LEE, C. M., R. H. WOOD, and M. A. WELSCH. Influence of Short-Term Endurance Exercise Training on Heart Rate Variability. Med. Sci. Sports Exerc., Vol. 35, No. 6, pp. 961–969, 2003.
Purpose: To examine the influence of 2 wk (eight sessions) of endurance training on cardiac autonomic modulation, as measured by heart rate variability (HRV).
Methods: Twenty-four males (mean age: 23.1 yr) were randomized to an exercise (EX;N = 12) or control group (CT;N = 12). EX trained for eight sessions (4× wk−1, 40 min, 80–85% HRreserve) on a cycle ergometer. ECG tracings were collected during 5 min of paced breathing (12 breaths·min−1 (PB)), 5 min of spontaneous breathing (SB1), 5 min of 70° head-up tilt (TILT), and a second 5-min period of spontaneous breathing (SB2). Data were collected before (test 1), during (tests 2–4), and 48 h after (test 5) the 2-wk period. HRV was reported as the standard deviation of RR intervals, and as natural logarithm of the normalized units (NU) of high- and low-frequency power (lnHF and lnLF).
Results: EX exhibited a significant increase in peak oxygen consumption (8%). During PB and TILT conditions, ANOVA revealed a group × time interaction such that EX exhibited lower lnLFNU and lnLF/lnHF during test 5 compared with test 1.
Conclusion: These data suggest that eight endurance exercise-training sessions performed over 2 wk enhance the relative vagal modulation of the heart during PB and TILT, but not during SB.
Chronic endurance exercise training may alter cardiac autonomic balance as reflected by changes in heart rate variability (HRV). Studies involving young apparently healthy individuals suggest that endurance-training programs generally result in a shift toward a greater vagal modulation of the heart (2,6,15). For example, De Meersman (6) reported a significantly elevated respiratory sinus arrhythmia after 8 wk of running, and Levy et al. (15) reported significant elevations in the standard deviation of normal RR intervals after 6 months of endurance training. More recently, Al-Ani et al. (2) reported that 6 wk of cycling exercise at an intensity of 85% heart rate maximum elevated high-frequency (HF) modulations of the heart, believed to be associated with vagal activity (1,4). Although the exact mechanism of the shift in sympathovagal balance is not fully understood, evidence suggests the changes may be mediated through central, cardioreflex, and/or end-organ changes (7,8,10).
Interestingly, the time course of exercise-training induced changes in autonomic modulation of the heart has not been extensively studied. Most studies that have approached this issue have examined plasma catecholamines and/or heart rate at rest and during physical activity (17,18,26). From these findings, investigators have concluded reductions in heart rate and plasma norepinephrine (NE) and epinephrine (E) are evident within 6–10 training sessions (17,18,26). However, the inferences that can be drawn from these studies are somewhat limited, inasmuch as measures of heart rate data do not allow for inferences about the specific activity of the sympathetic and parasympathetic systems. Furthermore, plasma NE does not necessarily solely reflect the rate of release from the axon terminals. Rather, it is influenced by binding and rate of clearance. Thus, it is of interest to evaluate the time course of training-induced autonomic changes using HRV, which may be more sensitive to changes in sympathovagal balance (19) than HR or plasma catecholamines. Additionally, because poor HRV profiles are associated with an increased risk of cardiac arrhythmic events in patients with heart disease as well as among the general population (5,24), elucidation of a time course of endurance training-induced alterations in HRV may provide clinicians with useful information regarding exercise programming in healthy and at-risk populations.
Of concern when utilizing HRV to generate inferences about the autonomic modulation of the heart is the observation that HRV parameters appear to be influenced by acute laboratory conditions. For example, Hayano et al. (11) reported that paced breathing results in a shift in the HRV power spectrum toward the HF range, a phenomenon thought to be a result of the respiratory sinus arrhythmia. Moreover, autonomic reactivity as observed under stressors such as head-up tilt may provide insight beyond that which is gleaned from resting measures alone. Furlan et al. (9) reported that head-up tilt, which elicits a sympathetic outflow, results in a shift toward the LF range. At present, however, it is not clear as to how HRV measured under different laboratory conditions might influence the sensitivity of HRV to detect training induced alterations in autonomic modulation of the heart.
Therefore, the objectives of this investigation were to 1) examine adaptations in cardiac autonomic modulation as measured by HRV under different conditions, including paced breathing and head-up tilt, in response to 2 wk of exercise training; and 2) evaluate a possible time course of any training-induced alterations in autonomic of the heart. It is hypothesized that participants undergoing 2 wk of high-intensity exercise training will exhibit a greater relative vagal modulation of the heart, as identified by an increase in HF power, and a concurrent decrease in LF power, expressed as normalized units. Furthermore, in accordance with previous studies evaluating endurance training on markers of autonomic activity, it is hypothesized that these training-induced alterations will be evident by the sixth training session.