@article{mahon_farris_sawicki_lewek_2015, title={Individual limb mechanical analysis of gait following stroke}, volume={48}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2015.02.006}, abstractNote={The step-to-step transition of walking requires significant mechanical and metabolic energy to redirect the center of mass. Inter-limb mechanical asymmetries during the step-to-step transition may increase overall energy demands and require compensation during single-support. The purpose of this study was to compare individual limb mechanical gait asymmetries during the step-to-step transitions, single-support and over a complete stride between two groups of individuals following stroke stratified by gait speed (≥0.8 m/s or <0.8 m/s). Twenty-six individuals with chronic stroke walked on an instrumented treadmill to collect ground reaction force data. Using the individual limbs method, mechanical power produced on the center of mass was calculated during the trailing double-support, leading double-support, and single-support phases of a stride, as well as over a complete stride. Robust inter-limb asymmetries in mechanical power existed during walking after stroke; for both groups, the non-paretic limb produced significantly more positive net mechanical power than the paretic limb during all phases of a stride and over a complete stride. Interestingly, no differences in inter-limb mechanical power asymmetry were noted between groups based on walking speed, during any phase or over a complete stride. Paretic propulsion, however, was different between speed-based groups. The fact that paretic propulsion (calculated from anterior–posterior forces) is different between groups, but our measure of mechanical work (calculated from all three directions) is not, suggests that limb power output may be dominated by vertical components, which are required for upright support.}, number={6}, journal={JOURNAL OF BIOMECHANICS}, author={Mahon, Caitlin E. and Farris, Dominic J. and Sawicki, Gregory S. and Lewek, Michael D.}, year={2015}, month={Apr}, pages={984–989} }
 @article{robertson_farris_sawicki_2015, title={More is not always better: Modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics (vol 9, 046018, 2014)}, volume={10}, number={1}, journal={Bioinspiration & Biomimetics}, author={Robertson, B. D. and Farris, D. J. and Sawicki, G. S.}, year={2015} }
 @article{farris_hampton_lewek_sawicki_2015, title={Revisiting the mechanics and energetics of walking in individuals with chronic hemiparesis following stroke: from individual limbs to lower limb joints}, volume={12}, ISSN={["1743-0003"]}, DOI={10.1186/s12984-015-0012-x}, abstractNote={Previous reports of the mechanics and energetics of post-stroke hemiparetic walking have either not combined estimates of mechanical and metabolic energy or computed external mechanical work based on the limited combined limbs method. Here we present a comparison of the mechanics and energetics of hemiparetic and unimpaired walking at a matched speed. Mechanical work done on the body centre of mass (COM) was computed by the individual limbs method and work done at individual leg joints was computed with an inverse dynamics analysis. Both estimates were converted to average powers and related to simultaneous estimates of net metabolic power, determined via indirect calorimetry. Efficiency of positive work was calculated as the ratio of average positive mechanical power $$ {\overline{P}}^{+} $$ to net metabolic power. Total $$ {\overline{P}}^{+} $$ was 20% greater for the hemiparetic group (H) than for the unimpaired control group (C) (0.49 vs. 0.41 W · kg−1). The greater $$ {\overline{P}}^{+} $$ was partly attributed to the paretic limb of hemiparetic walkers not providing appropriately timed push-off $$ {\overline{P}}^{+} $$ in the step-to-step transition. This led to compensatory non-paretic limb hip and knee $$ {\overline{P}}^{+} $$ which resulted in greater total mechanical work. Efficiency of positive work was not different between H and C. Increased work, not decreased efficiency, explains the greater metabolic cost of hemiparetic walking post-stroke. Our results highlighted the need to target improving paretic ankle push-off via therapy or assistive technology in order to reduce the metabolic cost of hemiparetic walking.}, journal={JOURNAL OF NEUROENGINEERING AND REHABILITATION}, author={Farris, Dominic James and Hampton, Austin and Lewek, Michael D. and Sawicki, Gregory S.}, year={2015}, month={Feb} }
 @article{robertson_farris_sawicki_2014, title={More is not always better: modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics}, volume={9}, ISSN={["1748-3190"]}, DOI={10.1088/1748-3182/9/4/046018}, abstractNote={Development of robotic exoskeletons to assist/enhance human locomotor performance involves lengthy prototyping, testing, and analysis. This process is further convoluted by variability in limb/body morphology and preferred gait patterns between individuals. In an attempt to expedite this process, and establish a physiological basis for actuator prescription, we developed a simple, predictive model of human neuromechanical adaptation to a passive elastic exoskeleton applied at the ankle joint during a functional task. We modeled the human triceps surae–Achilles tendon muscle tendon unit (MTU) as a single Hill-type muscle, or contractile element (CE), and series tendon, or series elastic element (SEE). This modeled system was placed under gravitational load and underwent cyclic stimulation at a regular frequency (i.e. hopping) with and without exoskeleton (Exo) assistance. We explored the effect that both Exo stiffness k Exo ?> and muscle activation A stim ?> had on combined MTU and Exo (MTU + Exo), MTU, and CE/SEE mechanics and energetics. Model accuracy was verified via qualitative and quantitative comparisons between modeled and prior experimental outcomes. We demonstrated that reduced A stim ?> can be traded for increased k Exo ?> to maintain consistent MTU + Exo mechanics (i.e. average positive power P ¯ mech + ?> output) from an unassisted condition (i.e. k Exo = 0 kN · m − 1 ?> ). For these regions of parameter space, our model predicted a reduction in MTU force, SEE energy cycling, and metabolic rate P ¯ met ?> , as well as constant CE P ¯ mech + ?> output compared to unassisted conditions. This agreed with previous experimental observations, demonstrating our model’s predictive ability. Model predictions also provided insight into mechanisms of metabolic cost minimization, and/or enhanced mechanical performance, and we concluded that both of these outcomes cannot be achieved simultaneously, and that one must come at the detriment of the other in a spring-assisted compliant MTU.}, number={4}, journal={BIOINSPIRATION & BIOMIMETICS}, author={Robertson, Benjamin D. and Farris, Dominic J. and Sawicki, Gregory S.}, year={2014}, month={Dec} }
 @article{farris_hicks_delp_sawicki_2014, title={Musculoskeletal modelling deconstructs the paradoxical effects of elastic ankle exoskeletons on plantar-flexor mechanics and energetics during hopping}, volume={217}, ISSN={["1477-9145"]}, DOI={10.1242/jeb.107656}, abstractNote={Abstract}, number={22}, journal={JOURNAL OF EXPERIMENTAL BIOLOGY}, author={Farris, Dominic James and Hicks, Jennifer L. and Delp, Scott L. and Sawicki, Gregory S.}, year={2014}, month={Nov}, pages={4018–4028} }
 @article{farris_robertson_sawicki_2013, title={Elastic ankle exoskeletons reduce soleus muscle force but not work in human hopping}, volume={115}, ISSN={["1522-1601"]}, DOI={10.1152/japplphysiol.00253.2013}, abstractNote={Inspired by elastic energy storage and return in tendons of human leg muscle-tendon units (MTU), exoskeletons often place a spring in parallel with an MTU to assist the MTU. However, this might perturb the normally efficient MTU mechanics and actually increase active muscle mechanical work. This study tested the effects of elastic parallel assistance on MTU mechanics. Participants hopped with and without spring-loaded ankle exoskeletons that assisted plantar flexion. An inverse dynamics analysis, combined with in vivo ultrasound imaging of soleus fascicles and surface electromyography, was used to determine muscle-tendon mechanics and activations. Whole body net metabolic power was obtained from indirect calorimetry. When hopping with spring-loaded exoskeletons, soleus activation was reduced (30–70%) and so was the magnitude of soleus force (peak force reduced by 30%) and the average rate of soleus force generation (by 50%). Although forces were lower, average positive fascicle power remained unchanged, owing to increased fascicle excursion (+4–5 mm). Net metabolic power was reduced with exoskeleton assistance (19%). These findings highlighted that parallel assistance to a muscle with appreciable series elasticity may have some negative consequences, and that the metabolic cost associated with generating force may be more pronounced than the cost of doing work for these muscles.}, number={5}, journal={JOURNAL OF APPLIED PHYSIOLOGY}, author={Farris, Dominic James and Robertson, Benjamin D. and Sawicki, Gregory S.}, year={2013}, month={Sep}, pages={579–585} }
 @article{farris_sawicki_2012, title={Linking the mechanics and energetics of hopping with elastic ankle exoskeletons}, volume={113}, number={12}, journal={Journal of Applied Physiology}, author={Farris, D. J. and Sawicki, G. S.}, year={2012}, pages={1862–1872} }
 @article{farris_trewartha_mcguigan_2012, title={The effects of a 30-min run on the mechanics of the human Achilles tendon}, volume={112}, number={2}, journal={European Journal of Applied Physiology}, author={Farris, D. J. and Trewartha, G. and McGuigan, M. P.}, year={2012}, pages={653–660} }
 @article{farris_buckeridge_trewartha_mcguigan_2012, title={The effects of orthotic heel lifts on achilles tendon force and strain during running}, volume={28}, number={5}, journal={Journal of Applied Biomechanics}, author={Farris, D. J. and Buckeridge, E. and Trewartha, G. and McGuigan, M. P.}, year={2012}, pages={511–519} }
 @article{farris_sawicki_2012, title={The mechanics and energetics of human walking and running: a joint level perspective}, volume={9}, ISSN={["1742-5662"]}, DOI={10.1098/rsif.2011.0182}, abstractNote={Humans walk and run at a range of speeds. While steady locomotion at a given speed requires no net mechanical work, moving faster does demand both more positive and negative mechanical work per stride. Is this increased demand met by increasing power output at all lower limb joints or just some of them? Does running rely on different joints for power output than walking? How does this contribute to the metabolic cost of locomotion? This study examined the effects of walking and running speed on lower limb joint mechanics and metabolic cost of transport in humans. Kinematic and kinetic data for 10 participants were collected for a range of walking (0.75, 1.25, 1.75, 2.0 m s−1) and running (2.0, 2.25, 2.75, 3.25 m s−1) speeds. Net metabolic power was measured by indirect calorimetry. Within each gait, there was no difference in the proportion of power contributed by each joint (hip, knee, ankle) to total power across speeds. Changing from walking to running resulted in a significant (p= 0.02) shift in power production from the hip to the ankle which may explain the higher efficiency of running at speeds above 2.0 m s−1and shed light on a potential mechanism behind the walk–run transition.}, number={66}, journal={JOURNAL OF THE ROYAL SOCIETY INTERFACE}, author={Farris, Dominic James and Sawicki, Gregory S.}, year={2012}, month={Jan}, pages={110–118} }
 @article{farris_trewartha_mcguigan_2011, title={Could intra-tendinous hyperthermia during running explain chronic injury of the human Achilles tendon?}, volume={44}, number={5}, journal={Journal of Biomechanics}, author={Farris, D. J. and Trewartha, G. and McGuigan, M. P.}, year={2011}, pages={822–826} }