Motor unit activity under low tensions as muscle changes length

Twenty men between 24 and 38 years of age performed tracking tasks involving abduction, adduction, and holding the position of the little finger against constant adductive torques of 5, 10, and 15 percent of maximal abductive effort. Abduction and adduction were performed at 12 degrees per second between 0 and 30 degrees of abduction. Isometric tasks were performed at 10, 15, and 20 degrees. Myoelectric activity from the abductor digiti minimi muscle, recorded through fine wires, occurring between 10 and 20 degrees of abduction was analyzed in terms of global content (by rectifying and averaging the electromyograms) and in terms of identifiable action potentials of individual motor units. For the rectified electromyograms, averaged amplitude at a given load was greatest when the muscle was shortening (abduction), next greatest when the muscle was lengthening (adduction), and least when the muscle was isometric. Change in rectified electromyographic amplitude between 10 and 20 degrees of abduction was greater when the muscle was shortening than when it was lengthening or isometric. Mean firing rate for identified motor units was higher when the muscle was shortening than when lengthening or remaining isometric. The results of this study suggest that maintaining muscle activity against a small load is most easily done if the muscle is isometric, and least easily done if the muscle is shortening.