Hysteresis properties of movement in elbow joint of unanesthetized cats when antagonistic muscles are activated using different methods

Flexing and extending movements of the elbow joint were studied in unanesthetized cats. These movements were evoked by intracortical microstimulation and by vibration of the forepaw. After the animals were anesthetized the same movements were studied in response to direct stimulation of the antagonistic muscles. Interaction of the hysteresis effects in the antagonistic muscles under conditions of cortical-evoked movements in response to stimulation of two points of the cortex, one of which evoked flexion, and the other extension of the elbow joint, was studied using external local disturbance. Coactivation of the antagonists was shown to increase both the joint stiffness and the ambiguity of the equilibrium of the joint angle. This ambiguity was expressed in both the antagonistic actuating disturbances as well as in the change of the sequence for activating the antagonistic muscles. Comparison of the cortical-evoked movements and movements evoked by vibrating the forepaw when tested with an external load disturbance showed that with intracortical microstimulation the myotatic reflexes in the activated muscle are suppressed, but when vibration is used they are well defined in both antagonistic muscles. At the same time, in spite of the significantly different pathways for activating spinal neurons, the ambiguity effects displayed when determining the equilibrium of the joint angle were similar in both cases.