Neuromechanical adaptations of foot function when hopping on a damped surface.

To preserve motion, humans must adopt actuator-like dynamics to replace energy that is dissipated during contact with damped surfaces. Our ankle plantar flexors are credited as the primary source of work generation. Our feet and their intrinsic foot muscles also appear to be an important source of generative work, but their contributions to restoring energy to the body remain unclear. Here, we test the hypothesis that our feet help to replace work dissipated by a damped surface through controlled activation of the intrinsic foot muscles. We used custom-built platforms to provide both elastic and damped surfaces and asked participants to perform a bilateral hopping protocol on each. We recorded foot motion and ground reaction forces, alongside muscle activation, using intramuscular electromyography from flexor digitorum brevis, abductor hallucis, soleus, and tibialis anterior. Hopping in the Damped condition resulted in significantly greater positive work and contact-phase muscle activation compared with the Elastic condition. The foot contributed 25% of the positive work performed about the ankle, highlighting the importance of the foot when humans adapt to different surfaces.NEW & NOTEWORTHY Adaptable foot mechanics play an important role in how we adjust to elastic surfaces. However, natural substrates are rarely perfectly elastic and dissipate energy. Here, we highlight the important role of the foot and intrinsic foot muscles in contributing to replacing dissipated work on damped surfaces and uncover an important energy-saving mechanism that may be exploited by the designers of footwear and other wearable devices.

Walking with increasing acceleration is achieved by tuning ankle torque onset timing and rate of torque development.

Understanding the mechanics of torque production about the ankle during accelerative gait is key to designing effective clinical and rehabilitation practices, along with developing functional robotics and wearable assistive technologies. We aimed to explore how torque and work about the ankle is produced as walking acceleration increases from 0 to 100% maximal acceleration. We hypothesized that as acceleration increased, greater work about the ankle would not be solely due to ramping up plantar flexor torque, and instead would be a product of adjustments to relative timing of ankle torque and angular displacement. Fifteen healthy participants performed walking without acceleration (constant speed), as well as low, moderate and maximal accelerations, while motion capture and ground reaction force data were recorded. We employed vector coding in a novel application to overcome limitations of previously employed evaluation methods. As walking acceleration increased, there was reduced negative work and increased positive work about the ankle. Furthermore, early stance dorsiflexion had reducing plantar flexor torque due to delayed plantar flexor torque onset as acceleration increased, while mid-stance ankle plantar flexor torque was substantially increased with minimal ankle dorsiflexion, irrespective of acceleration magnitude. Assistive devices need to account for these changes during accelerative walking to facilitate functional gait.

Neuromechanical adaptations of foot function to changes in surface stiffness during hopping.

Humans choose work-minimizing movement strategies when interacting with compliant surfaces. Our ankles are credited with stiffening our lower limbs and maintaining the excursion of our body's center of mass on a range of surface stiffnesses. We may also be able to stiffen our feet through an active contribution from our plantar intrinsic muscles (PIMs) on such surfaces. However, traditional modeling of the ankle joint has masked this contribution. We compared foot and ankle mechanics and muscle activation on low, medium, and high stiffness surfaces during bilateral hopping using a traditional and anatomical ankle model. The traditional ankle model overestimated work and underestimated stiffness compared with the anatomical model. Hopping on a low stiffness surface resulted in less longitudinal arch compression with respect to the high stiffness surface. However, because midfoot torque was also reduced, midfoot stiffness remained unchanged. We observed lower activation of the PIMs, soleus, and tibialis anterior on the low and medium stiffness conditions, which paralleled the pattern we saw in the work performed by the foot and ankle. Rather than performing unnecessary work, participants altered their landing posture to harness the energy stored by the sprung surface in the low and medium conditions. These findings highlight our preference to minimize mechanical work when transitioning to compliant surfaces and highlight the importance of considering the foot as an active, multiarticular, part of the human leg.NEW & NOTEWORTHY When seeking to understand how humans adapt their movement to changes in substrate, the role of the human foot has been neglected. Using multi-segment foot modeling, we highlight the importance of adaptable foot mechanics in adjusting to surfaces of different compliance. We also show, via electromyography, that the adaptations are under active muscular control.

Foot stiffening during the push-off phase of human walking is linked to active muscle contraction, and not the windlass mechanism.

The rigidity of the human foot is often described as a feature of our evolution for upright walking and is bolstered by a thick plantar aponeurosis that connects the heel to the toes. Previous descriptions of human foot function consider stretch of the plantar aponeurosis via toe extension (windlass mechanism) to stiffen the foot as it is levered against the ground for push-off during walking. In this study, we applied controlled loading to human feet in vivo, and studied foot function during the push-off phase of walking, with the aim of carefully testing how the foot is tensioned during contact with the ground. Both experimental paradigms revealed that plantar aponeurosis strain via the 'windlass mechanism' could not explain the tensioning and stiffening of the foot that is observed with increased foot-ground contact forces and push-off effort. Instead, electromyographic recordings suggested that active contractions of ankle plantar flexors provide the source of tension in the plantar aponeurosis. Furthermore, plantar intrinsic foot muscles were also contributing to the developed tension along the plantar aspect of the foot. We conclude that active muscular contraction, not the passive windlass mechanism, is the foot's primary source of rigidity for push-off against the ground during bipedal walking.