Analysis of the human and ape foot during bipedal standing with implications for the evolution of the foot

The ratio of the power arm (the distance from the heel to the talocrural joint) to the load arm (that from the talocrural joint to the distal head of the metatarsals), or RPL, differs markedly between the human and ape foot. The arches are relatively higher in the human foot in comparison with those in apes. This study evaluates the effect of these two differences on biomechanical effectiveness during bipedal standing, estimating the forces acting across the talocrural and tarsometatarsal joints, and attempts to identify which type of foot is optimal for bipedal standing. A simple model of the foot musculoskeletal system was built to represent the geometric and force relationships in the foot during bipedal standing, and measurements for a variety of human and ape feet applied. The results show that: (1) an RPL of around 40% (as is the case in the human foot) minimizes required muscle force at the talocrural joint; (2) the presence of an high arch in the human foot reduces forces in the plantar musculature and aponeurosis; and (3) the human foot has a lower total of force in joints and muscles than do the ape feet. These results indicate that the proportions of the human foot, and the height of the medial arch are indeed better optimized for bipedal standing than those of apes, further suggesting that their current state is to some extent the product of positive selection for enhanced bipedal standing during the evolution of the foot.