Muscle sympathetic nerve activity during handgrip and post-handgrip muscle ischemia after exposure to simulated microgravity in humans.

To examine the effect of 6 degrees head-down bed rest (HDBR) on vasomotor sympathetic responses to isometric forearm exercise, 16 healthy male subjects aged 20-36 years performed voluntary isometric handgrip (HG) at 30% of maximal voluntary contraction until fatigue, followed by 2 min of post-handgrip muscle ischemia (PHGMI) with 250 mmHg of cuff inflation, before and after 14 days of HDBR. Time to fatigue and maximal voluntary HG force were not affected by HDBR. Pre-exercise baseline muscle sympathetic nerve activity (MSNA, measured by microneurography), heart rate (measured by electrocardiogram) and mean blood pressure (measured by Portapres) increased after HDBR. Increases in MSNA were similar during HG but significantly lower during PHGMI (P < 0.01) after HDBR. Responses of heart rate and mean blood pressure during HG and PHGMI were not affected by HDBR. These results suggest that the magnitude of muscle metaboreflex during isometric forearm exercise might be attenuated after 14 days of simulated microgravity.