KINEMATIC DETERMINANTS OF HUMAN LOCOMOTION

1. The aim of this study was to find kinematic patterns that are invar iant across the normal range of locomotion speeds. Subjects walked at different, freely chosen speeds ranging from 0.9 to 2.1 m s(-1), while motion and ground reaction forces on the right side of the body were recorded in three-dimensional space. 2. The time course of the anatomi cal angles of flexion-extension at the hip and ankle was variable not only across subjects, but even from trial to trial in the same subject . By contrast, the time course of the changes in the angles of elevati on of each limb segment (pelvis, thigh, shank and foot) relative to th e vertical was stereotyped across subjects. 3. To compare the waveform s across speeds, data were scaled in time relative to gait cycle durat ion. The pattern of ground reaction forces was highly speed dependent. Several distinct families of curves could be recognized in the flexio n-extension angles at the hip and ankle. Instead, the waveforms of glo bal length and elevation of the limb, elevation angles of all limb seg ments and flexion-extension at the knee were invariant with speed. 4. When gait trajectories at all speeds are plotted in the position space defined by the elevation angles of the limb segments, they describe r egular loops on a plane. The statistical characteristics of these angu lar covariations were quantified by means of principal component analy sis. The first two principal components accounted together for > 99% o f the total experimental variance, and were quantitatively comparable in all subjects. 5. This constraint of planar covariation of the eleva tion angles is closely reminiscent of that previously described for th e control of posture. The existence of laws of intersegmental co-ordin ation, common to the control of posture and locomotion, presumably ass ures the maintenance of dynamic equilibrium during forward progression , and the anticipatory adaptation to potentially destabilizing factors by means of co-ordinated kinematic synergies of the whole body.