1 Time - course of human motoneuron recovery 2 after sustained low - level voluntary activity

37 Motoneurons often fire repetitively and for long periods. In sustained voluntary contractions the 38 excitability of motoneurons declines. We provide the first detailed description of the time-course of 39 human motoneuron recovery after sustained activity at a constant discharge rate. We recorded the 40 discharge of single motor units (MUs, n=30) with intramuscular wire electrodes inserted into triceps 41 brachii during weak isometric contractions. Subjects (n=15) discharged single motor units at a 42 constant frequency (~10 Hz) with visual feedback for prolonged durations (3-7 minutes) until 43 rectified surface electromyogram (sEMG) of triceps brachii increased by ~100%. After a rest of 1-2, 44 15, 30, 60, 120 or 240 s, subjects briefly resumed the contraction with the target MU at the same 45 discharge rate. Each MU was tested with 3-4 rest periods. The magnitude of sEMG was increased 46 when contractions were resumed and the target motoneuron discharged at the test frequency 47 following rest intervals of 2-60 s (p = 0.001-0.038). The increased sEMG indicates that greater 48 excitatory drive was needed to discharge the motoneuron at the test rate. The increase in EMG 49 recovered exponentially with a time-constant of 28 s, but did not return to baseline even after a rest 50 period of ~240 s. Thus, the decline in motoneuron excitability from a weak contraction takes 51 several minutes to recover fully. 52 53 Abbreviations: sEMG, surface electromyography; MU, motor unit; MVC, maximal voluntary 54 contraction; CI, confidence interval; ISI, interspike interval 55