Joint-level kinetic redundancy is exploited to control limb-level forces during human hopping

Compensatory mechanisms can take advantage of neuromechanical redundancy to meet global task goals in spite of local injuries or perturbations. We hypothesized that joint-level kinetic redundancy is also exploited during intact, unperturbed human locomotion to accomplish limb-level force goals. The limb-level force goals of hopping in place at a constant frequency are minimizing cycle-to-cycle variance of vertical ground reaction force and varying horizontal (fore-aft) ground reaction force to make backward and forward corrections in position from cycle to cycle. Uncontrolled Manifold analysis of joint torque variance showed that hoppers exploited redundancy to minimize vertical force variance at landing, mid-stance, and takeoff, and to vary horizontal force at landing and takeoff. Timing fluctuations, however, increased vertical force variance. We conclude that joint torque variance is not random noise, but has functional relevance and is purposefully structured to meet specific locomotor goals.