Shifting gears to power human walking.

The musculoskeletal system features an abundance of functionally similar muscles, yet the performance benefits of this apparent redundancy remain unclear. Here, we show that muscles with similar function, but distinct moment arms, facilitate a dynamic gear-shifting mechanism in walking. Ankle power generation is shown to transition from large (triceps surae) to small (posterior tibialis, peroneals, and extrinsic toe flexors) moment arm muscles as joint velocity increases during pushoff. Computational models reveal that the sequential power phasing emerges naturally from muscle-tendon dynamics given the underlying differences in muscle geometry and architecture. Tendon kinetic measures during walking align with the model predictions and reveal that small plantarflexors contribute substantially more to powering human walking than previously recognized. These findings provide insights into the functional role of muscle redundancy, and suggest a fundamental principle governing the coordinated recruitment of synergistic muscles in locomotion.