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.
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.
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).
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.
1The Human Performance and Health Research Laboratory, Department of Human Health and Nutritional Sciences, University of Guelph, Guelph, ON, Canada; 2Department of Human Health and Nutritional Sciences, University of Guelph, Guelph, ON, Canada; 3Toronto General Research Institute, Toronto General Hospital, Toronto, ON, Canada
Address for Correspondence: Jamie F. Burr, Animal Science and Nutrition, 50 Stone Road E., University of Guelph, Guelph, ON, Canada, N1G 2W1, Phone 519-824-4120 x 52591. Email: firstname.lastname@example.org
This research was supported by a Natural Science and Engineering Research Council (NSERC) of Canada Discovery Grant (P.J.M. #06019; J.F.B. #03974), the Ontario Ministry of Research, Innovation, Science (P.J.M. #34379; J.F.B. 460597), and the Canada Foundation for Innovation (P.J.M.#34379; J.F.B. 460597). A.M.C. was supported by a Ontario Graduate Scholarship. A.V.I. and C.J.D. were supported by Queen Elizabeth II Graduate Scholarships in Science and Technology. J.D.S. was supported by a NSERC Undergraduate Student Research Award. The authors have no conflicts of interest to declare. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by ACSM.
Accepted for Publication: 28 November 2017