Foot-stiffening socks reduce the metabolic cost of walking

Greater foot longitudinal bending stiffness theoretically facilitates bipedal walking by reducing whole-body metabolic rate, yet empirical evidence is elusive. Here, we tested this theory by studying human walking with and without foot-stiffening socks. These custom socks applied more pressure to the user's forefoot than commercially available socks (p < 0.001), which in turn increased medial longitudinal arch stiffness through cross-axis coupling (p = 0.034). Walking with foot-stiffening socks reduced whole-body metabolic rate 2% versus the use of commercial socks (p = 0.008). Biomechanically, stiffening participants' feet increased ankle gear ratio (peak: p = 0.017; average: p = 0.045) and reduced the ankle's average angular velocity during the stance-phase of walking (p = 0.018). In congruence, leg muscle fascicle shortening velocities decreased 15–20% (soleus: p = 0.023; medial gastrocnemius: p = 0.005), contributing to the 9–12% reduction in estimated leg active muscle volume (model1: p = 0.015; model2: p = 0.142). Therefore, foot-stiffening socks make walking less effortful by eliciting more economical leg muscle contractions, supporting the notion that the evolved stiffening of human feet facilitate terrestrial bipedal locomotion.