Comparison of External Load Compensation During Rhythmic Arm Movements and Rhythmic Jaw Movements in Humans

1 Department of Oral Pathophysiology, Faculty of Medicine, University of Utrecht; and 2 Helmholtz Institute, Faculty of Physics and Astronomy, University of Utrecht, 3584 CC Utrecht, The Netherlands Abbink, J. H., A. van der Bilt, F. Bosman, H. W. van der Glas, C. J. Erkelens, and M.F.H. Klaassen. Comparison of External Load Compensation During Rhythmic Arm Movements and Rhythmic Jaw Movements in Humans. J. Neurophysiol. 82: 1209-1217, 1999. Experiments were performed on human elbow flexor and extensor muscles and jaw-opening and -closing muscles to observe the effect on rhythmic movements of sudden loading. The load was provided by an electromagnetic device, which simulated the appearance of a smoothly increasing spring-like load. The responses to this loading were compared in jaw and elbow movements and between expected and unexpected disturbances. All muscles showed electromyographic responses to unexpected perturbations, with latencies of ~65 ms in the arm muscles and 25 ms in the jaw. When loading was predictable, anticipatory responses started in arm muscles ~200 ms before and in jaw muscles 100 ms before the onset of loading. The reflex responses relative to the anticipatory responses were smaller for the arm muscles than for the jaw muscles. The reflex responses in the arm muscles were the same with unexpected and expected perturbations, whereas anticipation increased the reflex responses in the jaw muscles. Biceps brachii and triceps brachii showed similar sensory-induced responses and similar anticipatory responses. Jaw muscles differed, however, in that the reflex response was stronger in masseter than in digastric. It was concluded that reflex responses in the arm muscles cannot overcome the loading of the arm adequately, which is compensated by a large centrally programmed response when loading is predictable. The jaw muscles, particularly the jaw-closing muscles, tend to respond mainly through reflex loops, even when loading of the jaw is anticipated. The differences between the responses of the arm and the jaw muscles may be related to physical differences. For example, the jaw was decelerated more strongly by the load than the heavier arm. The jaw was decelerated strongly but briefly,