Human Gait Entrainment to Periodic Neuro-Mechanical Perturbations: An Emerging Approach to Understanding Locomotion

Human gait is a highly complex, rhythmic behavior which is known to synchronize to external rhythms. Entrainment is a specific form of synchronization wherein the frequency of a nonlinear limit-cycle oscillator converges to that of an external periodic input through mechanical coupling. Human gait resembles a limit-cycle oscillator and has exhibited entrainment to various periodic stimuli including neuro-mechanical perturbations. Understanding the intricacies of gait entrainment contributes to our understanding of human motor control and could lead to well-informed technologies for gait rehabilitation. The purpose of this narrative review was to consolidate our current understanding of gait entrainment with neuro-mechanical perturbations and to identify impactful directions for future research. The metrics used to measure/evaluate entrainment differed between studies, though most assessed entrainment across several strides. Entrainment was sensitive to perturbation magnitude, the difference in frequencies, and their interaction. Gait tended to synchronize with the perturbation at a phase which maximized the net mechanical work of the perturbation. Future work should investigate enhanced coupling models, entrainment in different modalities of gait, and with clinical populations. Gait entrainment may help us better understand locomotion and reveal clinical benefits of wearable robots that extend beyond reducing an individual's effort or energy expenditure.