Balance control is simplified by muscle-skeletal leg design

Different gaits with upright body configurations make human bipedal locomotion unique [1]. Upper body balance is key to human locomotion. To balance, humans may control the direction of the ground reaction force [2] and the center of pressure, either continuously or step by step. Older infants learn standing and walking upright while holding tight to stable objects [3], thereby simplifying balance by additional forces exerted at upper limbs. It is not clear how humans acquire their extraordinary balance skills. The mechanics of the segmented leg are not yet well understood but prerequisite for understanding neuro-muscular co-ordination [4]. Here we propose that design features related to balance may have led to evolutionary adaptation of leg geometry and muscle architecture.