KINEMATIC COORDINATION IN HUMAN GAIT - RELATION TO MECHANICAL ENERGY-COST

Twenty-four subjects walked at different, freely chosen speeds (V) ran ging from 0.4 to 2.6 m s(-1), while the motion and the ground reaction forces were recorded in three-dimensional space. We considered the ti me course of the changes of the angles of elevation of the trunk, pelv is, thigh, shank, and foot in the sagittal plane. These angles specify the orientation of each segment with respect to the vertical and to t he direction of forward progression. The changes of the trunk and pelv is angles are of limited amplitude and reflect the dynamics of both ri ght and left lower limbs. The changes of the thigh, shank, and foot el evation are ample, and they are coupled tightly among each other. When these angles are plotted one versus the others, they describe regular loops constrained on a plane. The plane of angular covariation rotate s, slightly but systematically, along the long axis of the gait loop w ith increasing V. The rotation, quantified by the change of the direct ion cosine of the normal to the plane with the thigh axis (u(3t)), is related to a progressive phase shift between the foot elevation and th e shank elevation with increasing V. As a next step in the analysis, w e computed the mass-specific mean absolute power (P-u) to obtain a glo bal estimate of the rate at which mechanical work is performed during the gait cycle. When plotted on logarithmic coordinates, P-u increases linearly with V. The slope of this relationship varies considerably a cross subjects, spanning a threefold range. We found that, at any give n V > 1 m s(-1), the value of the plane orientation (u(3t)) is correla ted with the corresponding value of the net mechanical power (P-u). On the average, the progressive rotation of the plane with increasing Vi s associated with a reduction of the increment of P-u that would occur if u(3t) remained constant at the value characteristic of low V. The specific orientation of the plane at any given speed is not the same i n all subjects, but there is an orderly shift of the plane orientation that correlates with the net power expended by each subject. In gener al, smaller values of u(3t) tend to be associated with smaller values of P-u and vice versa. We conclude that the parametric tuning of the p lane of angular covariation is a reliable predictor of the mechanical energy expenditure of each subject and could be used by the nervous sy stem for limiting the overall energy expenditure.