The Role of Entrainment in Human Walking: Energy Minimization in Oscillating Environments

During locomotion, humans often entrain (i.e. synchronize) their steps to external oscillations: e.g. swaying bridges, tandem walking, bouncy harnesses, vibrating treadmills, exoskeletons. Previous studies have discussed the role of nonlinear oscillators (e.g. central pattern generators) in facilitating entrainment. However, the underlying benefits of entrainment are unknown. Given substantial evidence that humans prioritize economy during locomotion, we tested whether reduced metabolic expenditure accompanies human entrainment to vertical force oscillations, where frequency and amplitude were prescribed via a custom mechatronics system during walking. Although metabolic cost was not significantly reduced during entrainment, individuals who experienced negative work from oscillations had a higher cost than those who experienced positive work, and subjects generally selected phase relationships indicating the latter. It is possible that individuals use mechanical cues to infer energy cost and inform effective gait strategies. If so, an accurate prediction may rely on the relative stability of interactions with the environment. Our results suggest that entrainment is preferred over a wide range of oscillation parameters, though not as a direct priority for minimizing metabolic cost. Instead, entrainment may act to stabilize interactions with the environment, thus increasing predictability for the effective implementation of internal models that guide energy minimization.