MUSCLE SYMPATHETIC-NERVE RESPONSES TO STATIC LEG EXERCISE

Previous studies of muscle sympathetic nerve activity (MSNA) during static exercise have employed predominantly the arms. These studies have revealed striking increases in arm and leg MSNA during static handgrip (SHG) and postexercise circulatory arrest (PECA). The purpose of this study was to examine MSNA during static leg exercise (SLE) at intensities and duration commonly used during SHG followed by PECA. During 2 min of SLE (static knee extension) at 10% of maximal voluntary contraction (MVC; n = 18) in the sitting position, mean arterial pressure and heart rate increased significantly. Surprisingly, MSNA in the contralateral leg did not increase above control levels during SLE but rather decreased (23 +/- 5%; P < 0.05) during the 1st min of SLE at 10% MVC. We compared MSNA responses to SHG and SLE (n = 8) at 30% MVC. SHG and SLE elicited comparable increases (P < 0.05) in arterial pressure and heart rate, but SHG elicited significant increases in MSNA, whereas SLE did not. During PECA after SHG and SLE, mean arterial pressure remained significantly above control. However, MSNA was unchanged during PECA after SLE but was significantly greater than control during PECA after SHG. Because previous studies have indicated differences in MSNA responses to the arm and leg, we measured arm and leg MSNA simultaneously in six subjects during SLE at 20% MVC and PECA. During SLE and PECA, MSNA in the contralateral arm and leg did not differ significantly from each other. In summary, 1) SLE produces a biphasic response in MSNA in both the leg and arm with a decrease during the 1st min of exercise followed by a return to control levels during the 2nd min, and 2) MSNA responses to SLE are markedly different from MSNA responses during SHG when performed at the same intensity (30% MVC) and duration (2 min).