MUSCLE SYMPATHETIC-NERVE RESPONSES TO PROLONGED ONE-LEGGED EXERCISE

We previously demonstrated that brief dynamic one-legged knee extensions (DKE) performed in the upright position decreased muscle sympathetic nerve activity (MSNA). The present study was undertaken to investigate MSNA responses to prolonged DKE. DKE was performed for 40 min in the upright (sitting) position, and MSNA was recorded in the peroneal nerve of the contralateral leg by microneurography. DKE elicited a significant increase in mean arterial pressure (MAP) from 83 +/- 3 to 101 +/- 6 (SE) mmHg during the 5th min of exercise. The increase in MAP persisted for the remainder of exercise. In contrast, heart rate (HR) showed a gradual increase throughout the 40 min of exercise (80 +/- 3 to 110 +/- 5 beats/min at 40 min of exercise; P < 0.05). MSNA decreased from 34 +/- 3 to 28 +/- 3 bursts/min (P < 0.05) from control to the 5th min of DKE and then returned to control levels by 20 min (35 +/- 3 bursts/min) and remained unchanged for the remainder of exercise. During recovery, MSNA was significantly increased above control for 10 min postexercise (40 bursts/min) when HR and MAP had returned to control levels. These results confirm our previous finding that MSNA is decreased during the early stage of upright DKE. The two new findings are as follows: 1) prolonged DKE failed to increase MSNA above control levels, and 2) during recovery when HR and MAP are at control levels, MSNA is significantly elevated.