Three-dimensional kinematics and dynamics of the foot during walking: a model of central control mechanisms.

The foot is a critical interface between the body and supporting surface during walking, but there is no coherent framework on which to model the dynamics of the stance and swing phases. To establish this framework, we studied the rotational and translational dynamics of foot movement in three dimensions with a motion detection system (OPTOTRAK), while subjects walked on a treadmill. Positions, velocities, and durations were normalized to leg-length and gravity. Foot position and rotation at toe-off were closely related to walking velocity. Foot pitch at toe clearance increased with walking velocity, but the medial-lateral and vertical toe positions were unaltered. Phase-plane trajectories along the fore-aft direction, i.e., plots of toe velocity versus position, were circular during the swing phases, with radii proportional to walking velocity. Peak forward, lateral, and upward velocities were linearly related to corresponding excursions, forming main sequences. A second order model predicted the changes in toe position and velocity, and the approximately hyperbolic decrements in duration as a function of walking velocity. The model indicates that the foot is controlled in an overdamped manner during the stance phase and as a feedback-controlled undamped pendulum during the swing. The data and model suggest that the state of the foot at toe-off, set by walking velocity during the stance phase, determines the dynamics of the swing phase. Thus, in addition to determining locomotion kinematics, walking velocity plays a critical role in determining the phase-plane trajectories and main sequence relationships of foot movements during the swing phases.