EFFECTS OF DISCONTINUING REGULAR AEROBIC TRAINING ON HEART RATE DYNAMICS

Abstract

Vagal outflow to the heart is to physical fitness. New analysis methods of R-R in-tervals, based on fractal scaling, are observed to reveal automic regulation. PURPOSE: This study was designed to assess the effects of aerobic training on autonomic regulation analyzed by spectral and by fractal scaling methods during 8 weeks intervention and 1 year follow-up. METHODS: The healthy sedentary men were randomized to the training (n = 18), and the control group (n = 6). A heart ate (HR) controlled aerobic training in tervention was 8 weeks including 5 sessions a week (45 ± 15 min) at the intensity of 70–80% of the HRmax. The training group was encouraged to continue aerobic training thereafter independently for 10 months. At the end of one year follow-up the training group was divided into an active (n = 7) and a passive (n = 11) group. The subjects were defined as a passive when VO2max was decreased more than 5% during the follow-up. Short-term fractal scaling exponent (α1) calculated by Detrended Fluctuation Analysis method along with low (LF) and high frequency (HF) spectral components of HR variability were analyzed over 24-hour at baseline, after 8 weeks intervention, and after one year follow-up. RESULTS: In the active group VO2max (mean ± SD) increased after 8 weeks training (43 ± 7, p < 0.001) and remained at the high level after one year (48 ± 7, p < 0.001). There were statistically no significant changes in 24-h HR (67 ± 8, 63 ± 5, 64 ± 6, respectively) or LF/HF-ratio (3.3 ± 1.7, 2.5 ± 1.0, 2.6 ± 1.1, respectively). Fractal scaling decreased after 8 weeks training from 1.33 ± 0.17 to 1.25 ± 0.14 (p < 0.01) and remained below the baseline level (1.28 ± 0.14, p < 0.05) after follow-up. In the passive group VO2max increased after 8 weeks training (41 ± 4 vs. 45 ± 4, p < 0.001) but returned to the baseline after one year follow-up (40 ± 4, p < 0.001). HR decreased (72 ± 7 vs. 64 ± 7, p < 0.001) but increased to the baseline level after one year (68 ± 9, NS). LF/HF-ratio decreased after 8 weeks training (3.6 ± 2.1 vs. 2.9 ± 1.4, p < 0.05) but increased back to baseline after one year (3.7 ± 1.9 NS). α1 decreased after 8 weeks training from 1.38 ± 0.18 to 1.28 ± 0.17 (p < 0.001) and increased back to the baseline level (1.37 ± 0.18, NS) after one year follow-up. There were no changes in any variables for the control group. CONCLUSION: Aerobic training causes a change in quality properties of HR dynamics towards increased beat-to-beat variability as seen in enhanced vagal dominance. This effect of HR indices disappear after withdrawal from regular training. Fractal scaling exponent α1 may be more sensitive than conventional measures to detect these subtle quality changes in HR dynamics. Supported by Ministry of Education (Helsinki, Finland), Academy of Finland (Helsinki, Finland) and Polar Electro Oy (Kempele, Finland).