Purpose: The aims of this study were 1) to assess day-to-day stability of resting heart rate variability (HRV) in healthy adults; 2) to determine whether the reliability of these measures is influenced by self-reported habitual physical activity level; and 3) determine whether the magnitude of HRV is related to self-reported habitual physical activity level.
Methods: A resting electrocardiogram was obtained on five consecutive mornings in 37 men (age = 33+/-6 yr) grouped according to their self-reported level of weekly physical activity (LOW, N = 15; MOD, N = 10; HIGH, N = 12). All measurements were obtained while subjects breathed at 10 breaths x min(-1) (0.167 Hz).
Results: HRV was assessed using time and frequency domain measures. Most measures were highly reproducible (R > 0.90) regardless of activity level. After adjusting for differences in age and body mass index, weekly physical activity level was not a significant predictor of any measure of HRV. However, all time and frequency domain measures of HRV tended to be higher in active (MOD and HIGH) compared with inactive (LOW) subjects. No measure of HRV differed between the two most active groups.
Conclusions: Heart rate and HRV are highly reproducible, regardless of physical activity level. Additionally, although time and frequency domain measures of HRV may be greater in active than sedentary individuals, HRV does not appear to be increased in a dose-dependent manner with increasing levels of physical activity.