Resistance exercise training enhances sympathetic nerve activity during fatigue-inducing isometric handgrip trials.

Muscle sympathetic nerve activity (MSNA) was investigated 1 week before (pre-training), 1 week after, and 4-6 weeks after strength training using fatigue-inducing handgrip exercises and post-exercise forearm occlusion. Eighteen volunteers underwent forearm training, which consisted of 30 maximal effort, 10-s-duration static handgrips, 4 days per week for 4 weeks. A second group of 18 volunteers served as a control. MSNA was recorded from the tibial nerve by microneurography. Maximal handgrip force increased at 1 week post-training. The MSNA response during fatigued handgrip also increased at 1 week post-training, as compared to pre-training (52.6 +/- 5.8 vs. 40.6 +/- 4.4 bursts min(-1) (mean +/- SEM), respectively). However, at 4 weeks post-training, MSNA activity returned to the pre-training level (44.0 +/- 5.2 bursts min(-1); p < 0.0001 by ANOVA), while the control group showed no changes throughout this period. The MSNA response during post-exercise forearm occlusion was constant throughout the experiment in both groups. Our results indicate that an increased MSNA response after strength training is likely to be the result of central neural factors rather than the muscle metaboreflex.