A KINEMATIC AND KINETIC-ANALYSIS OF LOCOMOTION DURING VOLUNTARY GAIT MODIFICATION IN THE CAT

As part of a study to characterize the postural reactions that occur d uring voluntary gait modification, we examined the kinematic, electrom yographic (EMG), and kinetic responses that occurred when cats stepped over an obstacle placed in their path. Analyses of the kinematics as each of the forelimbs stepped over the obstacle showed that changes in joint angles were most pronounced at the elbow of the first (lead) li mb, and at the shoulder of the second (trailing) limb. In the hindlimb s, there was a pronounced change in the knee joint angle in both the l eading and trailing limbs. Examination of the horizontal and vertical velocities of the tip of the forepaw suggests that the movements can b e divided into two phases: one in which the limb is rapidly lifted abo ve and over the obstacle, and a slower one during which the limb is ca refully repositioned on the floor. On the basis of the velocity profil es, we suggest that the repositioning of the paw on the support surfac e is more critically controlled for the forelimb than for the hindlimb . Analysis of the changes in the ground reaction forces in the support ing limbs during these gait modifications showed that there were two m ajor increases in vertical reaction force. One of these occurred as th e two forelimbs were straddling the obstacle, the other when the two h indlimbs were straddling it. There was also a net increase in the ante roposterior force that resulted in a small increase in propulsion as t he cat stepped over the obstacle. Each change in the vertical ground r eaction force was paralleled by a similar change in the amplitude of t he EMG recorded from the respective extensor muscles. An analysis of t he vertical displacement of the scapula and of the pelvis showed that there was a slight increase in the height of the scapula in the suppor t limb just prior to and during the swing phase of the trailing foreli mb, and a more pronounced and progressive change in the height of the pelvis prior to and during the passage of both hindlimbs over the obst acle. We suggest that the increases in vertical ground reaction force raise the height of the body sufficiently to allow, respectively, pass age of the trail forelimb and each of the hindlimbs over the obstacle. The results are discussed with respect to both the biomechanical chan ges underlying these gait modifications and the neuronal mechanisms im plicated in their control.