Closed-loop control of muscle length through motor unit recruitment in load-moving conditions

Neuroprostheses aimed at restoring lost movement in the limbs of spinal cor d injured individuals are being developed in this laboratory. As part of th is program, we have designed a digital proportional integral-derivative con troller integrated with a stimulation system which effects recruitment of m otor units according to the size principle. This system is intended to cont rol muscle length while shortening against fixed loads. Feline sciatic nerv es were exposed and stimulated with ramp, triangular, sinusoidal, staircase and random signals as test inputs. Changes in muscle length and effective time delay under different conditions were measured and analyzed. Differenc es of tracking quality between open- and closed-loop conditions were examin ed through analysis of variance as well as the differences between small (2 50 g) and large (1 kg) loads. The results showed that parameters used to co mpare muscle length output to the input signals were dramatically improved in the closed-loop trials as compared to the open-loop condition. Mean squa red correlation coefficients between input and output signals for ramp sign als increased by 0.019, and for triangular signals by 0.12. Mean peak cross correlation between input and output signals for sinusoidal waveforms incr eased by 0.06, with decreases in time to peak cross correlation (effective time delay) from 195 to 38 ms. In slow random signals (power up to 0.5 Hz), peak cross correlation went from 0.74 to 0.89, and time-to-peak cross corr elation decreased from 205 to 55 ms. In fast random signals (power up to 1 Hz), peak cross correlation went from 0.82 to 0.89, and time-to-peak cross correlation from 200 to 65 ms. For staircase signals, both rise times and m ean steady-state errors decreased. It was found that, once the length range was set, the load weight had no effect on tracking performance. Analysis o f mean square error demonstrated that for all signals tested, the feedback decreased the tracking error significantly, whereas, again, load had no eff ect. The results suggest that tracking is vastly improved by using a closed -loop system to control muscle length, and that load does not affect the qu ality of signal tracking as measured by standard control system analysis me thods. (C) 2000 Elsevier Science Ltd. All rights reserved.