Preventing ankle twists: the role of the human foot arch in enhancing lateral dynamic stability

Maintaining dynamic stability poses a significant challenge during the evolution from quadrupedal to bipedal locomotion in humans. Unlike our ancestors and primate relatives with flat feet, humans have evolved a unique foot arch structure which is known for providing foot stiffness, thereby facilitating activities like running and jumping. Nevertheless, its impact on dynamic stability, particularly lateral stability and ankle twists, remains uncertain. We show that modern humans possess an optimized foot arch height, which provides enhanced lateral dynamic stability when compared to individuals with either flat or high-arch feet. To quantify this lateral stability, we introduce a novel dimensionless stability indicator for single-legged landings, utilizing a nonlinear multi-Degree-of-Freedom mathematical model and an energy-based approach. The typical human foot arch adapts optimally to combing factors such as landing speed, inclination angle, and external disturbances. By contrast, high-arch foot is inept in handling inclinations; whereas flat foot resists inclination but lacks stability at high landing speeds, making it more suitable for foraging primates rather than human hunters. The generalizable nonlinear mathematical model and precise stability indicator for the lower limb have the potential to assist in identifying individuals at a high risk of ankle injuries, informing the design of footwear and robotic feet, and inspiring further research into postural stability.