year={2017}, pages={201–205} }
 @article{franz_francis_allen_thelen_2017, title={Visuomotor Entrainment and the Frequency-Dependent Response of Walking Balance to Perturbations}, volume={25}, ISSN={["1558-0210"]}, DOI={10.1109/tnsre.2016.2603340}, abstractNote={Visuomotor entrainment, or the synchronization of motor responses to visual stimuli, is a naturally emergent phenomenon in human standing. Our purpose was to investigate the prevalence and resolution of visuomotor entrainment in walking and the frequency-dependent response of walking balance to perturbations. We used a virtual reality environment to manipulate optical flow in ten healthy young adults during treadmill walking. A motion capture system recorded trunk, sacrum, and heel marker trajectories during a series of 3-min conditions in which we perturbed a virtual hallway mediolaterally with systematic changes in the driving frequencies of perceived motion. We quantified visuomotor entrainment using spectral analyses and changes in balance control using trunk sway, gait variability, and detrended fluctuation analyses (DFA). ML kinematics were highly sensitive to visual perturbations, and instinctively synchronized (i.e., entrained) to a broad range of driving frequencies of perceived ML motion. However, the influence of visual perturbations on metrics of walking balance was frequency-dependent and governed by their proximity to stride frequency. Specifically, we found that a driving frequency nearest to subjects’ average stride frequency uniquely compromised trunk sway, gait variability, and step-to-step correlations. We conclude that visuomotor entrainment is a robust and naturally emerging phenomenon during human walking, involving coordinated and frequency-dependent adjustments in trunk sway and foot placement to maintain balance at the whole-body level. These findings provide mechanistic insight into how the visuomotor control of walking balance is disrupted by visual perturbations and important reference values for the emergence of balance deficits due to age, injury, or disease.}, number={8}, journal={IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING}, author={Franz, Jason R. and Francis, Carrie A. and Allen, Matthew S. and Thelen, Darryl G.}, year={2017}, month={Aug}, pages={1135–1142} }
 @article{franz_thelen_2016, title={Imaging and simulation of Achilles tendon dynamics: Implications for walking performance in the elderly}, volume={49}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2016.04.032}, abstractNote={The Achilles tendon (AT) is a complex structure, consisting of distinct fascicle bundles arising from each triceps surae muscle that may act as mechanically independent structures. Advances in tissue imaging are rapidly accelerating our understanding of the complexities of functional Achilles tendon behavior, with potentially important implications for musculoskeletal injury and performance. In this overview of our recent contributions to these efforts, we present the results of complementary experimental and computational approaches to investigate AT behavior during walking and its potential relevance to reduced triceps surae mechanical performance due to aging. Our experimental evidence reveals that older tendons exhibit smaller differences in tissue deformations than young adults between regions of the AT presumed to arise from the gastrocnemius and soleus muscles. These observations are consistent with a reduced capacity for inter-fascicle sliding within the AT, which could have implications for the mechanical independence of the triceps surae muscles. More uniform AT deformations are also correlated with hallmark biomechanical features of elderly gait – namely, a loss of net ankle moment, power, and positive work during push-off. Simulating age-related reductions in the capacity for inter-fascicle sliding in the AT during walking predicts detriments in gastrocnemius muscle-tendon mechanical performance coupled with underlying shifts in fascicle kinematics during push-off. AT compliance, also suspected to vary due to age, systematically modulates those effects. By integrating in vivo imaging with computational modeling, we have gained theoretical insight into multi-scale biomechanical changes due to aging, hypotheses regarding their functional effects, and opportunities for experiments that validate or invalidate these assertions.}, number={9}, journal={JOURNAL OF BIOMECHANICS}, author={Franz, Jason R. and Thelen, Darryl G.}, year={2016}, month={Jun}, pages={1403–1410} }
 @misc{franz_2016, title={The Age-Associated Reduction in Propulsive Power Generation in Walking}, volume={44}, ISSN={["1538-3008"]}, DOI={10.1249/jes.0000000000000086}, abstractNote={
                           Propulsive power generation during push-off in walking decreases with advancing age. A common explanation is an accommodation for sarcopenia and muscle weakness. Yet, muscle strengthening often yields disappointing outcomes for walking performance. We examine the hypothesis that declines in force or power generating capacity of propulsive leg muscles cannot fully explain the age-related reduction in propulsive power generation during walking.
                        }, number={4}, journal={EXERCISE AND SPORT SCIENCES REVIEWS}, author={Franz, Jason R.}, year={2016}, month={Oct}, pages={129–136} }