Relative contribution of walking velocity and stepping frequency to the neural control of locomotion.

Stepping frequency is tightly coupled to walking velocity during natural locomotion. In a recent model, we demonstrated that walking velocity determines stride frequency, governs the active feedback control of the swing and determines the swing phase dynamics that governs foot movement. Here, we questioned whether the swing phase dynamics reflect independent effects of stride frequency and walking velocity. Foot movements were measured with a motion detection system (Optotrak) while subjects walked at 0.6-2.1 m/s on a treadmill. Stepping frequencies of 1.3-2.8 Hz were generated with pacing cues at each walking velocity. In the 'iso-velocity' condition, peak forward toe velocity during the swing phases was related to walking velocity and did not vary with alterations in stride frequency. In the 'iso-frequency' condition, in contrast, stepping frequency altered the relationship between toe acceleration and toe position in the fore-aft direction. The cycle frequency, main sequence (peak velocity vs. amplitude) relationships, and the shape of the phase-plane trajectories of the swing phases also reflected this relationship. The data were modeled by decoupling stepping frequency from walking velocity, while maintaining active feedback control dependent on frequency. The latter predicted both the dominant shape of the phase plane trajectories and the main sequence relationships. Thus, according to the model, walking velocity and stride frequency are independent central variables that control the dynamics of the swing phases and stepping. The ability to decouple stride frequency from walking velocity may help in navigating over uneven terrain or when executing curved trajectories while maintaining a constant velocity.