CENTRAL COMMAND INCREASES MUSCLE SYMPATHETIC-NERVE ACTIVITY DURING INTENSE INTERMITTENT ISOMETRIC-EXERCISE IN HUMANS

During sustained isometric exercise, central command has very little e ffect on muscle sympathetic nerve activity (MSNA). To determine if cen tral command has a greater effect on MSNA during intermittent than dur ing sustained contractions, MSNA was recorded with microelectrodes (pe roneal nerve) during intermittent isometric handgrip at 25%, 50%, and 75% maximum voluntary contraction (MVC) in 9 human subjects with paced breathing. Similar experiments were performed in 11 additional subjec ts before and after partial neuromuscular blockade (intravenous curare ) to isolate the influence of central command while minimizing force o utput and thus muscle afferent feedback. Before curare, handgrip at 25 % and 50% MVC had no effect on MSNA, whereas handgrip at 75% MVC synch ronized the MSNA to the handgrip such that MSNA was 5.7+/-1.3 times hi gher (mean+/-SEM, P<.001) during the contraction periods than during t he relaxation periods. After curare, this synchronization of MSNA pers isted without attenuation, even though force output fell to <25% of th e initial MVC. From these observations, we conclude that central comma nd causes synchronization of motor activity and muscle sympathetic act ivity during intense intermittent isometric exercise.