Sensitivity and range of human gait synchronization with a machine oscillator system

Abstract Humans sometimes synchronize their steps to mechanical oscillations in the environment (e.g., when walking on a swaying bridge or with a wearable robot). Previous studies have discovered discrete frequencies and/or amplitudes where individuals spontaneously synchronize to external oscillations, but these parameters are often chosen arbitrarily or for convenience of a successful experiment and are sparsely sampled due to time constraints on subject availability. As a result, the parameter space under which human gait synchronization occurs is still relatively underexplored. Here we systematically measure synchronization over a broad range of parameters in machine oscillations, applied vertically to the body center of mass during walking. Two complementary experiments were utilized to characterize the amplitudes (i.e., sensitivity) and frequencies (i.e., range) where gait synchronization occurs. We found that individuals were more sensitive to oscillation frequencies near their baseline step frequency and synchronized to a broader range of frequencies with larger oscillation amplitudes. Subjects also had greater sensitivity and range when they synchronized with oscillation frequencies below (rather than above) their baseline step frequencies. The results of this study provide a comprehensive mapping of parameters where synchronization occurs and could inform the design of exoskeletons, rehabilitation devices and other gait-assistive technologies.