Transient lateral stability of the lower limb: role of the foot arch in preventing ankle twists

Abstract Maintaining stability represents a substantial challenge as humans undergo the transition from quadrupedal to bipedal locomotion. Unlike other primates with flat feet, humans evolved a unique foot arch structure that is known to enable running and jumping by providing foot flexibility. However, its contribution to stability, especially lateral dynamic (transient) stability involving potential ankle twists, remains uncertain. Here we show that modern humans exhibit an optimized foot arch height, which offers superior lateral stability compared to those with flat or high arches during locomotion. We introduced a novel, straightforward dimensionless indicator for lower limb stability, named transient postural stability, representing the maximum perturbation that can be resisted without losing stability. This indicator is derived from a nonlinear multi-body model that incorporates actual unstable scenarios and is assessed using an energy landscape-based approach. Basins of attraction validate the indicator and provide detailed information on the system’s capacity to withstand multiple disturbances. The normal human foot arch demonstrates optimal adaptability when confronted with a combination of factors, including landing speed, inclination angle, and external disturbances. By contrast, high-arch foot is inept in handling inclinations; whereas flat foot offers resistance to inclination but lacks stability at high landing speeds. The research presents an energy-based assessment of postural stability in the lower limb, serving as a foundational framework for further investigations.