Elastic ankle muscle-tendon interactions are adjusted to produce acceleration during walking in humans

This is the final version. Available from Company of Biologists via the DOI in this recordHumans and other cursorial mammals have distal leg muscles with high in-series compliance that aid locomotor economy. This muscle-tendon design is considered sub-optimal for injecting net positive mechanical work. However, humans change speed frequently when walking and any acceleration requires net positive ankle work. The present study unveiled how the muscle-tendon interaction of human ankle plantar flexors are adjusted and integrated with body mechanics to provide net positive work during accelerative walking. We found that for accelerative walking, a greater amount of active plantar flexor fascicle shortening early in the stance phase occurred and was transitioned through series elastic tissue stretch and recoil. Reorientation of the leg during early stance for acceleration allowed the ankle and whole soleus muscle-tendon complex to remain isometric while its fascicles actively shortened, stretching in-series elastic tissues for subsequent recoil and net positive joint work. This muscle-tendon behaviour is fundamentally different from constant-speed walking, where the ankle and soleus muscle-tendon complex undergo a period of negative work to store energy in series elastic tissues before subsequent recoil, minimizing net joint work. Muscles with high in-series compliance can therefore contribute to net positive work for accelerative walking and here we show a mechanism for how in human ankle muscles.D.J.F. was funded by a post-doctoral fellowship granted by the Australian Sports Commission. This study was also funded by an Early Career Researcher Grant awarded to D.J.F. by The University of Queensland

Elastic ankle exoskeletons reduce soleus muscle force but not work in human hopping

This is the author accepted manuscript. The final version is available from the American Physiological Society via the DOI in this recordInspired 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.This study was in part funded by US Israel Binational Science Foundation Start Up Grant 2011152 awarded to G. S. Sawicki

The influences of added mass on muscle activation and contractile mechanics during submaximal and maximal countermovement jumping in humans

This is the final version. Available from the Company of Biologists via the DOI in this recordMuscle contractile mechanics induced by the changing demands of human movement have the potential to influence our movement strategies. This study examined fascicle length changes of the triceps surae during jumping with added mass or increasing jump height to determine whether the chosen movement strategies were associated with relevant changes in muscle contractile properties. Sixteen participants jumped at sub-maximal and maximal intensities while total net work was matched via two distinct paradigms: (1) adding mass to the participant or (2) increasing jump height. Electromyography (EMG) and ultrasound analyses were performed to examine muscle activation, fascicle length and fascicle velocity changes of the triceps surae during jumping. Integrated EMG was significantly higher in the added mass paradigm with no difference in mean or maximal EMG, indicating that the muscle was activated for a significantly longer period of time but not activated to a greater intensity. Fascicle shortening velocity was slower with added mass compared than with increasing jump height; therefore, intrinsic force–velocity properties probably enabled increased force production. Improved fascicle contractile mechanics paired with a longer activation period probably produced a consistently larger fascicle force, enabling a greater impulse about the ankle joint. This may explain why previous research found that participants used an ankle-centred strategy for work production in the added mass paradigm and not in the jump height paradigm. The varied architecture of muscles within the lower limb may influence which muscles we choose to employ for work production under different task constraints.Australian Postgraduate Awar

The Influence of Foot-Strike Technique on the Neuromechanical Function of the Foot

This is the author accepted manuscript. The final version is available from Lippincott, Williams & Wilkins via the DOI in this recordPURPOSE: The aim of this study was to investigate the influence of foot-strike technique on longitudinal arch mechanics and intrinsic foot muscle function during running. METHODS: Thirteen healthy participants ran barefoot on a force-instrumented treadmill at 2.8 ms with a forefoot (FFS) and rearfoot (RFS; habitual) running technique, whereas kinetic, kinematic, and electromyographic data from the intrinsic foot muscles were collected simultaneously. The longitudinal arch was modeled as a single "midfoot" joint representing motion of the rearfoot (calcaneus) relative to the forefoot (metatarsals). An inverse dynamic analysis was performed to estimate joint moments generated about the midfoot, as well as mechanical work and power. RESULTS: The midfoot was more plantar flexed (higher arch) at foot contact when running with a forefoot running technique (RFS 0.2 ± 1.8 vs FFS 6.9 ± 3.0°, effect size (ES) = 2.7); however, there was no difference in peak midfoot dorsiflexion in stance (RFS -11.6 ± 3.0 vs FFS -11.4 ± 3.4°, ES = 0.63). When running with a forefoot technique, participants generated greater moments about the midfoot (27% increase, ES = 1.1) and performed more negative work (240% increase, ES = 2.2) and positive work (42% increase, ES = 1.1) about the midfoot. Average stance-phase muscle activation was greater for flexor digitorum brevis (20% increase, ES = 0.56) and abductor hallucis (17% increase, ES = 0.63) when running with a forefoot technique. CONCLUSIONS: Forefoot running increases loading about the longitudinal arch and also increases the mechanical work performed by the intrinsic foot muscles. These findings have substantial implications in terms of injury prevention and management for runners who transition from a rearfoot to a forefoot running technique.Funding for this study was provided via an industry research grant from Asics Oceania (grant identification number 2014000885

Revisiting the mechanics and energetics of walking in individuals with chronic hemiparesis following stroke: from individual limbs to lower limb joints

This is the final version. Available on open access from BMC via the DOI in this recordBACKGROUND: 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. METHODS: 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 [Formula: see text] to net metabolic power. RESULTS: Total [Formula: see text] 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 [Formula: see text] was partly attributed to the paretic limb of hemiparetic walkers not providing appropriately timed push-off [Formula: see text] in the step-to-step transition. This led to compensatory non-paretic limb hip and knee [Formula: see text] which resulted in greater total mechanical work. Efficiency of positive work was not different between H and C. CONCLUSIONS: 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.This research was funded by the following grants: NC TraCs Institute grant number 50KR41018; National Institutes of Health award #R24 HD 050821 (through the Rehabilitation Institute of Chicago); and Eunice Kennedy Shriver National Institute of Child Health & Development of the National Institutes of Health award #R21 HD072588 all to G.S.S

Individual limb mechanical analysis of gait following stroke

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordThe 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.This work was supported by the Foundation for Physical Therapy, Incorporated Geriatric Endowment Fund, the American Heart Association (09BGIA2210015), and the Joint University of North Carolina at Chapel Hill and North Carolina State University Rehabilitation Engineering Center seed grant

The effect of cadence on the muscle-tendon mechanics of the gastrocnemius muscle during walking

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this recordHumans naturally select a cadence that minimizes metabolic cost at a constant walking velocity. The aim of this study was to examine the effects of cadence on the medial gastrocnemius (MG) muscle and tendon interaction, and examine how this might influence lower limb energetics. We hypothesized that cadences higher than preferred would increase MG fascicle shortening velocity because of the reduced stride time. Furthermore, we hypothesized that cadences lower than preferred would require greater MG fascicle shortening to achieve increased muscle work requirements. We measured lower limb kinematics and kinetics, surface electromyography of the triceps surae and MG fascicle length, via ultrasonography, during walking at a constant velocity at the participants' preferred cadence and offsets of ±10%, ±20%, and ±30%. There was a significant increase in MG fascicle shortening with decreased cadence. However, there was no increase in the MG fascicle shortening velocity at cadences higher than preferred. Cumulative MG muscle activation per minute was significantly increased at higher cadences. We conclude that low cadence walking requires more MG shortening work, while MG muscle and tendon function changes little for each stride at higher cadences, driving up cumulative activation costs due to the increase in steps per minute.Scott Brennan is supported by an Australian Postgraduate Scholarship. Dominic Farris is supported by the Australian Sports Commission

Intrinsic foot muscles contribute to elastic energy storage and return in the human foot

 This is the author accepted manuscript. The final version is available from American Physiological Society via the DOI in this recordData Availability: Data from this study is available at https://www.dropbox.com/sh/okbsab120jsc2az/AAD05Q1dtaY6MLdi_dkaqUjsa?dl=0The human foot is uniquely stiff to enable forward propulsion, yet also possesses sufficient elasticity to act as an energy store, recycling mechanical energy during locomotion. Historically this dichotomous function has been attributed to the passive contribution of the plantar aponeurosis. However, recent evidence highlights the potential for muscles to actively modulate the energetic function of the foot. Here we test the hypothesis that the central nervous system can actively control the foot's energetic function, via activation of the muscles within the foot's longitudinal arch. We used a custom-built loading apparatus to deliver cyclical loads to human feet in-vivo, in order to deform the arch in a manner similar to that observed in locomotion. We recorded foot motion and forces, alongside muscle activation and ultrasound images from flexor digitorum brevis (FDB), an intrinsic foot muscle that spans the arch. When active, the FDB muscle fascicles contracted in an isometric manner, facilitating elastic energy storage in the tendon, in addition to the energy stored within the plantar aponeurosis. We propose that the human foot is akin to an active suspension system for the human body, with mechanical and energetic properties that can be actively controlled by the central nervous system.Australian Research CouncilNational Health & Medical Research Council (NHMRC

Shoes alter the spring-like function of the human foot during running

This is the author accepted manuscript. The final version is available from the Royal Society via the DOI in this recordThe capacity to store and return energy in legs and feet that behave like springs is crucial to human running economy. Recent comparisons of shod and barefoot running have led to suggestions that modern running shoes may actually impede leg and foot-spring function by reducing the contributions from the leg and foot musculature. Here we examined the effect of running shoes on foot longitudinal arch (LA) motion and activation of the intrinsic foot muscles. Participants ran on a force-instrumented treadmill with and without running shoes. We recorded foot kinematics and muscle activation of the intrinsic foot muscles using intramuscular electromyography. In contrast to previous assertions, we observed an increase in both the peak (flexor digitorum brevis +60%) and total stance muscle activation (flexor digitorum brevis +70% and abductor hallucis +53%) of the intrinsic foot muscles when running with shoes. Increased intrinsic muscle activation corresponded with a reduction in LA compression (-25%). We confirm that running shoes do indeed influence the mechanical function of the foot. However, our findings suggest that these mechanical adjustments are likely to have occurred as a result of increased neuromuscular output, rather than impaired control as previously speculated. We propose a theoretical model for foot-shoe interaction to explain these novel findings.Funding for this study was provided by Asics Oceania Pty Ltd. Grant ID number 2014000885

The role of human ankle plantar flexor muscle-tendon interaction and architecture in maximal vertical jumping examined in vivo

This is the final version. Available from Company of Biologists via the DOI in this record.Humans utilise elastic tendons of lower limb muscles to store and return energy during walking, running and jumping. Anuran and insect species use skeletal structures and/or dynamics in conjunction with similarly compliant structures to amplify muscle power output during jumping. We sought to examine whether human jumpers use similar mechanisms to aid elastic energy usage in the plantar flexor muscles during maximal vertical jumping. Ten male athletes performed maximal vertical squat jumps. Three-dimensional motion capture and a musculoskeletal model were used to determine lower limb kinematics that were combined with ground reaction force data in an inverse dynamics analysis. B-mode ultrasound imaging of the lateral gastrocnemius (GAS) and soleus (SOL) muscles was used to measure muscle fascicle lengths and pennation angles during jumping. Our results highlighted that both GAS and SOL utilised stretch and recoil of their series elastic elements (SEEs) in a catapult-like fashion, which likely serves to maximise ankle joint power. The resistance of supporting of body weight allowed initial stretch of both GAS and SOL SEEs. A proximal-to-distal sequence of joint moments and decreasing effective mechanical advantage early in the extension phase of the jumping movement were observed. This facilitated a further stretch of the SEE of the biarticular GAS and delayed recoil of the SOL SEE. However, effective mechanical advantage did not increase late in the jump to aid recoil of elastic tissues.D.J.F. is supported by a post-doctoral fellowship funded by the Australian Sports Commission