The adaptive function of the human ankle joint complex during walking on uneven terrains with implications for hominin locomotion

Hominins evolved in a vast diversity of natural environments with terrains that pose different biomechanical challenges, including uneven surfaces that can impact balance control during bipedal walking. Previous experimental research has identified an ‘ankle strategy’ for maintaining balance, wherein motion at the human ankle joint complex (AJC; the subtalar and talocrural joints) is used to help position the foot under the body. However, the role of the ankle strategy during uneven surface walking is currently unknown, and elucidating this role could help us understand the evolution of the hominin AJC and its potential adaptive function in bipedal walking on natural terrains. Here, we collected three-dimensional kinematic, kinetic, and electromyographic data from 17 human participants who walked barefoot on a flat, even surface and on an uneven surface consisting of tiered blocks of ethylene-vinyl acetate foam, with a repeating 0.8–2.4 cm height variation. We developed linear mixed-effect models, incorporating participant identity as a random effect and walking surface condition as a fixed effect. Type 3 analyses of variance were employed to evaluate differences across surface conditions. On the uneven surface, participants averaged 38% and 28% greater AJC frontal plane ranges of motion and joint moments, respectively, during the first half of the stance phase (p < 0.001) and had greater coactivation between the fibularis longus and tibialis anterior muscles during single-limb support (p < 0.02). These results suggest that AJC mobility is critical for balance on uneven surfaces, and therefore, hominins may have experienced selective pressure to maintain some of the joint compliance of more arboreally adapted ancestors. However, this retained mobility comes with the trade-off of susceptibility to joint injury, meaning that hominins must rely on muscles like the fibularis longus to stabilize the ankle when walking on natural terrains.