Rg. Victor et al., CENTRAL COMMAND INCREASES MUSCLE SYMPATHETIC-NERVE ACTIVITY DURING INTENSE INTERMITTENT ISOMETRIC-EXERCISE IN HUMANS, Circulation research, 76(1), 1995, pp. 127-131
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.