Open-loop control of the freely-swinging paralyzed leg

An experimental model was used to study issues that are relevant to the use of electrical stimulation to help paralyzed individuals walk. Modulated stimulation sequences for the quadriceps muscles were manually selected using an iterative trial-and-error procedure to cause the knee angle to follow a specific movement pattern. Four paraplegic subjects were tested before and after an eight-week program in which the quadriceps were exercised daily with electrical simulation. It was found that 12.6+or-2.9 iterations were required to approximate the desired trajectory. The average error of the final match between the actual and desired trajectories was 2.1 degrees +or-0.7. Repeated responses were extremely consistent; the average difference between successive trails was less than 1 degrees in 81% of the trials. When the stimulation sequence was repeated every 3 s for 50 cycles, however, there was a progressive degradation in the response, even in exercised legs, that demonstrated the limitations of open-loop control.