Three Weeks of Overload Training Increases Resting Muscle Sympathetic Activity
Overload training is hypothesized to alter autonomic regulation, though interpretations using indirect measures of heart rate variability are conflicting. The aim of the present study was to examine the effects of overload training on muscle sympathetic nerve activity (MSNA), a direct measure of central sympathetic outflow, in recreational endurance athletes.Methods
Measurements of heart rate variability, cardiac baroreflex sensitivity (BRS), MSNA (microneurography), and sympathetic BRS were obtained in seventeen healthy triathletes and cyclists after 1-week of reduced training (baseline) and following 3-weeks of either regular (CON, n=7) or overload (OL, n=10) training.Results
Following training, the changes ([INCREMENT]) in peak power output (10 ± 10 vs. -12 ± 9 W, P < 0.001), maximal heart rate (-2 ± 4 vs. -8 ± 3 beats/min, P = 0.006), heart rate variability (standard deviations of normal-to-normal intervals: 27 ± 31 vs. -3 ± 25 ms, P = 0.04), and cardiac BRS (7 ± 6 vs. -2 ± 8 ms/mmHg, P = 0.02) differed between CON and OL groups. The change in MSNA burst frequency (-2 ± 2 vs. 4 ± 5 bursts/minute, P = 0.02) differed between groups. Across all participants, the changes in resting MSNA and peak power output were correlated negatively (r = -0.51, P = 0.04). No between-group differences in resting heart rate or blood pressure were observed (All P > 0.05).Conclusions
Overload training increased MSNA and attenuated increases in cardiac BRS and heart rate variability observed with regular training. These results support neural adaptations following overload training and suggest that increased central sympathetic outflow may be linked with decreased exercise performance.