Limb immobilization alters muscle activation patterns during a fatiguing isometric contraction

The purpose of the study was to determine the role of excitation-contractio n coupling in the increased endurance time for low-force contractions after 4 weeks of elbow joint immobilization. Twelve subjects participated in a p rotocol that required immobilization of the elbow joint in a fiberglass cas t for 4 weeks, and 4 subjects acted as controls. Measurements of muscle str ength, contractile properties, and fatigability were performed before and a fter 4 weeks of limb immobilization, and after 4 weeks of recovery. The imm obilization intervention produced significant reductions in the daily activ ity of the elbow flexor muscles, a 21% decline in the maximum voluntary con traction (MVC) force, and a 31% decrease in the maximum load that could be lifted once. Seven of the immobilized subjects exhibited an unusual pattern of muscle activity during the fatiguing contraction after immobilization, which was associated with an increase in the endurance time of the elbow fl exor muscles (mean = 220%) in these subjects. The unusual pattern of muscle activity involved lower relative activity of the brachialis muscle, no inc rease in the amplitude of the electromyogram (EMG) for the elbow flexor mus cles, and intermittent rather than continuous EMG. In contrast, the force-f requency relationship of biceps brachii was not altered by immobilization i n these subjects, suggesting that adaptations in excitation-contraction cou pling were not the primary cause of the prolonged endurance time after immo bilization. Rather, the results suggest that the prolonged endurance time e xhibited by some subjects after immobilization was largely due to adaptatio ns within the nervous system, (C) 2000 John Wiley & Sons, Inc.