@misc{blemker_brooks_esser_saul_2024, title={Fiber-type traps: revisiting common misconceptions about skeletal muscle fiber types with application to motor control, biomechanics, physiology, and biology}, volume={136}, ISSN={["1522-1601"]}, DOI={10.1152/japplphysiol.00337.2023}, abstractNote={ Skeletal muscle is a highly complex tissue that is studied by scientists from a wide spectrum of disciplines, including motor control, biomechanics, exercise science, physiology, cell biology, genetics, regenerative medicine, orthopedics, and engineering. Although this diversity in perspectives has led to many important discoveries, historically, there has been limited overlap in discussions across fields. This has led to misconceptions and oversimplifications about muscle biology that can create confusion and potentially slow scientific progress across fields. The purpose of this synthesis paper is to bring together research perspectives across multiple muscle fields to identify common assumptions related to muscle fiber type that are points of concern to clarify. These assumptions include 1) classification by myosin isoform and fiber oxidative capacity is equivalent, 2) fiber cross-sectional area (CSA) is a surrogate marker for myosin isoform or oxidative capacity, and 3) muscle force-generating capacity can be inferred from myosin isoform. We address these three fiber-type traps and provide some context for how these misunderstandings can and do impact experimental design, computational modeling, and interpretations of findings, from the perspective of a range of fields. We stress the dangers of generalizing findings about “muscle fiber types” among muscles or across species or sex, and we note the importance for precise use of common terminology across the muscle fields. }, number={1}, journal={JOURNAL OF APPLIED PHYSIOLOGY}, author={Blemker, Silvia S. and Brooks, Susan V. and Esser, Karyn A. and Saul, Katherine R.}, year={2024}, month={Jan}, pages={109–121} }
 @article{rubin_hinson_saul_filer_hu_huang_2024, title={Modified motor unit properties in residual muscle following transtibial amputation}, volume={21}, ISSN={["1741-2552"]}, DOI={10.1088/1741-2552/ad1ac2}, abstractNote={Abstract}, number={1}, journal={JOURNAL OF NEURAL ENGINEERING}, author={Rubin, Noah and Hinson, Robert and Saul, Katherine and Filer, William and Hu, Xiaogang and Huang, He}, year={2024}, month={Feb} }
 @misc{blemker_brooks_esser_saul_2024, title={Reply to Drs. Reggiani and Murgia}, volume={136}, ISSN={["1522-1601"]}, DOI={10.1152/japplphysiol.00037.2024}, number={2}, journal={JOURNAL OF APPLIED PHYSIOLOGY}, author={Blemker, Silvia S. and Brooks, Susan V. and Esser, Karyn A. and Saul, Katherine R.}, year={2024}, month={Feb}, pages={439–439} }
 @article{lantis_saul_schmidt_2024, title={Short-term effects of running exercise on pinch strength, grip strength, and manual dexterity of the dominant and non-dominant hands}, ISSN={["1366-5847"]}, DOI={10.1080/00140139.2024.2344688}, abstractNote={Occupations including first responders and military require manual tasks; therefore changes in hand strength and dexterity could affect performance. We hypothesised that pinch strength, grip strength, and dexterity will change after unloaded and loaded exercise. Twenty-four male (25 ± 4.0 yrs; 86.3 ± 9.3 kg) and 10 female (25 ± 6.0 yrs; 62.1 ± 5.9 kg) participants completed 3 conditions for 5 minutes: (1) no exercise (2) run with no load at 3.0 m/s and (3) run wearing a 9.1 kg belt. Heart rate was different among conditions (p ≤ 0.05). Pinch strength was significantly different for the non-dominant hand after exercise (p = 0.005) for male participants, but not for the dominant hand. Grip strength was significantly different for the non-dominant hand between loaded and unloaded run (p = 0.035) for male participants. Pinch and grip strength did not change after exercise for female participants. Dexterity times were not different after exercise, but female participants were significantly faster (p ≤ 0.039) than male participants.}, journal={ERGONOMICS}, author={Lantis, Kristen D. and Saul, Katherine and Schmidt, Deanna J.}, year={2024}, month={Apr} }
 @article{goins_fox_saul_servello_sullivan_2024, title={The Relationship Between Elbow Flexion Postures and Overhead Reaching in Birth Brachial Plexus Injuries}, volume={36}, ISSN={["1538-005X"]}, DOI={10.1097/PEP.0000000000001059}, abstractNote={
            Purpose:
            The aim of this study was to investigate the effect of alterations in muscle length of the biceps in various elbow postures during shoulder elevation and muscle activation.
          }, number={1}, journal={PEDIATRIC PHYSICAL THERAPY}, author={Goins, T. R. and Fox, John and Saul, Katherine and Servello, Cindy and Sullivan, Jane E.}, year={2024}, month={Jan}, pages={62–69} }
 @article{rubin_hinson_saul_hu_huang_2023, title={Ankle Torque Estimation With Motor Unit Discharges in Residual Muscles Following Lower-Limb Amputation}, volume={31}, ISSN={["1558-0210"]}, url={http://dx.doi.org/10.1109/tnsre.2023.3336543}, DOI={10.1109/TNSRE.2023.3336543}, abstractNote={There has been increased interest in using residual muscle activity for neural control of powered lower-limb prostheses. However, only surface electromyography (EMG)-based decoders have been investigated. This study aims to investigate the potential of using motor unit (MU)-based decoding methods as an alternative to EMG-based intent recognition for ankle torque estimation. Eight people without amputation (NON) and seven people with amputation (AMP) participated in the experiments. Subjects conducted isometric dorsi- and plantarflexion with their intact limb by tracing desired muscle activity of the tibialis anterior (TA) and gastrocnemius (GA) while ankle torque was recorded. To match phantom limb and intact limb activity, AMP mirrored muscle activation with their residual TA and GA. We compared neuromuscular decoders (linear regression) for ankle joint torque estimation based on 1) EMG amplitude (aEMG), 2) MU firing frequencies representing neural drive (ND), and 3) MU firings convolved with modeled twitch forces (MUDrive). In addition, sensitivity analysis and dimensionality reduction of optimization were performed on the MUDrive method to further improve its practical value. Our results suggest MUDrive significantly outperforms (lower root-mean-square error) EMG and ND methods in muscles of NON, as well as both intact and residual muscles of AMP. Reducing the number of optimized MUDrive parameters degraded performance. Even so, optimization computational time was reduced and MUDrive still outperformed aEMG. Our outcomes indicate integrating MU discharges with modeled biomechanical outputs may provide a more accurate torque control signal than direct EMG control of assistive, lower-limb devices, such as exoskeletons and powered prostheses.}, journal={IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING}, author={Rubin, Noah and Hinson, Robert and Saul, Katherine and Hu, Xiaogang and Huang, He}, year={2023}, pages={4821–4830} }
 @article{jadelis_ellis_kamper_saul_2023, title={Cosimulation of the index finger extensor apparatus with finite element and musculoskeletal models}, volume={157}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2023.111725}, abstractNote={Musculoskeletal modeling has been effective for simulating dexterity and exploring the consequences of disability. While previous approaches have examined motor function using multibody dynamics, existing musculoskeletal models of the hand and fingers have difficulty simulating soft tissue such as the extensor mechanism of the fingers, which remains underexplored. To investigate the extensor mechanism and its impact on finger motor function, we developed a finite element model of the index finger extensor mechanism and a cosimulation method that combines the finite element model with a multibody dynamic model. The finite element model and cosimulation were validated through comparison with experimentally derived tissue strains and fingertip endpoint forces respectively. Tissue strains predicted by the finite element model were consistent with the experimentally observed strains of the 9 postures tested in cadaver specimens. Fingertip endpoint forces predicted using the cosimulation were well aligned in both force (difference within 0.60 N) and direction (difference within 30°with experimental results. Sensitivity of the extensor mechanism to changes in modulus and adhesion configuration were evaluated for ± 50% of experimental moduli, presence of the radial and ulnar adhesions, and joint capsule. Simulated strains and endpoint forces were found to be minimally sensitive to alterations in moduli and adhesions. These results are promising and demonstrate the ability of the cosimulation to predict global behavior of the extensor mechanism, while enabling measurement of stresses and strains within the structure itself. This model could be used in the future to predict the outcomes for different surgical repairs of the extensor mechanism.}, journal={JOURNAL OF BIOMECHANICS}, author={Jadelis, Christopher T. and Ellis, Benjamin J. and Kamper, Derek G. and Saul, Katherine R.}, year={2023}, month={Aug} }
 @article{mccain_dalman_berno_libera_lewek_sawicki_saul_2023, title={The influence of induced gait asymmetry on joint reaction forces}, volume={153}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2023.111581}, abstractNote={Chronic injury- or disease-induced joint impairments result in asymmetric gait deviations that may precipitate changes in joint loading associated with pain and osteoarthritis. Understanding the impact of gait deviations on joint reaction forces (JRFs) is challenging because of concurrent neurological and/or anatomical changes and because measuring JRFs requires medically invasive instrumented implants. Instead, we investigated the impact of joint motion limitations and induced asymmetry on JRFs by simulating data recorded as 8 unimpaired participants walked with bracing to unilaterally and bilaterally restrict ankle, knee, and simultaneous ankle + knee motion. Personalized models, calculated kinematics, and ground reaction forces (GRFs) were input into a computed muscle control tool to determine lower limb JRFs and simulated muscle activations guided by electromyography-driven timing constraints. Unilateral knee restriction increased GRF peak and loading rate ipsilaterally but peak values decreased contralaterally when compared to walking without joint restriction. GRF peak and loading rate increased with bilateral restriction compared to the contralateral limb of unilaterally restricted conditions. Despite changes in GRFs, JRFs were relatively unchanged due to reduced muscle forces during loading response. Thus, while joint restriction results in increased limb loading, reductions in muscle forces counteract changes in limb loading such that JRFs were relatively unchanged.}, journal={JOURNAL OF BIOMECHANICS}, author={McCain, Emily M. and Dalman, Morgan J. and Berno, Matthew E. and Libera, Theresa L. and Lewek, Michael D. and Sawicki, Gregory S. and Saul, Katherine R.}, year={2023}, month={May} }
 @article{howe_dixit_saul_fisher_2022, title={A Direct Comparison of Node and Element-Based Finite Element Modeling Approaches to Study Tissue Growth}, volume={144}, ISSN={["1528-8951"]}, DOI={10.1115/1.4051661}, abstractNote={Abstract}, number={1}, journal={JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME}, author={Howe, Danielle and Dixit, Nikhil N. and Saul, Katherine R. and Fisher, Matthew B.}, year={2022}, month={Jan} }
 @article{wong_moore_lewis_reid_saul_carey_2022, title={Ergonomic Assessment of Surgeon Characteristics and Laparoscopic Device Strain in Gynecologic Surgery}, volume={29}, ISSN={["1553-4669"]}, DOI={10.1016/j.jmig.2022.09.552}, abstractNote={To evaluate whether surgeon characteristics, including sex and hand size, were associated with grip strength decline with laparoscopic advanced energy devices.Prospective cohort study.Ergonomic simulation at an academic tertiary care site and the Society of Gynecologic Surgeons 47th Annual Meeting.Thirty-eight participants (19 women and 19 men) were recruited.Surgeon anthropometric measurements were collected. Each participant completed a 120-second trial of maximum voluntary effort with 3 laparoscopic advanced energy devices (LigaSure, HALO PKS, and ENSEAL). Grip strength was measured using a handheld dynamometer. Subjects completed the NASA Raw Task Load Index scale after each device trial. Grip strengths and ergonomic workload scores were compared using Student t tests and Wilcoxon rank sum tests where appropriate. Univariate and multivariate models analyzed hand size and ergonomic workload.Women had lower baseline grip strength (288 vs 451 N) than men, as did participants with glove size <7 compared with ≥7 (231 vs 397 N). Normalized grip strength was not associated with surgeon sex (p = .08), whereas it was significantly associated with surgeon glove size (p <.01). Grip strength decline was significantly greater for smaller compared to larger handed surgeons for LigaSure (p = .02) and HALO PKS devices (p <.01). The ergonomic workload of device use was significantly greater for smaller compared to larger handed surgeons (p <.01). Surgeon handspan significantly predicted grip strength decline with device use, even after accounting for potential confounders (R2 = .23, β = .8, p <.01).Surgeons with smaller hand size experienced a greater grip strength decline and greater ergonomic workload during repetitive laparoscopic device use. No relationship was found between surgeon sex and grip strength decline or ergonomic workload. Laparoscopic device type was also identified as a significant main effect contributing to grip strength decline. These findings point toward ergonomic strain stemming from an improper fit between the laparoscopic device and the surgeon's hand during device use.}, number={12}, journal={JOURNAL OF MINIMALLY INVASIVE GYNECOLOGY}, author={Wong, Jacqueline M. K. and Moore, Kristin J. and Lewis, Preston and Reid, Monique and Saul, Katherine and Carey, Erin T.}, year={2022}, month={Dec}, pages={1357–1363} }
 @article{dalman_liao_saul_2022, title={Evaluating anthropometric scaling of a generic adult model to represent pediatric shoulder strength}, volume={141}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2022.111170}, abstractNote={The structure of the developing musculoskeletal system during childhood and adolescence influences tissue loading and function. Anatomical features important for musculoskeletal loading such as muscle volume and limb proportion vary with age but limited available anatomical data for the developing limb makes predicting loads challenging. Our aim was to evaluate whether anthropometric scaling of an existing adult musculoskeletal upper limb model is sufficient to accurately represent pediatric strength. An adult upper limb model was scaled using two scale factors based on length features and max isometric force (MIF). Length features (e.g. limb and muscle length) were scaled based on linear regression for available literature reports of forearm length vs. height (N = 366 Pediatric, N = 107 Adults), while MIF was scaled based on relating body mass vs. total shoulder muscle volume (N = 6). Children-specific models were developed for 6 pediatric individuals whose height, body mass, and shoulder moment-generating capacity (a common measure of strength) were previously reported. These models were used to predict isometric shoulder moments for flexion/extension, internal/external rotation, and ad/abduction and compared with physical measurements previously reported. The predicted isometric shoulder moments were significantly correlated to measured moments for these same individuals (p < 0.04, r2 > 0.7). However, predicted moments tended to underestimate measured values; shoulder external rotation was most accurately predicted (slope: 1.1234) while shoulder adduction was most underestimated (slope: 0.4624). This work provides an initial basis for pediatric scaling but illustrates the important need for additional direct measures of muscle size and limb strength and function in a pediatric population.}, journal={JOURNAL OF BIOMECHANICS}, author={Dalman, Morgan and Liao, Ashlee and Saul, Katherine R.}, year={2022}, month={Aug} }
 @article{hinson jr_saul_kamper_huang_2022, title={Sensitivity analysis guided improvement of an electromyogram-driven lumped parameter musculoskeletal hand model}, volume={141}, ISSN={["1873-2380"]}, url={http://dx.doi.org/10.1016/j.jbiomech.2022.111200}, DOI={10.1016/j.jbiomech.2022.111200}, abstractNote={EMG-driven neuromusculoskeletal models have been used to study many impairments and hold great potential to facilitate human–machine interactions for rehabilitation. A challenge to successful clinical application is the need to optimize the model parameters to produce accurate kinematic predictions. In order to identify the key parameters, we used Monte-Carlo simulations to evaluate the sensitivities of wrist and metacarpophalangeal (MCP) flexion/extension prediction accuracies for an EMG-driven, lumped-parameter musculoskeletal model. Four muscles were modeled with 22 total optimizable parameters. Model predictions from EMG were compared with measured joint angles from 11 able-bodied subjects. While sensitivities varied by muscle, we determined muscle moment arms, maximum isometric force, and tendon slack length were highly influential, while passive stiffness and optimal fiber length were less influential. Removing the two least influential parameters from each muscle reduced the optimization search space from 22 to 14 parameters without significantly impacting prediction correlation (wrist: 0.90 ± 0.05 vs 0.90 ± 0.05, p = 0.96; MCP: 0.74 ± 0.20 vs 0.70 ± 0.23, p = 0.51) and normalized root mean square error (wrist: 0.18 ± 0.03 vs 0.19 ± 0.03, p = 0.16; MCP: 0.18 ± 0.06 vs 0.19 ± 0.06, p = 0.60). Additionally, we showed that wrist kinematic predictions were insensitive to parameters of the modeled MCP muscles. This allowed us to develop a novel optimization strategy that more reliably identified the optimal set of parameters for each subject (27.3 ± 19.5%) compared to the baseline optimization strategy (6.4 ± 8.1%; p = 0.004). This study demonstrated how sensitivity analyses can be used to guide model refinement and inform novel and improved optimization strategies, facilitating implementation of musculoskeletal models for clinical applications.}, journal={JOURNAL OF BIOMECHANICS}, publisher={Elsevier BV}, author={Hinson Jr, Robert Jr and Saul, Katherine and Kamper, Derek and Huang, He}, year={2022}, month={Aug} }
 @article{dixit_mccormick_cole_saul_2021, title={Influence of Brachial Plexus Birth Injury Location on Glenohumeral Joint Morphology}, volume={46}, ISSN={["1531-6564"]}, DOI={10.1016/j.jhsa.2020.10.019}, abstractNote={Patient presentation after brachial plexus birth injury (BPBI) is influenced by nerve injury location; more contracture and bone deformity occur at the shoulder in postganglionic injuries. Although bone deformity after postganglionic injury is well-characterized, the extent of glenohumeral deformity after preganglionic BPBI is unclear.Twenty Sprague-Dawley rat pups received preganglionic or postganglionic neurectomy on a single forelimb at postnatal days 3 to 4. Glenohumeral joints on affected and unaffected sides were analyzed using micro-computed tomography scans after death at 8 weeks after birth. Glenoid version, glenoid inclination, glenoid and humeral head radius of curvature, and humeral head thickness and width were measured bilaterally.The glenoid was significantly more declined in affected compared with unaffected shoulders after postganglionic (-17.7° ± 16.9°) but not preganglionic injury. Compared with the preganglionic group, the affected shoulder in the postganglionic group exhibited significantly greater declination and increased glenoid radius of curvature. In contrast, the humeral head was only affected after preganglionic but not postganglionic injury, with a significantly smaller humeral head radius of curvature (-0.2 ± 0.2 mm), thickness (-0.2 ± 0.3 mm), and width (-0.3 ± 0.4 mm) on the affected side compared with the unaffected side; changes in these metrics were significantly associated with each other.These findings suggest that glenoid deformities occur after postganglionic BPBI but not after preganglionic BPBI, whereas the humeral head is smaller after preganglionic injury, possibly suggesting an overall decreased biological growth rate in this group.This study expands understanding of the altered glenoid and humeral head morphologies after preganglionic BPBI and its comparisons with morphologies after postganglionic BPBI.}, number={6}, journal={JOURNAL OF HAND SURGERY-AMERICAN VOLUME}, author={Dixit, Nikhil N. and McCormick, Carolyn M. and Cole, Jacqueline H. and Saul, Katherine R.}, year={2021}, month={Jun} }
 @article{mccain_libera_berno_sawicki_saul_lewek_2021, title={Isolating the energetic and mechanical consequences of imposed reductions in ankle and knee flexion during gait}, volume={18}, ISSN={["1743-0003"]}, DOI={10.1186/s12984-021-00812-8}, abstractNote={Abstract}, number={1}, journal={JOURNAL OF NEUROENGINEERING AND REHABILITATION}, author={McCain, Emily M. and Libera, Theresa L. and Berno, Matthew E. and Sawicki, Gregory S. and Saul, Katherine R. and Lewek, Michael D.}, year={2021}, month={Feb} }
 @article{doshi_reid_dixit_fawcett_cole_saul_2021, title={Location of brachial plexus birth injury affects functional outcomes in a rat model}, volume={9}, ISSN={["1554-527X"]}, url={https://doi.org/10.1002/jor.25173}, DOI={10.1002/jor.25173}, abstractNote={Abstract}, journal={JOURNAL OF ORTHOPAEDIC RESEARCH}, publisher={Wiley}, author={Doshi, Raveena M. and Reid, Monique Y. and Dixit, Nikhil N. and Fawcett, Emily B. and Cole, Jacqueline H. and Saul, Katherine R.}, year={2021}, month={Sep} }
 @article{mccain_berno_libera_lewek_sawicki_saul_2021, title={Reduced joint motion supersedes asymmetry in explaining increased metabolic demand during walking with mechanical restriction}, volume={126}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2021.110621}, abstractNote={Recent research has highlighted the complex interactions among chronic injury- or disease-induced joint limitations, walking asymmetry, and increased metabolic cost. Determining the specific metabolic impacts of asymmetry or joint impairment in clinical populations is difficult because of concurrent neurological and physiological changes. This work investigates the metabolic impact of gait asymmetry and joint restriction by unilaterally (asymmetric) and bilaterally (symmetric) restricting ankle, knee, and combined ankle and knee ranges of motion in unimpaired individuals. We calculated propulsive asymmetry, temporal asymmetry, and step-length asymmetry for an average gait cycle; metabolic rate; average positive center of mass power using the individual limbs method; and muscle effort using lower limb electromyography measurements weighted by corresponding physiological cross-sectional areas. Unilateral restriction caused propulsive and temporal asymmetry but less metabolically expensive gait than bilateral restriction. Changes in asymmetry did not correlate with changes in metabolic cost. Interestingly, bilateral restriction increased average positive center of mass power compared to unilateral restriction. Further, increased average positive center of mass power correlated with increased energy costs, suggesting asymmetric step-to-step transitions did not drive metabolic changes. The number of restricted joints reduces available degrees of freedom and may have a larger metabolic impact than gait asymmetry, as this correlated significantly with increases in metabolic rate for 7/9 participants. These results emphasize symmetry is not by definition metabolically optimal, indicate that the mechanics underlying symmetry are meaningful, and suggest that available degrees of freedom should be considered in designing future interventions.}, journal={JOURNAL OF BIOMECHANICS}, author={McCain, Emily M. and Berno, Matthew E. and Libera, Theresa L. and Lewek, Michael D. and Sawicki, Gregory S. and Saul, Katherine R.}, year={2021}, month={Sep} }
 @article{wu_saul_huang_2021, title={Using Reinforcement Learning to Estimate Human Joint Moments From Electromyography or Joint Kinematics: An Alternative Solution to Musculoskeletal-Based Biomechanics}, volume={143}, ISBN={1528-8951}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85107163923&partnerID=MN8TOARS}, DOI={10.1115/1.4049333}, abstractNote={Abstract}, number={4}, journal={JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME}, author={Wu, Wen and Saul, Katherine R. and Huang, He}, year={2021} }
 @article{nelson_hall_saul_crouch_2020, title={Effect of Mechanically Passive, Wearable Shoulder Exoskeletons on Muscle Output During Dynamic Upper Extremity Movements: A Computational Simulation Study}, volume={36}, ISSN={["1543-2688"]}, DOI={10.1123/jab.2018-0369}, abstractNote={Wearable passive (ie, spring powered) shoulder exoskeletons could reduce muscle output during motor tasks to help prevent or treat shoulder musculoskeletal disorders. However, most wearable passive shoulder exoskeletons have been designed and evaluated for static tasks, so it is unclear how they affect muscle output during dynamic tasks. The authors used a musculoskeletal model and Computed Muscle Control optimization to estimate muscle output with and without a wearable passive shoulder exoskeleton during 2 simulated dynamic tasks: abduction and upward reach. To an existing upper extremity musculoskeletal model, the authors added an exoskeleton model with 3-dimensional representations of the exoskeleton components, including a spring, cam wheel, force-transmitting shoulder cable, and wrapping surfaces that permitted the shoulder cable to wrap over the shoulder. The exoskeleton reduced net muscle-generated moments in positive shoulder elevation by 28% and 62% during the abduction and upward reach, respectively. However, muscle outputs (joint moments and muscle effort) were higher with the exoskeleton than without at some points of the movement. Muscle output was higher with the exoskeleton because the exoskeleton moment opposed the muscle-generated moment in some postures. The results of this study highlight the importance of evaluating muscle output for passive exoskeletons designed to support dynamic movements to ensure that the exoskeletons assist, rather than impede, movement.}, number={2}, journal={JOURNAL OF APPLIED BIOMECHANICS}, author={Nelson, Allison J. and Hall, Patrick T. and Saul, Katherine R. and Crouch, Dustin L.}, year={2020}, month={Apr}, pages={59–67} }
 @article{mcfarland_brynildsen_saul_2020, title={Sensitivity of Neuromechanical Predictions to Choice of Glenohumeral Stability Modeling Approach}, volume={36}, ISSN={["1543-2688"]}, DOI={10.1123/jab.2019-0088}, abstractNote={Most upper-extremity musculoskeletal models represent the glenohumeral joint with an inherently stable ball-and-socket, but the physiological joint requires active muscle coordination for stability. The authors evaluated sensitivity of common predicted outcomes (instability, net glenohumeral reaction force, and rotator cuff activations) to different implementations of active stabilizing mechanisms (constraining net joint reaction direction and incorporating normalized surface electromyography [EMG]). Both EMG and reaction force constraints successfully reduced joint instability. For flexion, incorporating any normalized surface EMG data reduced predicted instability by 54.8%, whereas incorporating any force constraint reduced predicted instability by 43.1%. Other outcomes were sensitive to EMG constraints, but not to force constraints. For flexion, incorporating normalized surface EMG data increased predicted magnitudes of joint reaction force and rotator cuff activations by 28.7% and 88.4%, respectively. Force constraints had no influence on these predicted outcomes for all tasks evaluated. More restrictive EMG constraints also tended to overconstrain the model, making it challenging to accurately track input kinematics. Therefore, force constraints may be a more robust choice when representing stability.}, number={4}, journal={JOURNAL OF APPLIED BIOMECHANICS}, author={McFarland, Daniel C. and Brynildsen, Alexander G. and Saul, Katherine R.}, year={2020}, month={Aug}, pages={249–258} }
 @article{dixit_mcfarland_saul_2019, title={Computational analysis of glenohumeral joint growth and morphology following a brachial plexus birth injury}, volume={86}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2019.01.040}, abstractNote={Children affected with brachial plexus birth injury (BPBI) undergo muscle paralysis. About 33% of affected children experience permanent osseous deformities of the glenohumeral joint. Recent evidence suggests that some cases experience restricted muscle longitudinal growth in addition to paralysis and reduced range of motion at the shoulder and elbow. It is unknown whether altered loading due to paralysis, muscle growth restriction and contracture, or static loading due to disuse is the primary driver of joint deformity after BPBI. This study uses a computational framework integrating finite element analysis and musculoskeletal modeling to examine the mechanical factors contributing to changes in bone growth and morphometry following BPBI. Simulations of 8 weeks of glenohumeral growth in a rat model of BPBI predicted that static loading of the joint is primarily responsible for joint deformation consistent with experimental measures of bone morphology, whereas dynamic loads resulted in normal bone growth. Under dynamic loading, glenoid version angle (GVA), glenoid inclination angle (GIA), and glenoid radius of curvature (GRC) (−1.3°, 38.2°, 2.5 mm respectively) were similar to the baseline values (−1.8°, −38°, 2.1 mm respectively). In the static case with unrestricted muscle growth, these measures increased in magnitude (5.2°, −48°, 3.5 mm respectively). More severe joint deformations were observed in GIA and GRC when muscle growth was restricted (GVA: 3.6°, GIA: −55°, GRC: 4.0 mm). Predicted morphology was consistent with literature reports of in vivo glenoid morphology following postganglionic BPBI. This growth model provides a framework for understanding the most influential mechanical factors driving glenohumeral deformity following BPBI.}, journal={JOURNAL OF BIOMECHANICS}, author={Dixit, Nikhil N. and McFarland, Daniel C. and Saul, Katherine R.}, year={2019}, month={Mar}, pages={48–54} }
 @article{mccain_dick_giest_nuckols_lewek_saul_sawicki_2019, title={Mechanics and energetics of post-stroke walking aided by a powered ankle exoskeleton with speed-adaptive myoelectric control}, volume={16}, ISSN={["1743-0003"]}, DOI={10.1186/s12984-019-0523-y}, abstractNote={Ankle exoskeletons offer a promising opportunity to offset mechanical deficits after stroke by applying the needed torque at the paretic ankle. Because joint torque is related to gait speed, it is important to consider the user's gait speed when determining the magnitude of assistive joint torque. We developed and tested a novel exoskeleton controller for delivering propulsive assistance which modulates exoskeleton torque magnitude based on both soleus muscle activity and walking speed. The purpose of this research is to assess the impact of the resulting exoskeleton assistance on post-stroke walking performance across a range of walking speeds.Six participants with stroke walked with and without assistance applied to a powered ankle exoskeleton on the paretic limb. Walking speed started at 60% of their comfortable overground speed and was increased each minute (n00, n01, n02, etc.). We measured lower limb joint and limb powers, metabolic cost of transport, paretic and non-paretic limb propulsion, and trailing limb angle.Exoskeleton assistance increased with walking speed, verifying the speed-adaptive nature of the controller. Both paretic ankle joint power and total limb power increased significantly with exoskeleton assistance at six walking speeds (n00, n01, n02, n03, n04, n05). Despite these joint- and limb-level benefits associated with exoskeleton assistance, no subject averaged metabolic benefits were evident when compared to the unassisted condition. Both paretic trailing limb angle and integrated anterior paretic ground reaction forces were reduced with assistance applied as compared to no assistance at four speeds (n00, n01, n02, n03).Our results suggest that despite appropriate scaling of ankle assistance by the exoskeleton controller, suboptimal limb posture limited the conversion of exoskeleton assistance into forward propulsion. Future studies could include biofeedback or verbal cues to guide users into limb configurations that encourage the conversion of mechanical power at the ankle to forward propulsion.N/A.}, journal={JOURNAL OF NEUROENGINEERING AND REHABILITATION}, author={McCain, Emily M. and Dick, Taylor J. M. and Giest, Tracy N. and Nuckols, Richard W. and Lewek, Michael D. and Saul, Katherine R. and Sawicki, Gregory S.}, year={2019}, month={May} }
 @article{mcfarland_mccain_poppo_saul_2019, title={Spatial Dependency of Glenohumeral Joint Stability During Dynamic Unimanual and Bimanual Pushing and Pulling}, volume={141}, ISSN={["1528-8951"]}, DOI={10.1115/1.4043035}, abstractNote={Degenerative wear to the glenoid from repetitive loading can reduce effective concavity depth and lead to future instability. Workspace design should consider glenohumeral stability to prevent initial wear. While glenohumeral stability has been previously explored for activities of daily living including push–pull tasks, whether stability is spatially dependent is unexplored. We simulated bimanual and unimanual push–pull tasks to four horizontal targets (planes of elevation: 0 deg, 45 deg, 90 deg, and 135 deg) at 90 deg thoracohumeral elevation and three elevation targets (thoracohumeral elevations: 20 deg, 90 deg, 170 deg) at 90 deg plane of elevation. The 45 deg horizontal target was most stable regardless of exertion type and would be the ideal target placement when considering stability. This target is likely more stable because the applied load acts perpendicular to the glenoid, limiting shear force production. The 135 deg horizontal target was particularly unstable for unimanual pushing (143% less stable than the 45 deg target), and the applied force for this task acts parallel to the glenoid, likely creating shear forces or limiting compressive forces. Pushing was less stable than pulling (all targets except sagittal 170 deg for both task types and horizontal 45 deg for bimanual) (p < 0.01), which is consistent with prior reports. For example, unimanual pushing at the 90 deg horizontal target was 197% less stable than unimanual pulling. There were limited stability benefits to task placement for pushing, and larger stability benefits may be seen from converting tasks from push to pull rather than optimizing task layout. There was no difference in stability between bimanual and unimanual tasks, suggesting no stability benefit to bimanual operation.}, number={5}, journal={JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME}, author={McFarland, Daniel C. and McCain, Emily M. and Poppo, Michael N. and Saul, Katherine R.}, year={2019}, month={May} }
 @article{vidt_santago_marsh_hegedus_tuohy_poehling_freehill_miller_saul_2018, title={Modeling a rotator cuff tear: Individualized shoulder muscle forces influence glenohumeral joint contact force predictions}, volume={60}, ISSN={["1879-1271"]}, DOI={10.1016/j.clinbiomech.2018.10.004}, abstractNote={Background Rotator cuff tears in older individuals may result in decreased muscle forces and changes to force distribution across the glenohumeral joint. Reduced muscle forces may impact functional task performance, altering glenohumeral joint contact forces, potentially contributing to instability or joint damage risk. Our objective was to evaluate the influence of rotator cuff muscle force distribution on glenohumeral joint contact force during functional pull and axilla wash tasks using individualized computational models. Methods Fourteen older individuals (age 63.4 yrs. (SD 1.8)) were studied; 7 with rotator cuff tear, 7 matched controls. Muscle volume measurements were used to scale a nominal upper limb model's muscle forces to develop individualized models and perform dynamic simulations of movement tracking participant-derived kinematics. Peak resultant glenohumeral joint contact force, and direction and magnitude of force components were compared between groups using ANCOVA. Findings Results show individualized muscle force distributions for rotator cuff tear participants had reduced peak resultant joint contact force for pull and axilla wash (P ≤ 0.0456), with smaller compressive components of peak resultant force for pull (P = 0.0248). Peak forces for pull were within the glenoid. For axilla wash, peak joint contact was directed near/outside the glenoid rim for three participants; predictions required individualized muscle forces since nominal muscle forces did not affect joint force location. Interpretation Older adults with rotator cuff tear had smaller peak resultant and compressive forces, possibly indicating increased instability or secondary joint damage risk. Outcomes suggest predicted joint contact force following rotator cuff tear is sensitive to including individualized muscle forces.}, journal={CLINICAL BIOMECHANICS}, author={Vidt, Meghan E. and Santago, Anthony C., II and Marsh, Anthony P. and Hegedus, Eric J. and Tuohy, Christopher J. and Poehling, Gary G. and Freehill, Michael T. and Miller, Michael E. and Saul, Katherine R.}, year={2018}, month={Dec}, pages={20–29} }
 @article{hennen_crouch_hutchinson_li_saul_2018, title={Relationship between glenoid deformity and gait characteristics in a rat model of neonatal brachial plexus injury}, volume={36}, ISSN={0736-0266}, url={http://dx.doi.org/10.1002/JOR.23836}, DOI={10.1002/JOR.23836}, abstractNote={ABSTRACT}, number={7}, journal={Journal of Orthopaedic Research®}, publisher={Wiley}, author={Hennen, Kelsey and Crouch, Dustin L. and Hutchinson, Ian D. and Li, Zhongyu and Saul, Katherine}, year={2018}, month={Jan}, pages={1991–1997} }
 @article{mcfarland_poppo_mccain_saul_2018, title={Spatial dependency of shoulder muscle demand during dynamic unimanual and bimanual pushing and pulling}, volume={73}, ISSN={0003-6870}, url={http://dx.doi.org/10.1016/J.APERGO.2018.07.011}, DOI={10.1016/J.APERGO.2018.07.011}, abstractNote={Work involving extensive pushing and pulling is associated with higher frequency of shoulder complaints. While reports of shoulder muscle demand during submaximal isometric tasks are abundant, dynamic submaximal push-pull exertions are not well understood. We evaluated how muscle demand (weighted EMG average) of surface glenohumeral muscles varies with task type and target. Seventeen healthy young adults performed seated unimanual and bimanual pushes and pulls to 3 thoracohumeral elevations (20°, 90°, 170°) and 4 elevation planes (0°, 45°, 90°, 135°) with loading at 15% of isometric push-pull capacity. Pulling required less demand than pushing (p < 0.0001). Muscle demand varied more with elevation than elevation plane. The lowest target had highest demand for pulling (p < 0.01), and the most elevated target had highest demand for pushing (p < 0.0001). Working above the shoulder is known to increase demand during isometric tasks, however, these results suggest that for dynamic tasks working against gravity has a larger effect on demand than task target.}, journal={Applied Ergonomics}, publisher={Elsevier BV}, author={McFarland, Daniel C. and Poppo, Michael N. and McCain, Emily M. and Saul, Katherine R.}, year={2018}, month={Nov}, pages={199–205} }
 @article{santago_vidt_li_tuohy_poehling_freehill_saul_2017, title={Shoulder Strength Requirements for Upper Limb Functional Tasks: Do Age and Rotator Cuff Tear Status Matter?}, volume={33}, ISSN={["1543-2688"]}, DOI={10.1123/jab.2016-0116}, abstractNote={Understanding upper limb strength requirements for daily tasks is imperative for early detection of strength loss that may progress to disability due to age or rotator cuff tear. We quantified shoulder strength requirements for 5 upper limb tasks performed by 3 groups: uninjured young adults and older adults, and older adults with a degenerative supraspinatus tear prior to repair. Musculoskeletal models were developed for each group representing age, sex, and tear-related strength losses. Percentage of available strength used was quantified for the subset of tasks requiring the largest amount of shoulder strength. Significant differences in strength requirements existed across tasks: upward reach 105° required the largest average strength; axilla wash required the largest peak strength. However, there were limited differences across participant groups. Older adults with and without a tear used a larger percentage of their shoulder elevation (p < .001, p < .001) and external rotation (p < .001, p = .017) strength than the young adults, respectively. Presence of a tear significantly increased percentage of internal rotation strength compared to young (p < .001) and uninjured older adults (p = .008). Marked differences in strength demand across tasks indicate the need for evaluating a diversity of functional tasks to effectively detect early strength loss, which may lead to disability.}, number={6}, journal={JOURNAL OF APPLIED BIOMECHANICS}, author={Santago, Anthony C., II and Vidt, Meghan E. and Li, Xiaotong and Tuohy, Christopher J. and Poehling, Gary G. and Freehill, Michael T. and Saul, Katherine R.}, year={2017}, month={Dec}, pages={446–452} }
 @article{li_santago_vidt_saul_2016, title={Analysis of effects of loading and postural demands on upper limb reaching in older adults using statistical parametric mapping}, volume={49}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2016.06.018}, abstractNote={Continuous time-series data are frequently distilled into single values and analyzed using discrete statistical methods, underutilizing large datasets. Statistical parametric mapping (SPM) allows hypotheses over the entire spectrum, but consistency with discrete analyses of kinematic data is unclear. We applied SPM to evaluate effect of load and postural demands during reaching on thoracohumeral kinematics in older and young adults, and examined consistency between one-dimensional SPM and discrete analyses of the same dataset. We hypothesized that older adults would choose postures that bring the humerus anterior to the frontal plane (towards flexion) even for low demand tasks, and that SPM would reveal differences persisting over larger temporal portions of the reach. Ten healthy older (72.4±3.1yrs) and 16 young (22.9±2.5yrs) adults reached upward and forward with high and low loads. SPM and discrete t-tests were used to analyze group effects for elevation plane, elevation, and axial rotation joint angles and velocity. Older adults used more positive (anterior) elevation plane and less elevated postures to initiate and terminate reaching (p<0.008), with long duration differences during termination. When reaching upward, differences in elevation persisted over longer temporal periods at midreach for high loads (32-58% of reach) compared to low load (41-45%). SPM and discrete analyses were consistent, but SPM permitted clear identification of temporal periods over which differences persisted, while discrete methods allowed analysis of extracted values, like ROM. This work highlights the utility of SPM to analyze kinematics time series data, and emphasizes importance of task selection when assessing age-related changes in movement.}, number={13}, journal={JOURNAL OF BIOMECHANICS}, author={Li, Xiaotong and Santago, Anthony C., II and Vidt, Meghan E. and Saul, Katherine R.}, year={2016}, month={Sep}, pages={2806–2816} }
 @article{vidt_santago_tuohy_poehling_freehill_kraft_marsh_hegedus_miller_saul_2016, title={Assessments of Fatty Infiltration and Muscle Atrophy From a Single Magnetic Resonance Image Slice Are Not Predictive of 3-Dimensional Measurements}, volume={32}, ISSN={["1526-3231"]}, DOI={10.1016/j.arthro.2015.06.035}, abstractNote={Purpose To (1) determine whether standard clinical muscle fatty infiltration and atrophy assessment techniques using a single image slice for patients with a rotator cuff tear (RCT) are correlated with 3-dimensional measures in older individuals (60+ years) and (2) to determine whether age-associated changes to muscle morphology and strength are compounded by an RCT. Methods Twenty older individuals were studied: 10 with an RCT of the supraspinatus (5 men and 5 women) and 10 matched controls. Clinical imaging assessments (Goutallier and Fuchs scores and cross-sectional area ratio) were performed for participants with RCTs. Three-dimensional measurements of rotator cuff muscle and fat tissues were obtained for all participants using magnetic resonance imaging (MRI). Isometric joint moment was measured at the shoulder. Results There were no significant associations between single-image assessments and 3-dimensional measurements of fatty infiltration for the supraspinatus and infraspinatus muscles. Compared with controls, participants with RCTs had significantly increased percentages of fatty infiltration for each rotator cuff muscle (all P ≤ .023); reduced whole muscle volume for the supraspinatus, infraspinatus, and subscapularis muscles (all P ≤ .038); and reduced fat-free muscle volume for the supraspinatus, infraspinatus, and subscapularis muscles (all P ≤ .027). Only the teres minor (P = .017) fatty infiltration volume was significantly greater for participants with RCTs. Adduction, flexion, and external rotation strength (all P ≤ .021) were significantly reduced for participants with RCTs, and muscle volume was a significant predictor of strength for all comparisons. Conclusions Clinical scores using a single image slice do not represent 3-dimensional muscle measurements. Efficient methods are needed to more effectively capture 3-dimensional information for clinical applications. Participants with RCTs had increased fatty infiltration percentages that were likely driven by muscle atrophy rather than increased fat volume. The significant association of muscle volume with strength production suggests that treatments to preserve muscle volume should be pursued for older patients with RCTs. Level of Evidence Level II, diagnostic study, with development of diagnostic criteria on the basis of consecutive patients with universally applied reference gold standard. To (1) determine whether standard clinical muscle fatty infiltration and atrophy assessment techniques using a single image slice for patients with a rotator cuff tear (RCT) are correlated with 3-dimensional measures in older individuals (60+ years) and (2) to determine whether age-associated changes to muscle morphology and strength are compounded by an RCT. Twenty older individuals were studied: 10 with an RCT of the supraspinatus (5 men and 5 women) and 10 matched controls. Clinical imaging assessments (Goutallier and Fuchs scores and cross-sectional area ratio) were performed for participants with RCTs. Three-dimensional measurements of rotator cuff muscle and fat tissues were obtained for all participants using magnetic resonance imaging (MRI). Isometric joint moment was measured at the shoulder. There were no significant associations between single-image assessments and 3-dimensional measurements of fatty infiltration for the supraspinatus and infraspinatus muscles. Compared with controls, participants with RCTs had significantly increased percentages of fatty infiltration for each rotator cuff muscle (all P ≤ .023); reduced whole muscle volume for the supraspinatus, infraspinatus, and subscapularis muscles (all P ≤ .038); and reduced fat-free muscle volume for the supraspinatus, infraspinatus, and subscapularis muscles (all P ≤ .027). Only the teres minor (P = .017) fatty infiltration volume was significantly greater for participants with RCTs. Adduction, flexion, and external rotation strength (all P ≤ .021) were significantly reduced for participants with RCTs, and muscle volume was a significant predictor of strength for all comparisons. Clinical scores using a single image slice do not represent 3-dimensional muscle measurements. Efficient methods are needed to more effectively capture 3-dimensional information for clinical applications. Participants with RCTs had increased fatty infiltration percentages that were likely driven by muscle atrophy rather than increased fat volume. The significant association of muscle volume with strength production suggests that treatments to preserve muscle volume should be pursued for older patients with RCTs.}, number={1}, journal={ARTHROSCOPY-THE JOURNAL OF ARTHROSCOPIC AND RELATED SURGERY}, author={Vidt, Meghan E. and Santago, Anthony C., II and Tuohy, Christopher J. and Poehling, Gary G. and Freehill, Michael T. and Kraft, Robert A. and Marsh, Anthony P. and Hegedus, Eric J. and Miller, Michael E. and Saul, Katherine R.}, year={2016}, month={Jan}, pages={128–139} }
 @article{vidt_santago_hegedus_marsh_tuohy_poehling_freehill_miller_saul_2016, title={Can self-report instruments of shoulder function capture functional differences in older adults with and without a rotator cuff tear?}, volume={29}, ISSN={["1873-5711"]}, DOI={10.1016/j.jelekin.2015.06.009}, abstractNote={Rotator cuff tears (RCT) are prevalent in older individuals and may compound age-associated functional declines. Our purpose was to determine whether self-report measures of perceived functional ability are valid for older patients with RCT. Twenty five subjects participated (12M/13F; age=63.9±3.0years); 13 with RCT and 12 controls (CON). Participants completed self-report measures of shoulder function (SST, ASES, WORC) and health-related quality of life (SF-36). Isometric joint moment and range of motion (ROM) were measured at the shoulder. Relationships among functional self-reports, and between these measures and joint moment and ROM were assessed; group differences for total and subcategory scores were evaluated. There were significant correlations among self-reports (rs=0.62-0.71, p⩽0.02). For RCT subjects, ASES was associated with all joint moments except adduction (p⩽0.02); SST, ASES, and WORC were associated with abduction and external rotation ROM (p⩽0.04). For RCT subjects, SST and WORC were associated with SF-36 physical function subcategory scores (p⩽0.05). The RCT group scored worse than CON on all functional self-reports (p<0.01) and WORC and ASES subcategories (p<0.01). In conclusion, SST, ASES, and WORC demonstrate utility and discriminant validity for older individuals by distinguishing those with RCT, but this work suggests prioritizing ASES given its stronger association with functional group strength.}, journal={JOURNAL OF ELECTROMYOGRAPHY AND KINESIOLOGY}, author={Vidt, Meghan E. and Santago, Anthony C., II and Hegedus, Eric J. and Marsh, Anthony P. and Tuohy, Christopher J. and Poehling, Gary G. and Freehill, Michael T. and Miller, Michael E. and Saul, Katherine R.}, year={2016}, month={Aug}, pages={90–99} }
 @article{caruthers_thompson_chaudhari_schmitt_best_saul_siston_2016, title={Muscle Forces and Their Contributions to Vertical and Horizontal Acceleration of the Center of Mass During Sit-to-Stand Transfer in Young, Healthy Adults}, volume={32}, ISSN={["1543-2688"]}, DOI={10.1123/jab.2015-0291}, abstractNote={Sit-to-stand transfer is a common task that is challenging for older adults and others with musculoskeletal impairments. Associated joint torques and muscle activations have been analyzed two-dimensionally, neglecting possible three-dimensional (3D) compensatory movements in those who struggle with sit-to-stand transfer. Furthermore, how muscles accelerate an individual up and off the chair remains unclear; such knowledge could inform rehabilitation strategies. We examined muscle forces, muscleinduced accelerations, and interlimb muscle force differences during sit-to-stand transfer in young, healthy adults. Dynamic simulations were created using a custom 3D musculoskeletal model; static optimization and induced acceleration analysis were used to determine muscle forces and their induced accelerations, respectively. The gluteus maximus generated the largest force (2009.07 ± 277.31 N) and was a main contributor to forward acceleration of the center of mass (COM) (0.62 ± 0.18 m/s2), while the quadriceps opposed it. The soleus was a main contributor to upward (2.56 ± 0.74 m/s2) and forward acceleration of the COM (0.62 ± 0.33 m/s2). Interlimb muscle force differences were observed, demonstrating lower limb symmetry cannot be assumed during this task, even in healthy adults. These findings establish a baseline from which deficits and compensatory strategies in relevant populations (eg, elderly, osteoarthritis) can be identified.}, number={5}, journal={JOURNAL OF APPLIED BIOMECHANICS}, author={Caruthers, Elena J. and Thompson, Julie A. and Chaudhari, Ajit M. W. and Schmitt, Laura C. and Best, Thomas M. and Saul, Katherine R. and Siston, Robert A.}, year={2016}, month={Oct}, pages={487–503} }
 @article{santago_vidt_tuohy_poehling_freehill_jordan_kraft_saul_2016, title={Quantitative Analysis of Three-Dimensional Distribution and Clustering of Intramuscular Fat in Muscles of the Rotator Cuff}, volume={44}, ISSN={["1573-9686"]}, DOI={10.1007/s10439-015-1488-z}, abstractNote={The purpose of this study was to (1) develop and present a technique to quantitatively assess three-dimensional distribution and clustering of intramuscular fat and (2) use the technique to compare spatial characteristics of intramuscular fat in rotator cuff muscles of older adults with and without a supraspinatus tear. Moran’s Index (I), an existing quantitative measure of clustering, was extended for use with MRI to allow comparisons across individuals with different size muscles. Sixteen older adults (>60 years) with (N = 6) and without (N = 10) a degenerative supraspinatus tear participated. Following 3D Dixon MRIs of the shoulder, which separates fat from water, rotator cuff muscles were segmented and sectioned and fat% and Moran’s I were calculated to assess distribution and clustering, respectively. Moran’s I ranged was 0.40–0.92 and 0.39–0.76 for the tear and control subjects, respectively. Compared to uninjured controls, tear subjects demonstrated increased fat distribution (p = 0.036) and clustering (p = 0.020) distally in the supraspinatus. Tear subjects had more pronounced distribution (p < 0.001) and clustering distally (p < 0.001) than proximally. Other rotator cuff muscles exhibited different patterns of fat clustering and distribution. This technique, which we applied to quantify spatial characteristics of intramuscular fat, can be applied to assess clustering of fat in other pathologies and tissues.}, number={7}, journal={ANNALS OF BIOMEDICAL ENGINEERING}, author={Santago, Anthony C., II and Vidt, Meghan E. and Tuohy, Christopher J. and Poehling, Gary G. and Freehill, Michael T. and Jordan, Jennifer H. and Kraft, Robert A. and Saul, Katherine R.}, year={2016}, month={Jul}, pages={2158–2167} }
 @article{crouch_santago_plate_li_saul_2016, title={Relationship between maximum isometric joint moment and functional task performance in patients with brachial plexus injury: A pilot study}, volume={44}, ISSN={["1879-2219"]}, DOI={10.1016/j.gaitpost.2015.12.038}, abstractNote={We evaluated whether subjects with brachial plexus injury (BPI) adapted their movements to reduce the mechanical demand on their impaired upper extremity. In 6 subjects with unilateral BPI with C5 and C6 involvement, we measured bilateral maximum isometric shoulder and elbow strength, and computed joint kinematics and net muscle-generated joint moments during 7 unimanual functional tasks. Compared to the unimpaired extremity, maximum strength in shoulder abduction, extension, and external rotation was 60% (p = 0.02), 49% (p = 0.02), and 75% (p = 0.02) lower, respectively, on the impaired side. Significant kinematic and kinetic differences were observed only when reaching to the back of the head. However, because of substantially reduced strength in their impaired upper extremities, subjects used a significantly higher percentage of their maximum strength during several tasks and along several directions of movement. The peak percentage of maximal strength subjects used across tasks was 32% (p = 0.03) and 29% (p = 0.03) more on their impaired side in shoulder extension and external rotation, respectively. Subjects had less reserve strength available for performing upper extremity tasks and, therefore, may be less adaptive to strength declines due to injury progression and normal aging. Quantitatively measuring maximal strength may help clinicians ensure that patients maintain sufficient upper extremity strength to preserve long-term functional ability.}, journal={GAIT & POSTURE}, author={Crouch, Dustin L. and Santago, Anthony C., II and Plate, Johannes F. and Li, Zhongyu and Saul, Katherine R.}, year={2016}, month={Feb}, pages={238–244} }
 @article{vidt_santago_marsh_hegedus_tuohy_poehling_freehill_miller_saul_2016, title={The effects of a rotator cuff tear on activities of daily living in older adults: A kinematic analysis}, volume={49}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2016.01.029}, abstractNote={Rotator cuff tears (RCT) in older individuals may compound age-associated physiological changes and impact their ability to perform daily functional tasks. Our objective was to quantify thoracohumeral kinematics for functional tasks in 18 older adults (mean age=63.3±2.2), and compare findings from nine with a RCT to nine matched controls. Motion capture was used to record kinematics for 7 tasks (axilla wash, forward reach, functional pull, hair comb, perineal care, upward reach to 90°, upward reach to 105°) spanning the upper limb workspace. Maximum and minimum joint angles and motion excursion for the three thoracohumeral degrees of freedom (elevation plane, elevation, axial rotation) were identified for each task and compared between groups. The RCT group used greater minimum elevation angles for axilla wash and functional pull (p≤0.0124) and a smaller motion excursion for functional pull (p=0.0032) compared to the control group. The RCT group also used a more internally rotated maximum axial rotation angle than controls for functional reach, functional pull, hair comb, and upward reach to 105° (p≤0.0494). The most differences between groups were observed for axial rotation, with the RCT group using greater internal rotation to complete functional tasks, and significant differences between groups were identified for all three thoracohumeral degrees of freedom for functional pull. We conclude that older adults with RCT used more internal rotation to perform functional tasks than controls. The kinematic differences identified in this study may have consequences for progression of shoulder damage and further functional impairment in older adults with RCT.}, number={4}, journal={JOURNAL OF BIOMECHANICS}, author={Vidt, Meghan E. and Santago, Anthony C. and Marsh, Anthony P. and Hegedus, Eric J. and Tuohy, Christopher J. and Poehling, Gary G. and Freehill, Michael T. and Miller, Michael E. and Saul, Katherine R.}, year={2016}, month={Feb}, pages={611–617} }
 @article{santago_plate_shively_register_smith_saul_2015, title={Age-related structural changes in upper extremity muscle tissue in a nonhuman primate model}, volume={24}, ISSN={["1058-2746"]}, DOI={10.1016/j.jse.2015.03.025}, abstractNote={Longitudinal studies of upper extremity aging in humans include logistical concerns that animal models can overcome. The vervet is a promising species with which to study aging-related processes. However, age-related changes in upper extremity muscle structure have not been quantified in this species. This study measured age-related changes to muscle structure, examined relationships between muscle structure and measures of physical performance, and evaluated the presence of rotator cuff tears.Muscle structure (volume, optimal fiber length, and physiologic cross-sectional area (PCSA)) of 10 upper extremity muscles was quantified from the right upper limb of 5 middle-aged and 6 older adult female vervets.Total measured PCSA was smaller (P = .001) in the older adult vervets than in the middle-aged vervets. Muscle volume reduction predominate the age-related reductions in PCSA. Total measured PCSA was not correlated to any measures of physical performance. No rotator cuff tears were observed. Supraspinatus volume was relatively larger and deltoid volume relatively smaller in the vervet compared with a human.The vervet is an appropriate translational model for age-related upper extremity muscle volume loss. Functional measures were not correlated to PCSA, suggesting the vervets may have enough strength for normal function despite loss of muscle tissue. Reduced relative demand on the supraspinatus may be responsible for the lack of naturally occurring rotator cuff tears.}, number={10}, journal={JOURNAL OF SHOULDER AND ELBOW SURGERY}, author={Santago, Anthony C., II and Plate, Johannes F. and Shively, Carol A. and Register, Thomas C. and Smith, Thomas L. and Saul, Katherine R.}, year={2015}, month={Oct}, pages={1660–1668} }
 @article{saul_hu_goehler_vidt_daly_velisar_murray_2015, title={Benchmarking of dynamic simulation predictions in two software platforms using an upper limb musculoskeletal model}, volume={18}, ISSN={["1476-8259"]}, DOI={10.1080/10255842.2014.916698}, abstractNote={Several opensource or commercially available software platforms are widely used to develop dynamic simulations of movement. While computational approaches are conceptually similar across platforms, technical differences in implementation may influence output. We present a new upper limb dynamic model as a tool to evaluate potential differences in predictive behavior between platforms. We evaluated to what extent differences in technical implementations in popular simulation software environments result in differences in kinematic predictions for single and multijoint movements using EMG- and optimization-based approaches for deriving control signals. We illustrate the benchmarking comparison using SIMM–Dynamics Pipeline–SD/Fast and OpenSim platforms. The most substantial divergence results from differences in muscle model and actuator paths. This model is a valuable resource and is available for download by other researchers. The model, data, and simulation results presented here can be used by future researchers to benchmark other software platforms and software upgrades for these two platforms.}, number={13}, journal={COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING}, author={Saul, Katherine R. and Hu, Xiao and Goehler, Craig M. and Vidt, Meghan E. and Daly, Melissa and Velisar, Anca and Murray, Wendy M.}, year={2015}, month={Oct}, pages={1445–1458} }
 @article{cheng_cornwall_crouch_li_saul_2015, title={Contributions of Muscle Imbalance and Impaired Growth to Postural and Osseous Shoulder Deformity Following Brachial Plexus Birth Palsy: A Computational Simulation Analysis}, volume={40}, ISSN={["1531-6564"]}, DOI={10.1016/j.jhsa.2015.02.025}, abstractNote={Two potential mechanisms leading to postural and osseous shoulder deformity after brachial plexus birth palsy are muscle imbalance between functioning internal rotators and paralyzed external rotators and impaired longitudinal growth of paralyzed muscles. Our goal was to evaluate the combined and isolated effects of these 2 mechanisms on transverse plane shoulder forces using a computational model of C5-6 brachial plexus injury.We modeled a C5-6 injury using a computational musculoskeletal upper limb model. Muscles expected to be denervated by C5-6 injury were classified as affected, with the remaining shoulder muscles classified as unaffected. To model muscle imbalance, affected muscles were given no resting tone whereas unaffected muscles were given resting tone at 30% of maximal activation. To model impaired growth, affected muscles were reduced in length by 30% compared with normal whereas unaffected muscles remained normal in length. Four scenarios were simulated: normal, muscle imbalance only, impaired growth only, and both muscle imbalance and impaired growth. Passive shoulder rotation range of motion and glenohumeral joint reaction forces were evaluated to assess postural and osseous deformity.All impaired scenarios exhibited restricted range of motion and increased and posteriorly directed compressive glenohumeral joint forces. Individually, impaired muscle growth caused worse restriction in range of motion and higher and more posteriorly directed glenohumeral forces than did muscle imbalance. Combined muscle imbalance and impaired growth caused the most restricted joint range of motion and the highest joint reaction force of all scenarios.Both muscle imbalance and impaired longitudinal growth contributed to range of motion and force changes consistent with clinically observed deformity, although the most substantial effects resulted from impaired muscle growth.Simulations suggest that treatment strategies emphasizing treatment of impaired longitudinal growth are warranted for reducing deformity after brachial plexus birth palsy.}, number={6}, journal={JOURNAL OF HAND SURGERY-AMERICAN VOLUME}, author={Cheng, Wei and Cornwall, Roger and Crouch, Dustin L. and Li, Zhongyu and Saul, Katherine R.}, year={2015}, month={Jun}, pages={1170–1176} }
 @article{edmonds_santago_saul_2015, title={Functional loss with displacement of medial epicondyle humerus fractures: A computer simulation study}, volume={35}, DOI={10.1097/bpo.0000000000000371}, abstractNote={Background: Assessment and management of the medial humeral epicondyle fracture remains controversial, with conflicting reports of displacement direction and consequent functional deficits unclear. The purpose of this study was to define biomechanically likely directions of medial epicondyle fracture displacement and to determine possible changes in muscle function related to that displacement. Methods: A 3-dimensional computer model of the upper extremity was used to simulate the consequences of medial epicondyle fracture displacements from 1 to 20 mm in the anterior, medial, and inferior directions relative to the humerus with the elbow at 90-degree flexion and neutral forearm rotation (a replication of accepted positions for clinical strength testing). Muscle length and force were calculated following displacement. Maximum isometric wrist flexion moments were calculated over the full range of wrist motion based on known force-generating properties of the muscles. Results: Anterior displacement resulted in shortened muscles and reduced wrist flexion moment, with a decrease in strength averaging 2% for every 1 mm of anterior fragment displacement at neutral wrist position (maximum decrease of 39% with 20 mm displacement). In contrast, displacement in the medial and inferior directions resulted in stretched muscles and increased wrist flexion moments and therefore are not biomechanically likely. Conclusions: Computer simulation of a medial epicondyle fracture suggests that anterior displacement could result in a dramatic loss of initial muscle strength and function. Medial displacement is unlikely to occur in vivo due to consequential muscle lengthening, suggesting that alternatives to the historical use of AP radiographs to assess displacement of this fracture are needed. Clinical Relevance: Our work provides a biomechanical explanation for anterior displacement of medial epicondyle fractures observed radiographically and motivates alternative methods of fracture assessment. A functional basis for determining acceptable displacement of medial epicondyle fractures is suggested; however, all individual clinical factors should be considered.}, number={7}, journal={Journal of Pediatric Orthopaedics}, author={Edmonds, E. W. and Santago, A. C. and Saul, K. R.}, year={2015}, pages={666–671} }
 @article{saul_vidt_gold_murray_2015, title={Upper Limb Strength and Muscle Volume in Healthy Middle-Aged Adults}, volume={31}, ISSN={["1543-2688"]}, DOI={10.1123/jab.2014-0177}, abstractNote={Our purpose was to characterize shoulder muscle volume and isometric moment, as well as their relationship, for healthy middle-aged adults. Muscle volume and maximum isometric joint moment were assessed for 6 functional muscle groups of the shoulder, elbow, and wrist in 10 middle-aged adults (46–60 y, 5M, 5F). Compared with young adults, shoulder abductors composed a smaller percentage of total muscle volume (P = .0009) and there was a reduction in shoulder adductor strength relative to elbow flexors (P = .012). We observed a consistent ordering of moment-generating capacity among functional groups across subjects. Although total muscle volume spanned a 2.3-fold range, muscle volume was distributed among functional groups in a consistent manner across subjects. On average, 72% of the variation in joint moment could be explained by the corresponding functional group muscle volume. These data are useful for improved modeling of upper limb musculoskeletal performance in middle-aged subjects, and may improve computational predictions of function for this group.}, number={6}, journal={JOURNAL OF APPLIED BIOMECHANICS}, author={Saul, Katherine R. and Vidt, Meghan E. and Gold, Garry E. and Murray, Wendy M.}, year={2015}, month={Dec}, pages={484–491} }
 @article{crouch_plate_li_saul_2014, title={Computational Sensitivity Analysis to Identify Muscles That Can Mechanically Contribute to Shoulder Deformity Following Brachial Plexus Birth Palsy}, volume={39}, ISSN={["1531-6564"]}, DOI={10.1016/j.jhsa.2013.10.027}, abstractNote={Purpose Two mechanisms, strength imbalance or impaired longitudinal muscle growth, potentially cause osseous and postural shoulder deformity in children with brachial plexus birth palsy. Our objective was to determine which muscles, via either deformity mechanism, were mechanically capable of producing forces that could promote shoulder deformity. Methods In an upper limb computational musculoskeletal model, we simulated strength imbalance by allowing each muscle crossing the shoulder to produce 30% of its maximum force. To simulate impaired longitudinal muscle growth, the functional length of each muscle crossing the shoulder was reduced by 30%. We performed a sensitivity analysis to identify muscles that, through either simulated deformity mechanism, increased the posteriorly directed, compressive glenohumeral joint force consistent with osseous deformity or reduced the shoulder external rotation or abduction range of motion consistent with postural deformity. Results Most of the increase in the posterior glenohumeral joint force by the strength imbalance mechanism was caused by the subscapularis, latissimus dorsi, and infraspinatus. Posterior glenohumeral joint force increased the most owing to impaired growth of the infraspinatus, subscapularis, and long head of biceps. Through the strength imbalance mechanism, the subscapularis, anterior deltoid, and pectoralis major muscles reduced external shoulder rotation by 28°, 17°, and 10°, respectively. Shoulder motion was reduced by 40° to 56° owing to impaired growth of the anterior deltoid, subscapularis, and long head of triceps. Conclusions The infraspinatus, subscapularis, latissimus dorsi, long head of biceps, anterior deltoid, pectoralis major, and long head of triceps were identified in this computational study as being the most capable of producing shoulder forces that may contribute to shoulder deformity following brachial plexus birth palsy. Clinical relevance The muscles mechanically capable of producing deforming shoulder forces should be the focus of experimental studies investigating the musculoskeletal consequences of brachial plexus birth palsy and are potentially critical targets for treating shoulder deformity. Two mechanisms, strength imbalance or impaired longitudinal muscle growth, potentially cause osseous and postural shoulder deformity in children with brachial plexus birth palsy. Our objective was to determine which muscles, via either deformity mechanism, were mechanically capable of producing forces that could promote shoulder deformity. In an upper limb computational musculoskeletal model, we simulated strength imbalance by allowing each muscle crossing the shoulder to produce 30% of its maximum force. To simulate impaired longitudinal muscle growth, the functional length of each muscle crossing the shoulder was reduced by 30%. We performed a sensitivity analysis to identify muscles that, through either simulated deformity mechanism, increased the posteriorly directed, compressive glenohumeral joint force consistent with osseous deformity or reduced the shoulder external rotation or abduction range of motion consistent with postural deformity. Most of the increase in the posterior glenohumeral joint force by the strength imbalance mechanism was caused by the subscapularis, latissimus dorsi, and infraspinatus. Posterior glenohumeral joint force increased the most owing to impaired growth of the infraspinatus, subscapularis, and long head of biceps. Through the strength imbalance mechanism, the subscapularis, anterior deltoid, and pectoralis major muscles reduced external shoulder rotation by 28°, 17°, and 10°, respectively. Shoulder motion was reduced by 40° to 56° owing to impaired growth of the anterior deltoid, subscapularis, and long head of triceps. The infraspinatus, subscapularis, latissimus dorsi, long head of biceps, anterior deltoid, pectoralis major, and long head of triceps were identified in this computational study as being the most capable of producing shoulder forces that may contribute to shoulder deformity following brachial plexus birth palsy.}, number={2}, journal={JOURNAL OF HAND SURGERY-AMERICAN VOLUME}, author={Crouch, Dustin L. and Plate, Johannes F. and Li, Zhongyu and Saul, Katherine R.}, year={2014}, month={Feb}, pages={303–311} }
 @article{plate_bates_mannava_smith_jorgensen_register_stehle_high_shively_kaplan_et al._2013, title={Age-related degenerative functional, radiographic, and histological changes of the shoulder in nonhuman primates}, volume={22}, ISSN={1058-2746}, url={http://dx.doi.org/10.1016/j.jse.2012.11.004}, DOI={10.1016/j.jse.2012.11.004}, abstractNote={Background Nonhuman primates have similar shoulder anatomy and physiology compared to humans, and may represent a previously underutilized model for shoulder research. This study sought to identify naturally occurring bony and muscular degeneration in the shoulder of nonhuman primates and to assess relationships between structural and functional aspects of the shoulder and measures of physical function of the animals. We hypothesized that age-related degenerative changes in the shoulders of nonhuman primates would resemble those observed in aging humans. Methods Middle-aged (n = 5; ages 9.4-11.8 years) and elderly (n = 6; ages 19.8-26.4 years) female vervet monkeys were studied for changes in mobility and shoulder function, and radiographic and histologic signs of age-related degeneration. Results Four out of 6 (4/6) elderly animals had degenerative changes of the glenoid compared to 0/5 of the middle-aged animals (P = .005). Elderly animals had glenoid retroversion, decreased joint space, walked slower, and spent less time climbing and hanging than middle-aged vervets (P < .05). Physical mobility and shoulder function correlated with glenoid version angle (P < .05). Supraspinatus muscles of elderly animals were less dense (P = .001), had decreased fiber cross-sectional area (P < .001), but similar amounts of nuclear material (P = .085). Degenerative rotator cuff tears were not observed in any of the eleven animals. Discussion and conclusion The vervet monkey naturally undergoes age-related functional, radiographic and histological changes of the shoulder, and may qualify as an animal model for selected translational research of shoulder osteoarthritis. Nonhuman primates have similar shoulder anatomy and physiology compared to humans, and may represent a previously underutilized model for shoulder research. This study sought to identify naturally occurring bony and muscular degeneration in the shoulder of nonhuman primates and to assess relationships between structural and functional aspects of the shoulder and measures of physical function of the animals. We hypothesized that age-related degenerative changes in the shoulders of nonhuman primates would resemble those observed in aging humans. Middle-aged (n = 5; ages 9.4-11.8 years) and elderly (n = 6; ages 19.8-26.4 years) female vervet monkeys were studied for changes in mobility and shoulder function, and radiographic and histologic signs of age-related degeneration. Four out of 6 (4/6) elderly animals had degenerative changes of the glenoid compared to 0/5 of the middle-aged animals (P = .005). Elderly animals had glenoid retroversion, decreased joint space, walked slower, and spent less time climbing and hanging than middle-aged vervets (P < .05). Physical mobility and shoulder function correlated with glenoid version angle (P < .05). Supraspinatus muscles of elderly animals were less dense (P = .001), had decreased fiber cross-sectional area (P < .001), but similar amounts of nuclear material (P = .085). Degenerative rotator cuff tears were not observed in any of the eleven animals. The vervet monkey naturally undergoes age-related functional, radiographic and histological changes of the shoulder, and may qualify as an animal model for selected translational research of shoulder osteoarthritis.}, number={8}, journal={Journal of Shoulder and Elbow Surgery}, publisher={Elsevier BV}, author={Plate, Johannes F. and Bates, Christopher M. and Mannava, Sandeep and Smith, Thomas L. and Jorgensen, Matthew J. and Register, Thomas C. and Stehle, John R. and High, Kevin P. and Shively, Carol A. and Kaplan, Jay R. and et al.}, year={2013}, month={Aug}, pages={1019–1029} }
 @article{crouch_plate_li_saul_2013, title={Biomechanical Contributions of Posterior Deltoid and Teres Minor in the Context of Axillary Nerve Injury: A Computational Study}, volume={38}, ISSN={0363-5023}, url={http://dx.doi.org/10.1016/j.jhsa.2012.11.007}, DOI={10.1016/j.jhsa.2012.11.007}, abstractNote={Purpose To determine whether transfer to only the anterior branch of the axillary nerve will restore useful function after axillary nerve injury with persistent posterior deltoid and teres minor paralysis. Methods We used a computational musculoskeletal model of the upper limb to determine the relative contributions of posterior deltoid and teres minor to maximum joint moment generated during a simulated static strength assessment and to joint moments during 3 submaximal shoulder movements. Movement simulations were performed with and without simulated posterior deltoid and teres minor paralysis to identify muscles that may compensate for their paralysis. Results In the unimpaired limb model, teres minor and posterior deltoid accounted for 16% and 14% of the total isometric shoulder extension and external rotation joint moments, respectively. During the 3 movement simulations, posterior deltoid produced as much as 20% of the mean shoulder extension moment, whereas teres minor accounted for less than 5% of the mean joint moment in all directions of movement. When we paralyzed posterior deltoid and teres minor, the mean extension moments generated by the supraspinatus, long head of triceps, latissimus dorsi, and middle deltoid increased to compensate. Compensatory muscles were not fully activated during movement simulations when posterior deltoid and teres minor were paralyzed. Conclusions Reconstruction of the anterior branch of the axillary nerve only is an appropriate technique for restoring shoulder abduction strength after isolated axillary nerve injury. When shoulder extension strength is compromised by extensive neuromuscular shoulder injury, reconstruction of both the anterior and posterior branches of the axillary nerve should be considered. Clinical relevance By quantifying the biomechanical role of muscles during submaximal movement, in addition to quantifying muscle contributions to maximal shoulder strength, we can inform preoperative planning and permit more accurate predictions of functional outcomes. To determine whether transfer to only the anterior branch of the axillary nerve will restore useful function after axillary nerve injury with persistent posterior deltoid and teres minor paralysis. We used a computational musculoskeletal model of the upper limb to determine the relative contributions of posterior deltoid and teres minor to maximum joint moment generated during a simulated static strength assessment and to joint moments during 3 submaximal shoulder movements. Movement simulations were performed with and without simulated posterior deltoid and teres minor paralysis to identify muscles that may compensate for their paralysis. In the unimpaired limb model, teres minor and posterior deltoid accounted for 16% and 14% of the total isometric shoulder extension and external rotation joint moments, respectively. During the 3 movement simulations, posterior deltoid produced as much as 20% of the mean shoulder extension moment, whereas teres minor accounted for less than 5% of the mean joint moment in all directions of movement. When we paralyzed posterior deltoid and teres minor, the mean extension moments generated by the supraspinatus, long head of triceps, latissimus dorsi, and middle deltoid increased to compensate. Compensatory muscles were not fully activated during movement simulations when posterior deltoid and teres minor were paralyzed. Reconstruction of the anterior branch of the axillary nerve only is an appropriate technique for restoring shoulder abduction strength after isolated axillary nerve injury. When shoulder extension strength is compromised by extensive neuromuscular shoulder injury, reconstruction of both the anterior and posterior branches of the axillary nerve should be considered.}, number={2}, journal={The Journal of Hand Surgery}, publisher={Elsevier BV}, author={Crouch, Dustin L. and Plate, Johannes F. and Li, Zhongyu and Saul, Katherine R.}, year={2013}, month={Feb}, pages={241–249} }
 @article{plate_wiggins_haubruck_scott_smith_saul_mannava_2013, title={Normal aging alters in vivo passive biomechanical response of the rat gastrocnemius-Achilles muscle–tendon unit}, volume={46}, ISSN={0021-9290}, url={http://dx.doi.org/10.1016/j.jbiomech.2012.11.007}, DOI={10.1016/j.jbiomech.2012.11.007}, abstractNote={Predisposition to Achilles tendon (AT) ruptures in middle-aged individuals may be associated with age-related changes to inherent passive biomechanical properties of the gastrocnemius-Achilles (GC-AT) muscle-tendon unit, due to known muscle-tendon structural changes in normal aging. The goal of this study was to determine whether the passive biomechanical response of the GC-AT muscle-tendon unit was altered with age in 6 young (8 months) and 6 middle-aged (24 months) F344xBN hybrid rats from the National Institute on Aging colony. Fung's quasilinear viscoelastic (QLV) model was used to determine in vivo history and time-dependent load-relaxation response of the GC-AT. Effective stiffness and modulus were also estimated using linear regression analysis. Fung's QLV revealed a significantly decreased magnitude of the relaxation response (parameter C, p=0.026) in middle-aged animals compared to young animals (0.108±0.007 vs. 0.144±0.015), with similar time-dependent viscous GC-AT properties (τ(1), τ(2)). The product of elastic parameters (A*B), which represents the initial slope of the elastic response, was significantly increased by 50% in middle-aged rats (p=0.014). Estimated GC-AT stiffness increased 28% at peak tensions in middle-aged rats (2.7±0.2 N/mm) compared to young rats (1.9±0.2 N/mm; p=0.036). While the limitations of this animal model must be considered, the changes we describe could be associated with the observation that GC-AT pathology and injury is more common in middle-aged individuals. Further studies are necessary to characterize the load-to-failure behavior of AT in middle-aged compared to young animals.}, number={3}, journal={Journal of Biomechanics}, publisher={Elsevier BV}, author={Plate, Johannes F. and Wiggins, Walter F. and Haubruck, Patrick and Scott, Aaron T. and Smith, Thomas L. and Saul, Katherine R. and Mannava, Sandeep}, year={2013}, month={Feb}, pages={450–455} }
 @article{mannava_plate_tuohy_seyler_whitlock_curl_smith_saul_2013, title={The science of rotator cuff tears: translating animal models to clinical recommendations using simulation analysis}, volume={21}, ISSN={0942-2056 1433-7347}, url={http://dx.doi.org/10.1007/S00167-012-2145-9}, DOI={10.1007/S00167-012-2145-9}, abstractNote={{"Label"=>"PURPOSE", "NlmCategory"=>"OBJECTIVE"} The purpose of this article is to review basic science studies using various animal models for rotator cuff research and to describe structural, biomechanical, and functional changes to muscle following rotator cuff tears. The use of computational simulations to translate the findings from animal models to human scale is further detailed. {"Label"=>"METHODS", "NlmCategory"=>"METHODS"} A comprehensive review was performed of the basic science literature describing the use of animal models and simulation analysis to examine muscle function following rotator cuff injury and repair in the ageing population. {"Label"=>"RESULTS", "NlmCategory"=>"RESULTS"} The findings from various studies of rotator cuff pathology emphasize the importance of preventing permanent muscular changes with detrimental results. In vivo muscle function, electromyography, and passive muscle-tendon unit properties were studied before and after supraspinatus tenotomy in a rodent rotator cuff injury model (acute vs chronic). Then, a series of simulation experiments were conducted using a validated computational human musculoskeletal shoulder model to assess both passive and active tension of rotator cuff repairs based on surgical positioning. {"Label"=>"CONCLUSION", "NlmCategory"=>"CONCLUSIONS"} Outcomes of rotator cuff repair may be improved by earlier surgical intervention, with lower surgical repair tensions and fewer electromyographic neuromuscular changes. An integrated approach of animal experiments, computer simulation analyses, and clinical studies may allow us to gain a fundamental understanding of the underlying pathology and interpret the results for clinical translation.}, number={7}, journal={Knee Surgery, Sports Traumatology, Arthroscopy}, publisher={Springer Science and Business Media LLC}, author={Mannava, Sandeep and Plate, Johannes F. and Tuohy, Christopher J. and Seyler, Thorsten M. and Whitlock, Patrick W. and Curl, Walton W. and Smith, Thomas L. and Saul, Katherine R.}, year={2013}, month={Jul}, pages={1610–1619} }
 @article{vidt_daly_miller_davis_marsh_saul_2012, title={Characterizing upper limb muscle volume and strength in older adults: A comparison with young adults}, volume={45}, ISSN={0021-9290}, url={http://dx.doi.org/10.1016/j.jbiomech.2011.10.007}, DOI={10.1016/j.jbiomech.2011.10.007}, abstractNote={Aging is associated with the loss of muscle volume (MV) and force leading to difficulties with activities of daily living. However, the relationship between upper limb MV and joint strength has not been characterized for older adults. Quantifying this relationship may help our understanding of the functional declines of the upper limb that older adults experience. Our objective was to assess the relationship between upper limb MV and maximal isometric joint moment-generating capacity (IJM) in a single cohort of healthy older adults (age ≥ 65 years) for 6 major functional groups (32 muscles). MV was determined from MRI for 18 participants (75.1±4.3 years). IJM at the shoulder (abduction/adduction), elbow (flexion/extension), and wrist (flexion/extension) was measured. MV and IJM measurements were compared to previous reports for young adults (28.6±4.5 years). On average older adults had 16.5% less total upper limb MV compared to young adults. Additionally, older adult wrist extensors composed a significantly increased percentage of upper limb MV. Older adult IJM was reduced across all joints, with significant differences for shoulder abductors (p<0.0001), adductors (p=0.01), and wrist flexors (p<0.0001). Young adults were strongest at the shoulder, which was not the case for older adults. In older adults, 40.6% of the variation in IJM was accounted for by MV changes (p≤0.027), compared to 81.0% in young adults. We conclude that for older adults, MV and IJM are, on average, reduced but the significant linear relationship between MV and IJM is maintained. These results suggest that older adult MV and IJM cannot be simply scaled from young adults.}, number={2}, journal={Journal of Biomechanics}, publisher={Elsevier BV}, author={Vidt, Meghan E. and Daly, Melissa and Miller, Michael E. and Davis, Cralen C. and Marsh, Anthony P. and Saul, Katherine R.}, year={2012}, month={Jan}, pages={334–341} }
 @article{mogk_johanson_hentz_saul_murray_2011, title={A simulation analysis of the combined effects of muscle strength and surgical tensioning on lateral pinch force following brachioradialis to flexor pollicis longus transfer}, volume={44}, ISSN={0021-9290}, url={http://dx.doi.org/10.1016/j.jbiomech.2010.11.004}, DOI={10.1016/j.jbiomech.2010.11.004}, abstractNote={Biomechanical simulations of tendon transfers performed following tetraplegia suggest that surgical tensioning influences clinical outcomes. However, previous studies have focused on the biomechanical properties of only the transferred muscle. We developed simulations of the tetraplegic upper limb following transfer of the brachioradialis (BR) to the flexor pollicis longus (FPL) to examine the influence of residual upper limb strength on predictions of post-operative transferred muscle function. Our simulations included the transfer, ECRB, ECRL, the three heads of the triceps, brachialis, and both heads of the biceps. Simulations were integrated with experimental data, including EMG and joint posture data collected from five individuals with tetraplegia and BR-FPL tendon transfers during maximal lateral pinch force exertions. Given a measured co-activation pattern for the non-paralyzed muscles in the tetraplegic upper limb, we computed the highest activation for the transferred BR for which neither the elbow nor the wrist flexor moment was larger than the respective joint extensor moment. In this context, the effects of surgical tensioning were evaluated by comparing the resulting pinch force produced at different muscle strength levels, including patient-specific scaling. Our simulations suggest that extensor muscle weakness in the tetraplegic limb limits the potential to augment total pinch force through surgical tensioning. Incorporating patient-specific muscle volume, EMG activity, joint posture, and strength measurements generated simulation results that were comparable to experimental results. Our study suggests that scaling models to the population of interest facilitates accurate simulation of post-operative outcomes, and carries utility for guiding and developing rehabilitation training protocols.}, number={4}, journal={Journal of Biomechanics}, publisher={Elsevier BV}, author={Mogk, Jeremy P.M. and Johanson, M. Elise and Hentz, Vincent R. and Saul, Katherine R. and Murray, Wendy M.}, year={2011}, month={Feb}, pages={669–675} }
 @article{crouch_li_barnwell_plate_daly_saul_2011, title={Computer Simulation of Nerve Transfer Strategies for Restoring Shoulder Function After Adult C5 and C6 Root Avulsion Injuries}, volume={36}, ISSN={0363-5023}, url={http://dx.doi.org/10.1016/j.jhsa.2011.07.019}, DOI={10.1016/j.jhsa.2011.07.019}, abstractNote={Purpose Functional ability after nerve transfer for upper brachial plexus injuries relies on both the function and magnitude of force recovery of targeted muscles. Following nerve transfers targeting either the axillary nerve, suprascapular nerve, or both, it is unclear whether functional ability is restored in the face of limited muscle force recovery. Methods We used a computer model to simulate flexing the elbow while maintaining a functional shoulder posture for 3 nerve transfer scenarios. We assessed the minimum restored force capacity necessary to perform the task, the associated compensations by neighboring muscles, and the effect of altered muscle coordination on movement effort. Results The minimum force restored by the axillary, suprascapular, and combined nerve transfers that was required for the model to simulate the desired movement was 25%, 40%, and 15% of the unimpaired muscle force capacity, respectively. When the deltoid was paralyzed, the infraspinatus and subscapularis muscles generated higher shoulder abduction moments to compensate for deltoid weakness. For all scenarios, movement effort increased as restored force capacity decreased. Conclusions Combined axillary and suprascapular nerve transfer required the least restored force capacity to perform the desired elbow flexion task, whereas single suprascapular nerve transfer required the most restored force capacity to perform the same task. Although compensation mechanisms allowed all scenarios to perform the desired movement despite weakened shoulder muscles, compensation increased movement effort. Dynamic simulations allowed independent evaluation of the effect of restored force capacity on functional outcome in a way that is not possible experimentally. Clinical relevance Simultaneous nerve transfer to suprascapular and axillary nerves yields the best simulated biomechanical outcome for lower magnitudes of muscle force recovery in this computer model. Axillary nerve transfer performs nearly as well as the combined transfer, whereas suprascapular nerve transfer is more sensitive to the magnitude of reinnervation and is therefore avoided. Functional ability after nerve transfer for upper brachial plexus injuries relies on both the function and magnitude of force recovery of targeted muscles. Following nerve transfers targeting either the axillary nerve, suprascapular nerve, or both, it is unclear whether functional ability is restored in the face of limited muscle force recovery. We used a computer model to simulate flexing the elbow while maintaining a functional shoulder posture for 3 nerve transfer scenarios. We assessed the minimum restored force capacity necessary to perform the task, the associated compensations by neighboring muscles, and the effect of altered muscle coordination on movement effort. The minimum force restored by the axillary, suprascapular, and combined nerve transfers that was required for the model to simulate the desired movement was 25%, 40%, and 15% of the unimpaired muscle force capacity, respectively. When the deltoid was paralyzed, the infraspinatus and subscapularis muscles generated higher shoulder abduction moments to compensate for deltoid weakness. For all scenarios, movement effort increased as restored force capacity decreased. Combined axillary and suprascapular nerve transfer required the least restored force capacity to perform the desired elbow flexion task, whereas single suprascapular nerve transfer required the most restored force capacity to perform the same task. Although compensation mechanisms allowed all scenarios to perform the desired movement despite weakened shoulder muscles, compensation increased movement effort. Dynamic simulations allowed independent evaluation of the effect of restored force capacity on functional outcome in a way that is not possible experimentally.}, number={10}, journal={The Journal of Hand Surgery}, publisher={Elsevier BV}, author={Crouch, Dustin L. and Li, Zhongyu and Barnwell, Jonathan C. and Plate, Johannes F. and Daly, Melissa and Saul, Katherine R.}, year={2011}, month={Oct}, pages={1644–1651} }
 @article{mannava_wiggins_saul_stitzel_smith_andrew koman_smith_tuohy_2011, title={Contributions of Neural Tone to In Vivo Passive Muscle–Tendon Unit Biomechanical Properties in a Rat Rotator Cuff Animal Model}, volume={39}, ISSN={0090-6964 1573-9686}, url={http://dx.doi.org/10.1007/S10439-011-0301-X}, DOI={10.1007/S10439-011-0301-X}, abstractNote={Passive viscoelastic properties of muscle-tendon units are key determinants of intra- and post-operative success. Atrophied, retracted, and stiff muscle-tendon units are technically challenging to manipulate and perform poorly after surgical repair. This study employs botulinum neurotoxin A (BoNT-A)-mediated inhibition of presynaptic acetylcholine release to examine in vivo neural contributions to soft-tissue biomechanical properties. In vivo load-relaxation and active muscle contractile force testing protocols were performed in the rat rotator cuff model. The passive properties were assessed using linear regression analysis and Fung's quasi-linear viscoelastic (QLV) model. BoNT-A injected muscle--tendon units had a significant reduction in force of contraction (p = 0.001). When compared to saline injected controls, the BoNT-A significantly decreased parameter 'A' of the QLV model, which represents the linear elastic response (p = 0.032). The viscous properties in the BoNT-A treatment group were not significantly different from saline injected controls, as determined by comparison of QLV model parameters 'C,' 'τ(1),' and 'τ(2).' In conclusion, neural tone contributes significantly to muscle-tendon unit passive biomechanical properties. Pre-surgical treatment with BoNT-A may improve the rehabilitation of muscle by altering its passive elastic properties. Accordingly, pharmacological modulation of skeletal muscle stiffness with BoNT-A increases flexibility, potentially improving function. Chemical denervation with BoNT-A may also improve the manipulation of stiff and difficult to mobilize muscles during surgical procedures.}, number={7}, journal={Annals of Biomedical Engineering}, publisher={Springer Science and Business Media LLC}, author={Mannava, Sandeep and Wiggins, Walter F. and Saul, Katherine R. and Stitzel, Joel D. and Smith, Beth P. and Andrew Koman, L. and Smith, Thomas L. and Tuohy, Christopher J.}, year={2011}, month={Mar}, pages={1914–1924} }
 @article{saul_hayon_smith_tuohy_mannava_2011, title={Postural dependence of passive tension in the supraspinatus following rotator cuff repair: A simulation analysis}, volume={26}, ISSN={0268-0033}, url={http://dx.doi.org/10.1016/j.clinbiomech.2011.04.005}, DOI={10.1016/j.clinbiomech.2011.04.005}, abstractNote={Background Despite surgical advances, repair of rotator cuff tears is associated with 20–70% incidence of recurrent tearing. The tension required to repair the torn tendon influences surgical outcomes and may be dependent on the gap length from torn tendon that must be spanned by the repair. Detailed understanding of forces throughout the range of motion (ROM) may allow surgeons to make evidence-based recommendations for post-operative care. Methods We used a computational shoulder model to assess passive tension and total moment-generating capacity in supraspinatus for repairs of gaps up to 3 cm throughout the shoulder (ROM). Findings In 60° abduction, increased gap length from 0.5 cm to 3 cm caused increases in passive force from 3 N to 58 N, consistent with those seen during clinical repair. For reduced abduction, passive forces increased substantially. For a 0.5 cm gap, tension throughout the ROM (elevation, plane of elevation, and rotation) is within reasonable limits, but larger gaps are associated with tensions that markedly exceed reported pull-out strength of sutures and anchors. Peak moment for a large 3 cm gap length was 5.09 Nm, a 53% reduction in moment-generating capacity compared to uninjured supraspinatus. Interpretation We conclude that shoulder posture is an important determinant of passive forces during rotator cuff repair surgery. Choosing postures that reduce forces intraoperatively to permit repair of larger gaps may lead to failure postoperatively when the shoulder is mobilized. For larger defects, loss of strength in supraspinatus may be substantial following repair even if retear is prevented.}, number={8}, journal={Clinical Biomechanics}, publisher={Elsevier BV}, author={Saul, Katherine R. and Hayon, Solomon and Smith, Thomas L. and Tuohy, Christopher J. and Mannava, Sandeep}, year={2011}, month={Oct}, pages={804–810} }
 @article{holzbaur_gold_murray_2007, title={Changes in Brachioradialis Muscle Volume and Length with spinal cord injury}, volume={40}, ISSN={0021-9290}, url={http://dx.doi.org/10.1016/S0021-9290(07)70066-0}, DOI={10.1016/S0021-9290(07)70066-0}, journal={Journal of Biomechanics}, publisher={Elsevier BV}, author={Holzbaur, Katherine and Gold, Garry and Murray, Wendy}, year={2007}, pages={S69} }
 @article{holzbaur_delp_gold_murray_2007, title={Moment-generating capacity of upper limb muscles in healthy adults}, volume={40}, ISSN={0021-9290}, url={http://dx.doi.org/10.1016/j.jbiomech.2006.11.013}, DOI={10.1016/j.jbiomech.2006.11.013}, abstractNote={Muscle strength and volume vary greatly among individuals. Maximum isometric joint moment, a standard measurement of strength, has typically been assessed in young, healthy subjects, whereas muscle volumes have generally been measured in cadavers. This has made it difficult to characterize the relationship between isometric strength and muscle size in humans. We measured maximum isometric moments about the shoulder, elbow, and wrist in 10 young, healthy subjects, ranging in size from a 20th percentile female to a 97th percentile male. The volumes of 32 upper limb muscles were determined from magnetic resonance images of these same subjects, and grouped according to their primary function. The maximum moments produced using the shoulder adductors (67.9±28.4 Nm) were largest, and were approximately 6.5(±1.2) times greater than those produced using the wrist extensors (10.2±4.6 Nm), which were smallest. While there were substantial differences in moment-generating capacity among these 10 subjects, moment significantly covaried with muscle volume of the appropriate functional group, explaining between 95% (p<0.0001; shoulder adductors) and 68% (p=0.004; wrist flexors) of the variation in the maximum isometric joint moments among subjects. While other factors, such as muscle moment arms or neural activation and coordination, can contribute to variation in strength among subjects, they either were relatively constant across these subjects compared to large differences in muscle volumes or they covaried with muscle volume. We conclude that differences in strength among healthy young adults are primarily a consequence of variation in muscle volume, as opposed to other factors.}, number={11}, journal={Journal of Biomechanics}, publisher={Elsevier BV}, author={Holzbaur, Katherine R.S. and Delp, Scott L. and Gold, Garry E. and Murray, Wendy M.}, year={2007}, month={Jan}, pages={2442–2449} }
 @article{holzbaur_murray_gold_delp_2007, title={Upper limb muscle volumes in adult subjects}, volume={40}, ISSN={0021-9290}, url={http://dx.doi.org/10.1016/j.jbiomech.2006.11.011}, DOI={10.1016/j.jbiomech.2006.11.011}, abstractNote={Muscle force-generating properties are often derived from cadaveric studies of muscle architecture. While the relative sizes of muscles at a single upper limb joint have been established in cadaveric specimens, the relative sizes of muscles across upper limb joints in living subjects remain unclear. We used magnetic resonance imaging to measure the volumes of the 32 upper limb muscles crossing the glenohumeral joint, elbow, forearm, and wrist in 10 young, healthy subjects, ranging from a 20th percentile female to a 97th percentile male, based on height. We measured the volume and volume fraction of these muscles. Muscles crossing the shoulder, elbow, and wrist comprised 52.5, 31.4, and 16.0% of the total muscle volume, respectively. The deltoid had the largest volume fraction (15.2%+/-1%) and the extensor indicis propius had the smallest (0.2%+/-0.05%). We determined that the distribution of muscle volume in the upper limb is highly conserved across these subjects with a three-fold variation in total muscle volumes (1427-4426cm(3)). When we predicted the volume of an individual muscle from the mean volume fraction, on average 85% of the variation among subjects was accounted for (average p=0.0008). This study provides normative data that forms the basis for investigating muscle volumes in other populations, and for scaling computer models to more accurately represent the muscle volume of a specific individual.}, number={4}, journal={Journal of Biomechanics}, publisher={Elsevier BV}, author={Holzbaur, Katherine R.S. and Murray, Wendy M. and Gold, Garry E. and Delp, Scott L.}, year={2007}, month={Jan}, pages={742–749} }
 @article{holzbaur_gold_johanson_murray_2006, title={MRI-based estimation of muscle volume and length following tendon transfer surgery}, volume={39}, ISSN={0021-9290}, url={http://dx.doi.org/10.1016/S0021-9290(06)83227-6}, DOI={10.1016/S0021-9290(06)83227-6}, journal={Journal of Biomechanics}, publisher={Elsevier BV}, author={Holzbaur, K.R.S. and Gold, G.E. and Johanson, M.E. and Murray, W.M.}, year={2006}, month={Jan}, pages={S84} }
 @article{holzbaur_delp_murray_2006, title={Moment-generating capacity of upper limb muscles}, volume={39}, ISSN={0021-9290}, url={http://dx.doi.org/10.1016/S0021-9290(06)83233-1}, DOI={10.1016/S0021-9290(06)83233-1}, journal={Journal of Biomechanics}, publisher={Elsevier BV}, author={Holzbaur, K.R.S. and Delp, S.L. and Murray, W.M.}, year={2006}, month={Jan}, pages={S85} }
 @article{holzbaur_murray_delp_2005, title={A Model of the Upper Extremity for Simulating Musculoskeletal Surgery and Analyzing Neuromuscular Control}, volume={33}, ISSN={0090-6964 1573-9686}, url={http://dx.doi.org/10.1007/S10439-005-3320-7}, DOI={10.1007/S10439-005-3320-7}, abstractNote={Biomechanical models of the musculoskeletal system are frequently used to study neuromuscular control and simulate surgical procedures. To be broadly applicable, a model must be accessible to users, provide accurate representations of muscles and joints, and capture important interactions between joints. We have developed a model of the upper extremity that includes 15 degrees of freedom representing the shoulder, elbow, forearm, wrist, thumb, and index finger, and 50 muscle compartments crossing these joints. The kinematics of each joint and the force-generating parameters for each muscle were derived from experimental data. The model estimates the muscle-tendon lengths and moment arms for each of the muscles over a wide range of postures. Given a pattern of muscle activations, the model also estimates muscle forces and joint moments. The moment arms and maximum moment-generating capacity of each muscle group (e.g., elbow flexors) were compared to experimental data to assess the accuracy of the model. These comparisons showed that moment arms and joint moments estimated using the model captured important features of upper extremity geometry and mechanics. The model also revealed coupling between joints, such as increased passive finger flexion moment with wrist extension. The computer model is available to researchers at http://nmbl.stanford.edu.}, number={6}, journal={Annals of Biomedical Engineering}, publisher={Springer Science and Business Media LLC}, author={Holzbaur, Katherine R. S. and Murray, Wendy M. and Delp, Scott L.}, year={2005}, month={Jun}, pages={829–840} }