FUNCTIONALLY COMPLEX MUSCLES OF THE CAT HINDLIMB .3. DIFFERENTIAL ACTIVATION WITHIN BICEPS-FEMORIS DURING POSTURAL PERTURBATIONS

The biceps femoris (BF) muscle is divided into three neuromuscular compartments defined by the innervation patterns of the main nerve branches (English and Weeks 1987). The goals of this study were i) to determine how different regions of the biceps femoris muscle are activated in the intact cat during a broad range of limb movements evoked by perturbations of stance posture, and ii) to determine the relationship between the anatomical compartments of biceps femoris and the functional units as defined in this task. Cats were trained to stand on a movable platform with each paw on a triaxial force plate. The animal's stance was perturbed by linear translation of the platform in each of sixteen different directions in the horizontal plane. EMG activity was recorded from eight sites across the width of the left biceps femoris muscle. During quiet stance only the anterior compartment was tonically active, presumably contributing to hip extensor torque in the maintenance of stance. During platform translation, evoked EMG activity was recorded from each electrode pair for a wide range of directions of perturbation; as direction changed progressively, the amplitude of evoked activity from any electrode pair increased to a maximum and then decreased. When the EMG amplitude was plotted in polar coordinates as a function of translation direction, the region of response formed a petal shaped area in the horizontal plane, termed the EMG tuning curve. The compartments of the BF muscle were not activated homogeneously. The tuning curve of the anterior BF compartment was similar to that of other hip extensors, and coincided with the region of postero-lateral force production by the hindlimb against the support. The tuning curve of the middle BF compartment was shifted in a counterclockwise direction from that of the anterior compartment, but overlapped extensively with it; the middle BF tuning curve was similar to that of anterior gracilis. The tuning curve of the posterior biceps compartment was rotated further counterclockwise and overlapped very. little with that of the middle BF compartment. The posterior BF was activated in a pattern similar to that of other knee flexors. The functional units of BF activation were not identical with the neuromuscular compartments defined by the main nerve branches. As direction of the perturbation changed, the region of BF that was activated moved progressively across the muscle. This progression of the active region was continuous across BFa and BFm, whereas there was a jump, or discontinuity at the border between BFm and BFp. Thus, differences in activation were observed not only across compartments, but also within compartments, and different regions of the BF muscle were activated independently during responses to postural perturbations.