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 21) and running (2.0, 2.25, 2.75, 3.25 m s 21 ) 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 ¼ 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 21 and shed light on a potential mechanism behind the walk -run transition.

Humans walk and run over a wide range of speeds with remarkable efficiency. For steady locomotion, moving at different speeds requires the muscle-tendon units of the leg to modulate the amount of mechanical power the limb absorbs and outputs in each step. How individual muscles adapt their behavior to modulate limb power output has been examined using computer simulation and animal models, but has not been studied in vivo in humans. In this study, we used a combination of ultrasound imaging and motion analysis to examine how medial gastrocnemius (MG) muscletendon unit behavior is adjusted to meet the varying mechanical demands of different locomotor speeds during walking and running in humans. The results highlighted key differences in MG fascicle-shortening velocity with both locomotor speed and gait. Fascicle-shortening velocity at the time of peak muscle force production increased with walking speed, impairing the ability of the muscle to produce high peak forces. Switching to a running gait at 2.0 m·s −1 caused fascicle shortening at the time of peak force production to shift to much slower velocities. This velocity shift facilitated a large increase in peak muscle force and an increase in MG power output. MG fascicle velocity may be a key factor that limits the speeds humans choose to walk at, and may explain the transition from walking to running. This finding is consistent with previous modeling studies.muscle mechanics | biomechanics | preferred transition speed A nkle plantar-flexor muscles are a vital source of mechanical power for human locomotion (1, 2). During walking, plantar-flexor muscles provide body weight support, contribute to propulsion, and accelerate the limb into swing (3). In running, the ankle acts in a spring-like manner, absorbing energy in plantarflexor muscle-tendon units during early stance and providing energy to accelerate the body in late stance (1). The mechanical work required to produce whole-body movement during walking and running varies between gaits and across speeds (4, 5). Thus, the plantar-flexors may need to adjust their mechanical work output with gait and speed to meet the changing demands on their contribution to total mechanical work.A recent experimental study in humans used an inverse-dynamics approach to examine how the mechanical power outputs of muscles acting at the hip, knee, and ankle joint were modulated for walking and running at a range of speeds (6). It was found that positive power output at the ankle, in conjunction with the knee and hip, increased with walking speed. Also, Hansen et al. (7) showed that at walking speeds above those preferred, the net positive work done at the ankle increased. When switching from walking to running gait, the relative contribution of ankle positive power output to total positive power output also increased (6). It was inferred from these data that plantar-flexor muscle mechanics were adjusted to accommodate faster walking speeds and then again with the switch to running gait. If this is truly the case, then it may hav...

1Background --Dynamic measurements of human muscle fascicle length from sequences of B--mode 2 ultrasound images have become increasingly prevalent in biomedical research. Manual digitisation of these 3 images is time consuming and algorithms for automating the process have been developed. Here we 4 present a freely available software implementation of a previously validated algorithm for semi--automated 5 tracking of muscle fascicle length in dynamic ultrasound image recordings, "UltraTrack". Methods --6UltraTrack implements an affine extension to an optic flow algorithm to track movement of the muscle 7 fascicle end--points throughout dynamically recorded sequences of images. The underlying algorithm has 8 been previously described and its reliability tested, but here we present the software implementation with 9 features for: tracking multiple fascicles in multiple muscles simultaneously; correcting temporal drift in 10 measurements; manually adjusting tracking results; saving and re--loading of tracking results and loading a 11 range of file formats. Results --Two example runs of the software are presented detailing the tracking of 12 fascicles from several lower limb muscles during a squatting and walking activity. Conclusion --We have 13 presented a software implementation of a validated fascicle--tracking algorithm and made the source code 14 and standalone versions freely available for download. 15

SignificanceHuman feet have evolved uniquely among primates, losing an opposable first digit in favor of a pronounced arch to enhance our ability to walk and run with an upright posture. Recent work suggests that muscles within our feet are key to how the foot functions during bipedal walking and running. Here we show direct evidence for the significance of these foot muscles in supporting the mechanical performance of the human foot. Contrary to expectations, the intrinsic foot muscles contribute minimally to supporting the arch of the foot during walking and running. However, these muscles do influence our ability to produce forward propulsion from one stride into the next, highlighting their role in bipedal locomotion.