The human foot is characterized by a pronounced longitudinal arch (LA) that compresses and recoils in response to external load during locomotion, allowing for storage and return of elastic energy within the passive structures of the arch and contributing to metabolic energy savings. Here, we examine the potential for active muscular contribution to the biomechanics of arch deformation and recoil. We test the hypotheses that activation of the three largest plantar intrinsic foot muscles, abductor hallucis, flexor digitorum and quadratus plantae is associated with muscle stretch in response to external load on the foot and that activation of these muscles (via electrical stimulation) will generate sufficient force to counter the deformation of LA caused by the external load. We found that recruitment of the intrinsic foot muscles increased with increasing load, beyond specific load thresholds. Interestingly, LA deformation and muscle stretch plateaued towards the maximum load of 150% body weight, when muscle activity was greatest. Electrical stimulation of the plantar intrinsic muscles countered the deformation that occurred owing to the application of external load by reducing the length and increasing the height of the LA. These findings demonstrate that these muscles have the capacity to control foot posture and LA stiffness and may provide a buttressing effect during foot loading. This active arch stiffening mechanism may have important implications for how forces are transmitted during locomotion and postural activities as well as consequences for metabolic energy saving.

The longitudinal arch (LA) of the human foot compresses and recoils in response to being cyclically loaded. This has typically been considered a passive process, however, it has recently been shown that the plantar intrinsic foot muscles have the capacity to actively assist in controlling LA motion. Here we tested the hypothesis that intrinsic foot muscles, abductor hallucis (AH), flexor digitorum brevis (FDB) and quadratus plantae (QP), actively lengthen and shorten during the stance phase of gait in response to loading of the foot. Nine participants walked at 1.25 m s 21 and ran at 2.78 and 3.89 m s 21 on a force-instrumented treadmill while foot and ankle kinematics were recorded according to a multisegment foot model. Muscle-tendon unit (MTU) lengths, determined from the foot kinematics, and intramuscular electromyography (EMG) signals were recorded from AH, FDB and QP. Peak EMG amplitude was determined during the stance phase for each participant at each gait velocity. All muscles underwent a process of slow active lengthening during LA compression, followed by a rapid shortening as the arch recoiled during the propulsive phase. Changes in MTU length and peak EMG increased significantly with increasing gait velocity for all muscles. This is the first in vivo evidence that the plantar intrinsic foot muscles function in parallel to the plantar aponeurosis, actively regulating the stiffness of the foot in response to the magnitude of forces encountered during locomotion. These muscles may therefore contribute to power absorption and generation at the foot, limit strain on the plantar aponeurosis and facilitate efficient foot ground force transmission.

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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.

The function of the human foot is described dichotomously as a compliant structure during mid-stance and a stiff lever during push-off. The arch-spring and the windlass mechanisms, respectively, describe each of these behaviours; however, their interaction has not been quantified to date. We hypothesized that by engaging the windlass mechanism with metatarsophalangeal joint (MTPJ) dorsiflexion, we would observe stiffening of the arch and reduced energy absorption and dissipation during dynamic compressions of the foot. Using a custom apparatus, the MTPJ angle was fixed at 30 degrees of plantarflexion, neutral or 30 degrees of dorsiflexion for nine participants, with the shank positioned similarly to the end of mid-stance. The arch was compressed at two speeds, with the faster speed comparable to walking around 1.5 m s Six cameras captured the compression and elongation of the arch, along with other kinematic variables, synchronously with the ground reaction force. Combining these measures, we computed the energy absorbed, returned and dissipated in the arch. Contrary to our hypothesis, when the windlass mechanism was engaged, the arch elongated more, and absorbed and dissipated more energy than when it was not engaged. This engagement of the windlass altered the rotational axis of the mid-foot, which probably oriented the arch-spanning structures closer to their resting length, increasing their compliance. This study provides novel evidence for an interplay between the windlass and arch-spring mechanisms that aids in regulation of energy storage within the foot.