Mechanical perturbations make it easier to walk... if you walk the right way

A control optimization model predicts that an oscillating force perturbation can substantially influence the energetic cost of walking. Cost is decreased when peak ground reaction forces are aligned with positive perturbation forces (in-phase) but increased with the opposite alignment (anti-phase). To test this empirically, we built a novel mechatronics oscillator system to apply sinusoidal vertical force perturbations to subjects walking on a treadmill. Given the freedom to adapt gait in real time, subjects consistently converged on the optimal interaction (in-phase), with force amplitudes ≥10% their own body weight. This indicates that individuals are sensitive to reduced cost opportunities available in their environment.