Modular Control of Kinematics in Prosthetic Gait: Low-Dimensional Description Based on the Planar Covariation Law

Amputation of a lower limb implies a re-organization in the strategies used to reach a stable walking pattern. The planar covariation law of elevation angles is a well-defined low-dimensional description of the kinematics of movement; according to this law, thigh, shank and foot elevation angle co-vary on a plane, defining a typical gait loop. However, a robust biomechanical interpretation of the outcomes of its related analysis is still missing. In this work, we tested the planar covariation law on a group of 14 trans-femoral amputees, comparing the results with the ones related to 12 healthy people. Moreover, by adopting a common covariance plane for all the subjects, we checked whether the projection of the original elevation angles on this plane is able to yield biomechanically meaningful information on the control of prosthetic gait. A common plane was able to describe the coordination of lower limb elevation angles in all subjects; on this plane, most of the differences among populations were identified on the time course of one principal component.