| Summary: | Reduced heart rate variability (HRV) is related to an increased risk of mortality. The primary aim of this dissertation was to determine the independent effects of regular exercise and maturation on HRV. It was hypothesized that (1) regular exercise increases HRV in adults and (2) HRV declines as a result of maturation. Resting HRV was measured using the (1) average change in HR (ΔHR) during each breathing cycle; (2) standard deviation of HR (HRSD); (3) root of mean squared differences of successive interbeat intervals (rMSSD); and (4) proportion of adjacent intervals that differed by more than 50 ms (pNN50). Spectral analysis was used to determine the power of oscillations in HR occurring at low (LF, 0.04–0.12 Hz), and high (HF, 0.12–0.40 Hz) frequencies, which are believed to represent cardiac sympathetic and parasympathetic influences, respectively. In Study I, HRV was compared in 40 adult males grouped according to their physical activity level (LOW, MOD, and HIGH). HRV appeared to be greater in the active groups (LOW < MOD = HIGH), but only pNN50 achieved statistical significance. Study II examined the effects of a 16 week moderate intensity endurance training program on HRV. Twelve adult males that exercised three times weekly, thirty minutes per session (EXP) were compared to a non-exercising control group (CON, N = 5). Aerobic capacity, pNN50, and rMSSD increased significantly in EXP (∼14%, 75%, and 37%, respectively) but not controls. There were no changes in ΔHR or LF power, but HF power increased in both EXP and CON. In Study III, HRV was compared between the adults from Study I and twelve boys. HRSD and ΔHR were greater in boys. All other measures of HRV appeared to be greater in boys, but none of these comparisons achieved statistical significance. It is concluded that (1) participation in regular aerobic exercise training appears to augment HRV; and (2) HRV declines as a result of maturation. When the data from studies I and II are considered in light of the existing literature, it appears that significant increases in HRV occur only after many weeks or months of endurance training.
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