OPEN-LOOP TRACKING PERFORMANCE OF A LIMB JOINT CONTROLLED BY RANDOM, PERIODIC, AND ABRUPT ELECTRICAL-STIMULATION INPUTS TO THE ANTAGONIST MUSCLE PAIR

The ability of the cat's ankle joint to track various input signals wh en controlled by electrically elicited motor unit recruitment, firing rate and antagonist muscle coactivation was examined. Pseudo-random, s inusoidal and staircase signals were used to control the soleus and ti bialis anterior muscles isometrically and with a 250-g pendulum. Track ing was evaluated through cross correlation for pseudo-random and sinu soidal signals, and by rise time and steady-state error in step signal s, Better tracking was obtained in isometric conditions than in load-m oving conditions. Pseudo-random signals resulted in 250-ms delay betwe en input and isometric torque output. For load-moving conditions, 340- ms and 400-ms delay in torque and angle were obtained. For sinusoids, delays decreased from 240 ms at 0.5 Hz, to 140 ms at 2 Hz in isometric conditions. Time delays for angle were between 300 and 400 ms, decrea sing as frequency increased. Poor cross correlation was found for torq ue in load-moving conditions, because of pendulum nonlinear dynamics. Step size was not uniform in staircase trials, with steady-state error s between 9% and 39%, and rise times between 200 and 1000 ms. It is co ncluded that open-loop joint control results in poor tracking, presuma bly because it is devoid of feedback mechanisms.