Influence of ischemia on the discharge rate in motor units during a sustained submaximal contraction
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Motor unit discharge patterns were observed under ischemic and non-ischemic conditions in the tibialis anterior during two sessions approximately 48 hours apart. Ten subjects completed three 5-second maximum voluntary contractions [MVCs) followed by a prolonged, submaximal, isometric contraction to induce fatigue, and three additional MVCs. This fatigue trial was completed under both ischemic and non-ischemic conditions. For the ischemic condition, a pressure cuff was placed above the knee, inflated to 180 mmHg, and arterial occlusion verified via Doppler ultrasound. During the fatigue task, a constant force of 20% MVC was maintained until endurance time. Single motor unit activity was recorded with intramuscular fine wire electrodes. Endurance time during the ischemic fatigue task (3.71 ± 0.58 min) was significantly less than the non-ischemic fatigue task (9.11 ± 0.56 min). Both tasks showed a significant decline in motor unit discharge rate. There was greater variability in the discharge rate during ischemic compared to non-ischemic conditions. Time to minimum discharge rate was greater during ischemic conditions. Overall, ischemic conditions resulted in decreased endurance time, greater rate of decline in discharge rate, and greater variability in discharge rate of motor units.