Torque–angle relationships of human toe flexor muscles highlight their capacity for propulsion in gait

Human proficiency for bipedal locomotion relies on the structure and function of our feet, including the interplay between active muscles and passive structures acting on the toes during the propulsive phase of gait. However, our understanding of the relative contributions of these different structures remains incomplete. We aimed to determine the distinct toe-flexion torque-angle relationships of the plantar intrinsic muscles (PIMs), extrinsic muscles and passive structures, therefore offering insight into their force-generating capabilities and importance for walking and running. Torque-angle data were twice collected from nine healthy individuals (6 males, 3 females; 28±5 years) using supramaximal transcutaneous electrical stimuli applied at two tibial nerve sites to distinguish between muscle-driven and passive toe-flexion torque about the metatarsophalangeal (MTP) joint. Innervating extrinsic muscles and PIMs concurrently produced peak torques (hallux=3.05±0.70 N m, MTP angle=48.0±13.6 deg; lesser digits=3.19±0.98 N m, MTP angle=42.6±13.4 deg) exceeding by 208% (hallux) and 150% (lesser digits), respectively, those from PIM stimulation alone. Notably, MTP joint angles pertinent to gait corresponded to the ascending limb of the active torque-angle relationship, with active muscle joint torques being the dominant contributor over passive torques. The latter finding suggests that human toe flexors are well adapted to generate the MTP joint torques that are necessary for walking and running. This further supports the notion that muscles acting within the foot play an important role in the foot's mechanical function and our ability to walk and run in an upright posture.