Common kinematic synergies of various human locomotor behaviours

Humans show a variety of locomotor behaviours in daily living, varying in locomotor modes and interaction styles with the external environment. However, how this excellent motor ability is formed, whether there are some invariants underlying various locomotor behaviours and simplifying their generation, and what factors contribute to the invariants remain unclear. Here we find three common kinematic synergies that form the six joint motions of one lower limb during walking, running, hopping, and sitting-down-standing-up (movement variance accounted for > 90%), through identifying the coordination characteristics of thirty-six lower limb motor tasks in diverse environments. This finding supports the notion that humans simplify the generation of various motor behaviours through reusing several basic motor modules, rather than developing entirely new modules for each behaviour. Moreover, a potential link is also found between these synergies and the unique biomechanical characteristics of the human musculoskeletal system (muscular-articular connective architecture and bone shape), and the patterns of inter-joint coordination are consistent with the energy-saving mechanism in locomotion by using biarticular muscles as efficient mechanical energy transducers between joints. Altogether, our work helps understand the formation mechanisms of human locomotion from a holistic viewpoint, and evokes inspirations for the development of artificial limbs imitating human motor ability.