Paraplegic standing controlled by functional neuromuscular stimulation. II. Computer simulation studies

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Abstract
We simulated two types of body motion. First, the body position is assumed to be initially perturbed from the upright position, and all muscles are assumed inactive at the initial position. The control law developed in the preceding paper drives the body segments to the standing position. Arm movements are then applied to the body to investigate how performance is affected by an external disturbance. Simulated body motion indicated that the current output-feedback control law functions well. The body can recover upright posture from a highly flexed position, and the controller can then maintain the body near the vertical during arm movements. The simulation results showed three consistent activation patterns based on energy minimization: 1) no antagonistic muscle pairs are coactivated, 2) strong muscles are recruited before weak ones, and 3) fast muscles are recruited before slow ones. The reason for the second and third observations is that energy liberation rate depends heavily on the relative amount of muscle activation. Since the current control law requires muscles to generate specific joint torques at a prescribed time, strong muscles do not have to be activated as much as weak ones, and recruiting a fast muscle at low activation level consumes less energy than recruiting a slow one at high activation level. Although the output-feedback control law functions well according to our simulation results, the static optimization process would, in practice, take too much computational time to make it practical. Based on the consistent activation patterns found in our simulations, we therefore developed a simpler (suboptimal) activation-distribution scheme that takes much less time and still gives nearly identical performance.