FORCE FEEDBACK-CONTROL OF MOTOR UNIT RECRUITMENT IN ISOMETRIC MUSCLE

The use of simple force feedback in an isometric muscle control system utilizing orderly recruitment of motor units is explored. Cat medial gastrocnemius motor units were stimulated with and without simple forc e feedback gain ranging from 0.7 to 0.9. Ramp, triangular, staircase, sinusoidal and bandwidth-limited pseudo-random input recruitment signa ls were used to study tracking accuracy through linear correlation in ramp and triangular signals, cross correlation in sinusoidal and rando m signals, and rise time and steady state error in staircase signals. Dramatic improvements were found in most tested tracking variables wit h the use of feedback; squared correlation coefficients increased from a mean of 0.93 to 0.99 for ramp signals and from 0.76 to 0.98 in tria ngular signals. Mean peak cross-correlations improved from 0.85 to 0.9 8 in random signals and from 0.93 to 0.98 for sinusoidal inputs, and m ean time to peak cross-correlations decreased from 144 to 24 ms in ran dom signals and from 156 to 25 ms in sine waves. Rise times in stairca se signals decreased from a mean of 520 to 175 ms, and mean steady sta te error decreased from 12 to 3%. Significant effects of the triangle cycle time, sinusoidal frequency and staircase step were found on the performance of the muscle force control system. In addition, the possi ble effects of intrinsic feedback mechanisms on the control system wer e examined by repeating the closed loop part of the study but with the sciatic nerve cut proximally. The tracking results were essentially a nd statistically the same as in the closed loop condition. It was conc luded that a simple feedback configuration provided superior tracking performance for a practical application in which orderly recruitment i s used to control muscles; furthermore, it was concluded that this typ e of system would be virtually immune to external disturbances such as spasticity resulting from intact spinal neural feedback mechanisms fo und in paralyzed individuals. (C) 1998 Elsevier Science Ltd. All right s reserved.