During human evolution, the foot underwent strong selective pressures. The pronounced medial arch of the human foot is considered a unique feature in the evolution of habitual bipedalism. A high medial arch in fossil hominins indicates the adaptations for the foot’s levering ability in push-off and mobility-enabled spring-like function.
Study: Mobility of the human foot’s medial arch helps enable upright bipedal locomotion. Image Credit: Eak.Temwanich/Shutterstock.com
Theoretically, the curved longitudinal arch acts as a nearly rigid lever for push-off, and the mobility of other joints enables the spring-like recoil of the arch-spanning tissues. There is a gap in research regarding how the arch recoil interacts with its lever function to promote movement.
Background
The arch in the human foot recoils in propulsion, and this phenomenon is known as plantarflexes. Notably, the midfoot joints of chimpanzees initially dorsiflex after heel lift. This is an interesting observation, as chimpanzees are the closest living relatives of humans. Interestingly, all non-human primates experience similar midfoot breaks.
The hindfoot lifts of non-primates form a “reverse arch,” where the midfoot remains below the plane connecting the heel and the toes. The fulcrum of the foot lever functions as midfoot instead of metatarsophalangeal joints. The shortening of the foot lever reduces mechanical benefits. Although midfoot mobility helps in climbing, it reduces push-off efficiency when walking bipedally.
The stability of the longitudinal arches in humans is manifested through robust plantar fascia, plantar ligaments, and the elevated arch structure. During heel lift, the midfoot above the plane remains connected at the heels and the toes. Although some humans experience marginal midfoot breaks, they are not as prominent as those of other primates. Therefore, the metatarsophalangeal joints act as the fulcrum of the human foot-lever providing humans the advantage of additional leverage compared to primates with a midfoot break.
It is still unclear how human arch plantarflexion affects its function as a lever. This understanding would help better understand the evolutionary divergence of humans from other primates. Scientists hypothesized that the recoil of the arch-spanning tissues provides the required mechanical work, which could be otherwise produced through a metabolic cost by muscles. Another hypothesis is that the arch contributes to the propulsion of the center of mass (COM).
The arch also contributes to running. Here, the leg-spring extends in propulsion by lifting the apex of the foot by approximately 10–15 mm. However, the arch recoil is small compared to COM excursion, which is closer to 80mm. During gait, the ankle posture is affected by static arch type. Compared to a low arch, a high arch aid in push-off postures. If the arch is not adequately high, the ankle may exhibit a reduced range of motion.
About the study
A recent Frontiers in Bioengineering and Biotechnology study hypothesized that the arch recoils to propel the COM, which affects the ankle posture. This hypothesis was tested via in vivo experiments using individual foot bone motion derived from biplanar videoradiography.
Initially, the timing of COM propulsion and arch recoil was compared. COM propulsion and arch recoil timing were predicted to be synchronized during walking and running. A model that mathematically restricted arch recoil was developed but preserved levering about the metatarsophalangeal joint (MTPJ).
The talus, as the apex of the arch, was expected to be higher at push-off in an arch that recoils compared to an arch that does not recoil much. These observations in an evolutionary context are based on data concerning the bipedal walking of chimpanzees. Previous studies have shown that chimpanzees have a midfoot that plantarflexes significantly less compared to humans at heel-lift.
The kinematic parameters, such as ankle plantarflexion and ground contact time, that affect ankle propulsion might be influenced by arch plantarflexion mobility. To understand whether the arch shape and dynamic motion influence the global position of the ankle during the propulsive phase of gait, participants with wide-ranging arch plantarflexion mobility and foot types were examined.
Study findings
The current study observed that neither global talar position nor ankle plantarflexion at push-off was associated with static arch height. This finding indicates that the ability of humans to locomote bipedally, particularly during push-off, is robustly linked to the plantarflexion mobility of the arch than its posture.
This study underscores that arch plantarflexion mobility is the key piece of the evolutionary puzzle. Compared to other primates, a functional link between arch structure and arch recoil in propulsion may aid bipedal locomotion in humans. The authors proposed two non-mutually exclusive reasons to explain the prominent medial arch in humans.
Firstly, the arch inherently orients the talus’ superior articular surface upright, such that even in the absence of arch recoil, it maintains an upright position compared to the non-arched feet of chimpanzees.
Secondly, the human arch-spanning tissues exhibit longer moment arms in the midfoot joints to develop more arch recoil than in chimpanzees. In chimpanzees, due to the lack of an arch, the midfoot exhibits less capacity to recoil and reorient the talus to remain upright.
Conclusions
Considering the overall experimental findings, the authors concluded that the evolution of a structural arch to function in tandem with the recoiling arch aids in upright bipedal locomotion in humans. More research is required to validate this observation.
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