A three-dimensional musculoskeletal model of the pelvis and lower limb of Australopithecus afarensis.

OBJECTIVES: Musculoskeletal modeling is a powerful approach for studying the biomechanics and energetics of locomotion. Australopithecus (A.) afarensis is among the best represented fossil hominins and provides critical information about the evolution of musculoskeletal design and locomotion in the hominin lineage. Here, we develop and evaluate a three-dimensional (3-D) musculoskeletal model of the pelvis and lower limb of A. afarensis for predicting muscle-tendon moment arms and moment-generating capacities across lower limb joint positions encompassing a range of locomotor behaviors. MATERIALS AND METHODS: A 3-D musculoskeletal model of an adult A. afarensis pelvis and lower limb was developed based primarily on the A.L. 288-1 partial skeleton. The model includes geometric representations of bones, joints and 35 muscle-tendon units represented using 43 Hill-type muscle models. Two muscle parameter datasets were created from human and chimpanzee sources. 3-D muscle-tendon moment arms and isometric joint moments were predicted over a wide range of joint positions. RESULTS: Predicted muscle-tendon moment arms generally agreed with skeletal metrics, and corresponded with human and chimpanzee models. Human and chimpanzee-based muscle parameterizations were similar, with some differences in maximum isometric force-producing capabilities. The model is amenable to size scaling from A.L. 288-1 to the larger KSD-VP-1/1, which subsumes a wide range of size variation in A. afarensis. DISCUSSION: This model represents an important tool for studying the integrated function of the neuromusculoskeletal systems in A. afarensis. It is similar to current human and chimpanzee models in musculoskeletal detail, and will permit direct, comparative 3-D simulation studies.

Computational performance of musculoskeletal simulation in OpenSim Moco using parallel computing.

Optimal control musculoskeletal simulation is a valuable approach for studying fundamental and clinical aspects of human movement. However, the high computational demand has long presented a substantial challenge, creating a need to improve simulation performance. The OpenSim Moco software package permits musculoskeletal simulation problems to be solved in parallel on multicore processors using the CasADi optimal control library, potentially reducing the computational demand. However, the computational performance of this framework has not been thoroughly examined. Thus, we aimed to investigate the computational speed-up obtained via multicore parallel computing relative to solving problems serially (i.e., using a single core) in optimal control simulations of human movement in OpenSim Moco. Simulations were solved using up to 18 cores with a variety of temporal mesh interval densities and using two different initial guess strategies. We examined a range of musculoskeletal models and movements that included two- and three-dimensional models, tracking and predictive simulations, and walking and reaching tasks. The maximum overall parallel speed-up was problem specific and ranged from 1.7 to 7.7 times faster than serial, with most of the speed-up achieved by about 6 processor cores. Parallel speed-up was generally greater on finer temporal meshes, while the initial guess strategy had minimal impact on speed-up. Considerable speed-up can be achieved for some optimal control simulation problems in OpenSim Moco by leveraging the multicore processors often available in modern computers. However, since improvements are problem specific, achieving optimal computational performance will require some degree of exploration by the end user.

Age-related changes in gait biomechanics and their impact on the metabolic cost of walking: Report from a National Institute on Aging workshop.

Changes in old age that contribute to the complex issue of an increased metabolic cost of walking (mass-specific energy cost per unit distance traveled) in older adults appear to center at least in part on changes in gait biomechanics. However, age-related changes in energy metabolism, neuromuscular function and connective tissue properties also likely contribute to this problem, of which the consequences are poor mobility and increased risk of inactivity-related disease and disability. The U.S. National Institute on Aging convened a workshop in September 2021 with an interdisciplinary group of scientists to address the gaps in research related to the mechanisms and consequences of changes in mobility in old age. The goal of the workshop was to identify promising ways to move the field forward toward improving gait performance, decreasing energy cost, and enhancing mobility for older adults. This report summarizes the workshop and brings multidisciplinary insight into the known and potential causes and consequences of age-related changes in gait biomechanics. We highlight how gait mechanics and energy cost change with aging, the potential neuromuscular mechanisms and role of connective tissue in these changes, and cutting-edge interventions and technologies that may be used to measure and improve gait and mobility in older adults. Key gaps in the literature that warrant targeted research in the future are identified and discussed.

Lower back kinetic demands during induced lower limb gait asymmetries.

BACKGROUND: Gait asymmetries are common in many clinical populations (e.g., amputation, injury, or deformities) and are associated with a high incidence of lower back pain. Despite this high incidence, the impact of gait asymmetries on lower back kinetic demands are not well characterized due to experimental limitations in these clinical populations. Therefore, we artificially and safely induced gait asymmetry during walking in healthy able-bodied participants to examine lower back kinetic demands compared to their normal gait. RESEARCH QUESTION: Are lower back kinetic demands different during artificially induced asymmetries than those during normal gait? METHODS: L5/S1 vertebral joint kinetics and trunk muscle forces were estimated during gait in twelve healthy men and women with a musculoskeletal lower back model that uniquely incorporated participant-specific responses using an EMG optimization approach. Five walking conditions were conducted on a force-measuring treadmill, including normal unperturbed "symmetrical" gait, and asymmetrical gait induced by unilaterally altering leg mass, leg length, and ankle joint motion in various combinations. Gait symmetry index and lower back kinetics were compared with repeated-measures ANOVAs and post hoc tests (α = .05). RESULTS: The perturbations were successful in producing different degrees of step length and stance time gait asymmetries (p < .01). However, lower back kinetic demands associated with asymmetrical gait were similar to, or only moderately different from normal walking for most conditions despite the observed asymmetries. SIGNIFICANCE: Our findings indicate that the high incidence of lower back pain often associated with gait asymmetries may not be a direct effect of increased lower back demands. If biomechanical demands are responsible for the high incidence of lower back pain in such populations, daily tasks besides walking may be responsible and warrant further investigation.

Are lower back demands reduced by improving gait symmetry in unilateral transtibial amputees?

BACKGROUND: Gait asymmetry and a high incidence of lower back pain are typical for people with unilateral lower limb amputation. A common therapeutic objective is to improve gait symmetry; however, it is unknown whether better gait symmetry reduces lower back pain risk. To begin investigating this important clinical question, we examined a preexisting dataset to explore whether L5/S1 vertebral joint forces in people with unilateral lower limb amputation can be improved with better symmetry. METHODS: L5/S1 compression and resultant shear forces were estimated in each participant with unilateral lower limb amputation (n = 5) with an OpenSim musculoskeletal model during different levels of guided gait asymmetry. The amount of gait asymmetry was defined by bilateral stance times and guided via real-time feedback. A theoretical lowest L5/S1 force was determined from the minimum of a best-fit quadratic curves of L5/S1 forces at levels of guided asymmetry ranging from -10 to +15%. The forces found at the theoretical lowest force and during the 0% asymmetry level were compared to forces at preferred levels of asymmetry and to those from an able-bodied group (n = 5). FINDINGS: Results indicated that the forces for the people with unilateral lower limb amputation group at the preferred level of asymmetry were not different then at their 0% asymmetry condition, theoretical lowest L5/S1 forces, or the able-bodied group (all p-values > .23). INTERPRETATION: These preliminary results challenge the premise that restoring symmetric gait in people with unilateral lower limb amputation will reduce risk of lower back pain.

EMG optimization in OpenSim: A model for estimating lower back kinetics in gait.

Participant-specific musculoskeletal models are needed to accurately estimate lower back internal kinetic demands and injury risk. In this study we developed the framework for incorporating an electromyography optimization (EMGopt) approach within OpenSim (https://simtk.org/projects/emg_opt_tool) and evaluated lower back demands estimated from the model during gait. Kinematic, external kinetic, and EMG data were recorded from six participants as they performed walking and carrying tasks on a treadmill. For evaluation, predicted lumbar vertebral joint forces were compared to those from a generic static optimization approach (SOpt) and to previous studies. Further, model-estimated muscle activations were compared to recorded EMG, and model sensitivity to day-to-day EMG variability was evaluated. Results showed the vertebral joint forces from the model were qualitatively similar in pattern and magnitude to literature reports. Compared to SOpt, the EMGopt approach predicted larger joint loads (p<.01) with muscle activations better matching individual participant EMG patterns. L5/S1 vertebral joint forces from EMGopt were sensitive to the expected variability of recorded EMG, but the magnitude of these differences (±4%) did not impact between-task comparisons. Despite limitations inherent to such models, the proposed musculoskeletal model and EMGopt approach appears well-suited for evaluating internal lower back demands during gait tasks.

Adaptations for bipedal walking: Musculoskeletal structure and three-dimensional joint mechanics of humans and bipedal chimpanzees (Pan troglodytes).

Humans are unique among apes and other primates in the musculoskeletal design of their lower back, pelvis, and lower limbs. Here, we describe the three-dimensional ground reaction forces and lower/hindlimb joint mechanics of human and bipedal chimpanzees walking over a full stride and test whether: 1) the estimated limb joint work and power during the stance phase, especially the single-support period, is lower in humans than bipedal chimpanzees, 2) the limb joint work and power required for limb swing is lower in humans than in bipedal chimpanzees, and 3) the estimated total mechanical power during walking, accounting for the storage of passive elastic strain energy in humans, is lower in humans than in bipedal chimpanzees. Humans and bipedal chimpanzees were compared at matched dimensionless and dimensional velocities. Our results indicate that humans walk with significantly less work and power output in the first double-support period and the single-support period of stance, but markedly exceed chimpanzees in the second double-support period (i.e., push-off). Humans generate less work and power in limb swing, although the species difference in limb swing power was not statistically significant. We estimated that total mechanical positive 'muscle fiber' work and power were 46.9% and 35.8% lower, respectively, in humans than in bipedal chimpanzees at matched dimensionless speeds. This is due in part to mechanisms for the storage and release of elastic energy at the ankle and hip in humans. Furthermore, these results indicate distinct 'heel strike' and 'lateral balance' mechanics in humans and bipedal chimpanzees and suggest a greater dissipation of mechanical energy through soft tissue deformations in humans. Together, our results document important differences between human and bipedal chimpanzee walking mechanics over a full stride, permitting a more comprehensive understanding of the mechanics and energetics of chimpanzee bipedalism and the evolution of hominin walking.

A muscle control strategy to alter pedal force direction under multiple constraints: A simulation study.

Humans can quickly adapt to different task demands in cycling. The motor system continuously manipulates applied pedal forces under the influence of gravitational and inertial forces, but the muscular control strategy remains unknown. The aim of this study was to investigate muscular control and coordination when altering pedal force patterns, using a musculoskeletal model with dynamic tracking optimization and induced acceleration analysis (IAA). The tracking data were pedaling kinematics and kinetics in recreational cyclists before and after learning to apply pedal force toward a tangential target direction in one-legged pedaling (Park et al., 2021). The gravity and inertial force contributions to pedal forces were relatively unchanged after practice due to the consistent rider posture and pedaling mechanics. Pedal force contributions induced by individual muscle-tendon units (MTUs) were also relatively consistent in direction before and after practice, likely due to similar joint positions and task constraints in the two conditions. However, the total applied pedal force from the sum of IAA component contributions was more closely directed towards the target due to coordinated changes in the magnitudes of contributions of the resultant pedal force by individual MTUs. The improvement in pedal force targeting seen in this complex coordination task may be possible through a control strategy of scaling muscle activity level. The rapid adaptation to a new pedal force pattern in this constrained task is facilitated by a relatively simple strategy of scaling muscle activation amplitude.

Metabolic cost of transport and stance time asymmetry in individuals with unilateral transtibial amputation using a passive prostheses while walking.

BACKGROUND: People with unilateral amputation typically walk with greater metabolic cost than able-bodied individuals, while preferring asymmetric walking characteristics. It is unclear if asymmetric walking is energetically optimal and how metabolic cost accounts for asymmetric patterns in people with amputation. The purpose of this study was to determine the effects of stance-time asymmetry on the metabolic cost of transport. METHODS: Fourteen participants (seven with amputation) completed two laboratory sessions where they walked on a treadmill while receiving real-time visual feedback about stance-time asymmetry. Expired gases were collected to determine the metabolic cost for a range of asymmetries (-15% to +15% in 5% increments, positive percentages represent more time on intact [dominant] limb). FINDINGS: Participants with amputation walked with greater (P = 0.008) stance-time asymmetry (4.34 ± 1.09%) compared with able-bodied participants (0.94 ± 2.44%). Stance-time asymmetry had a significant effect on metabolic cost (P < 0.001). The asymmetries coinciding with the predicted minimum metabolic cost for people with (3.23 ± 2.90%) and without (1.81 ± 2.18%) amputation were not different from preferred asymmetries (P = 0.365; p = 0.513), respectively. The cost of symmetric walking was 13.6% greater than near preferred walking for people with amputation (5% more time on intact limb). INTERPRETATION: Metabolic cost is not the only objective of walking, but like able-bodied individuals, it may influence how people with amputation walk. Rehabilitation typically tries to restore inter-limb symmetry after an injury, yet if the limbs are asymmetric, symmetric gait may not be optimal with current assistive devices.

A direct collocation framework for optimal control simulation of pedaling using OpenSim.

The direct collocation (DC) method has shown low computational costs in solving optimization problems in human movements, but it has rarely been used for solving optimal control pedaling problems. Thus, the aim of this study was to develop a DC framework for optimal control simulation of human pedaling within the OpenSim modeling environment. A planar bicycle-rider model was developed in OpenSim. The DC method was formulated in MATLAB to solve an optimal control pedaling problem using a data tracking approach. Using the developed DC framework, the optimal control pedaling problem was successfully solved in 24 minutes to ten hours with different objective function weightings and number of nodes from two different initial conditions. The optimal solutions for equal objective function weightings were successful in terms of tracking, with the model simulated pedal angles and pedal forces within ±1 standard deviation of the experimental data. With these weightings, muscle tendon unit (MTU) excitation patterns generally matched with burst timings and shapes observed in the experimental EMG data. Tracking quality and MTU excitation patterns were changed little by selection of node density above 31, and the optimal solution quality was not affected by initial guess used. The proposed DC framework could easily be turned into a predictive simulation with other objective functions such as fastest pedaling rate. This flexible and computationally efficient framework should facilitate the use of optimal control methods to study the biomechanics, energetics, and control of human pedaling.

The effects of posture on the three-dimensional gait mechanics of human walking in comparison with walking in bipedal chimpanzees.

Humans walk with an upright posture on extended limbs during stance and with a double-peaked vertical ground reaction force. Our closest living relatives, chimpanzees, are facultative bipeds that walk with a crouched posture on flexed, abducted hind limbs and with a single-peaked vertical ground reaction force. Differences in human and bipedal chimpanzee three-dimensional (3D) kinematics have been well quantified, yet it is unclear what the independent effects of using a crouched posture are on 3D gait mechanics for humans, and how they compare with chimpanzees. Understanding the relationships between posture and gait mechanics, with known differences in morphology between species, can help researchers better interpret the effects of trait evolution on bipedal walking. We quantified pelvis and lower limb 3D kinematics and ground reaction forces as humans adopted a series of upright and crouched postures and compared them with data from bipedal chimpanzee walking. Human crouched-posture gait mechanics were more similar to that of bipedal chimpanzee gait than to normal human walking, especially in sagittal plane hip and knee angles. However, there were persistent differences between species, as humans walked with less transverse plane pelvis rotation, less hip abduction, and greater peak anterior-posterior ground reaction force in late stance than chimpanzees. Our results suggest that human crouched-posture walking reproduces only a small subset of the characteristics of 3D kinematics and ground reaction forces of chimpanzee walking, with the remaining differences likely due to the distinct musculoskeletal morphologies of humans and chimpanzees.

Inclusion of actuator dynamics in simulations of assisted human movement.

Simulation of musculoskeletal systems using dynamic optimization is a powerful approach for studying the biomechanics of human movements and can be applied to human-robot interactions. The simulation results of human movements augmented by robotic devices may be used to evaluate and optimize the device design and controller. However, simulations are limited by the accuracy of the models which are usually simplified for computation efficiency. Typically, the powered robotic devices are often modeled as massless, ideal torque actuators that is without mass and internal dynamics, which may have significant impacts on the simulation results. This article investigates the effects of including the mass and internal dynamics of the device in simulations of assisted human movement. The device actuator was modeled in various ways with different detail levels. Dynamic optimization was used to find the muscle activations and actuator commands in motion tracking and predictive simulations. The results showed that while the effects of device mass and inertia can be small, the electrical dynamics of the motor can significantly impact the results. This outcome suggests the importance of using an accurate actuator model in simulations of human movement augmented by assistive devices. NOVELTY: Demonstrating the effects of including mass and internal dynamics of the actuator in simulations of assisted human movement A new OpenSim electric motor actuator class to capture the electromechanical dynamics for use in simulation of human movement assisted by powered robotic devices.

A model-based motion capture marker location refinement approach using inverse kinematics from dynamic trials.

Marker-based motion capture techniques are commonly used to measure human body kinematics. These techniques require an accurate mapping from physical marker position to model marker position. Traditional methods utilize a manual process to achieve marker positions that result in accurate tracking. In this work, we present an optimization algorithm for model marker placement to minimize marker tracking error during inverse kinematics analysis of dynamic human motion. The algorithm sequentially adjusts model marker locations in 3-D relative to the underlying rigid segment. Inverse kinematics is performed for a dynamic motion capture trial to calculate the tracking error each time a marker position is changed. The increase or decrease of the tracking error determines the search direction and number of increments for each marker coordinate. A final marker placement for the model is reached when the total search interval size for every coordinate falls below a user-defined threshold. Individual marker coordinates can be locked in place to prevent the algorithm from overcorrecting for data artifacts such as soft tissue artifact. This approach was used to refine model marker placements for eight able-bodied subjects performing walking trials at three stride frequencies. Across all subjects and stride frequencies, root mean square (RMS) tracking error decreased by 38.4% and RMS tracking error variance decreased by 53.7% on average. The resulting joint kinematics were in agreement with expected values from the literature. This approach results in realistic kinematics with marker tracking errors well below accepted thresholds while removing variance in the model-building procedure introduced by individual human tendencies.

Predictive Simulation of Human Walking Augmented by a Powered Ankle Exoskeleton.

The human ankle provides significant positive power during the stance phase of walking, which has resulted in studies focusing on methods to reduce the energetic walking cost by augmenting the ankle with exoskeletons. Recently, a few devices have successfully reduced the metabolic cost of walking by replacing part of the biological ankle plantar flexor torque. Despite these achievements, development of assistive ankle devices remains challenging, partly because the current practice of design and control of powered exoskeletons is highly time and effort consuming, which prevents quickly exploring different design and control parameters. Predictive simulations using musculoskeletal models coupled with robotic devices may facilitate the process of design and control of assistive devices. In this study, we simulate human walking augmented by a powered ankle exoskeleton. The walking problem was formulated as a predictive dynamic optimization in which both the optimal assistive device torque and the gait were solved simultaneously. Cases with exoskeletons assisting one ankle and both ankles were considered. The results showed that the energetic cost of walking could be reduced by 45% with one ankle augmented, and by 52% with both ankles augmented. This study contributes towards the goal of providing optimal assistive torque through external devices and theoretical peak reductions that could be expected from such devices.

Dynamic optimization of Gait with a Generalized Lower-Limb Prosthesis Model.

Predictive simulation of gait is a promising tool for robotic lower limb prosthesis design, but has been limited in its application to models of existing design types. We propose a modeling approach to find optimal prosthesis dynamics in gait simulations without constraining the prosthesis to follow kinematics allowed by a specific joint mechanism. To accomplish this, we render a transtibial prosthetic device as the composition of its resultant forces and moments as they act upon the prosthetic foot and socket and allow3 degree-of-freedom planar motion. The model is implemented into a human musculoskeletal model and used to solve dynamic optimizations of muscle and prosthesis controls to minimize muscle effort and loading on the residual limb during walking. The emphasis on muscle effort vs. limb loading is varied in the minimization objective and the resulting optimal prosthesis dynamics are compared. We found that muscle effort and socket loading measures were reduced for our prosthesis model compared to a revolute joint prosthesis model. We interpret large displacements in the linear axes to transfer energy to the plantarflexion action before toe-off and reduce loading at the socket-limb interface. Our results suggest this approach could assist in the design of non-biomimetic prostheses but requires experimental validation to assess our modeling assumptions, as well as progress toward increased fidelity of predictive simulation approaches more generally.

Bilevel Optimization for Cost Function Determination in Dynamic Simulation of Human Gait.

Predictive simulation based on dynamic optimization using musculoskeletal models is a powerful approach for studying human gait. Predictive musculoskeletal simulation may be used for a variety of applications from designing assistive devices to testing theories of motor control. However, the underlying cost function for the predictive optimization is unknown and is generally assumed a priori. Alternatively, the underlying cost function can be determined from among a family of possible cost functions, representing an inverse optimal control problem that may be solved using a bilevel optimization approach. In this study, a nested evolutionary approach is proposed to solve the bilevel optimization problem. The lower level optimization is solved by a direct collocation method, and the upper level is solved by a genetic algorithm. We demonstrate our approach to solve different bilevel optimization problems, including finding the weights among three common performance criteria in the cost function for normal human walking. The proposed approach was found to be effective at solving the bilevel optimization problems. This approach should provide practical utility in designing assistive devices to aid mobility, and could yield insights about the control of human walking.

Performance criteria for generating predictive optimal control simulations of bicycle pedaling.

The purpose of this study was to identify one or more performance-based criteria that may be used to generate predictive optimal control simulations of submaximal pedaling. Two-legged pedaling simulations were generated based on minimizing muscle activation, muscle stress, metabolic energy, time derivative of muscle force, and minimizing metabolic energy while pedaling smoothly. The simulations based on minimizing muscle activation and muscle stress most closely matched experimental pedaling data, with the activation criterion better matching experimental muscle activation timing. We conclude that predictive simulations of submaximal pedaling may be generated using a cost function based on minimizing muscle activation.

Three-dimensional kinematics and the origin of the hominin walking stride.

Humans are unique among apes and other primates in the musculoskeletal design of their lower back and pelvis. While the last common ancestor of the Pan-Homo lineages has long been thought to be 'African ape-like', including in its lower back and ilia design, recent descriptions of early hominin and Miocene ape fossils have led to the proposal that its lower back and ilia were more similar to those of some Old World monkeys, such as macaques. Here, we compared three-dimensional kinematics of the pelvis and hind/lower limbs of bipedal macaques, chimpanzees and humans walking at similar dimensionless speeds to test the effects of lower back and ilia design on gait. Our results indicate that locomotor kinematics of bipedal macaques and chimpanzees are remarkably similar, with both species exhibiting greater pelvis motion and more flexed, abducted hind limbs than humans during walking. Some differences between macaques and chimpanzees in pelvis tilt and hip abduction were noted, but they were small in magnitude; larger differences were observed in ankle flexion. Our results suggest that if Pan and Homo diverged from a common ancestor whose lower back and ilia were either 'African ape-like' or more 'Old World monkey-like', at its origin, the hominin walking stride likely involved distinct (i.e. non-human-like) pelvis motion on flexed, abducted hind limbs.

Preliminary study of a robotic foot-ankle prosthesis with active alignment.

Robotic prosthetic foot-ankle prostheses typically aim to replace the lost joint with revolute joints aimed at replicating normal joint biomechanics. In this paper, a previously developed robotic ankle prosthesis with active alignment is evaluated. It uses a four-bar mechanism to inject positive power into the gait cycle while altering the kinematics of the ankle joint and pylon segment to reduce loading on the residual limb. In a single-subject biomechanics analysis, there was a 10% reduction in peak limb pressures and evidence of greater gait symmetry in ground reaction forces when active alignment was implemented compared to walking with the daily use prosthesis. These results provide preliminary evidence that an alternative lower limb prosthesis may be capable of improving gait characteristics over traditional revolute designs.

Chimpanzee super strength and human skeletal muscle evolution.

Since at least the 1920s, it has been reported that common chimpanzees (Pan troglodytes) differ from humans in being capable of exceptional feats of "super strength," both in the wild and in captive environments. A mix of anecdotal and more controlled studies provides some support for this view; however, a critical review of available data suggests that chimpanzee mass-specific muscular performance is a more modest 1.5 times greater than humans on average. Hypotheses for the muscular basis of this performance differential have included greater isometric force-generating capabilities, faster maximum shortening velocities, and/or a difference in myosin heavy chain (MHC) isoform content in chimpanzee relative to human skeletal muscle. Here, we show that chimpanzee muscle is similar to human muscle in its single-fiber contractile properties, but exhibits a much higher fraction of MHC II isoforms. Unlike humans, chimpanzee muscle is composed of ∼67% fast-twitch fibers (MHC IIa+IId). Computer simulations of species-specific whole-muscle models indicate that maximum dynamic force and power output is 1.35 times higher in a chimpanzee muscle than a human muscle of similar size. Thus, the superior mass-specific muscular performance of chimpanzees does not stem from differences in isometric force-generating capabilities or maximum shortening velocities-as has long been suggested-but rather is due in part to differences in MHC isoform content and fiber length. We propose that the hominin lineage experienced a decline in maximum dynamic force and power output during the past 7-8 million years in response to selection for repetitive, low-cost contractile behavior.