Unaltered R-R interval variability and bradycardia in cyclists as compared with non-athletes.

PURPOSE: To test whether elite mountain bikers display a cardiac autonomic modulation pattern that is distinctive from that of active non-athletes.
BACKGROUND: The relationship between autonomic adaptation and bradycardia during physical exercise, including high-performance sports such as the mountain biking, remains to be elucidated.
METHODS: Twelve elite mountain bikers and 11 matched non-athletes controls were evaluated for time- and frequency-domain heart rate variability based on a 5-min ECG R-R intervals series obtained in both the supine and the orthostatic positions. Oxygen uptake and pulse rate were obtained at ventilatory thresholds and peak effort during an incremental cardiopulmonary exercise test. Significance of differences between medians (25th, 75th percentiles) from the two groups was evaluated by the Mann-Whitney test at p ≤ 0.05.
RESULTS: Athletes had lower heart rate [50 (47, 59) versus 63 (60, 69) bpm; p = 0.0004] and higher cardiopulmonary performance than controls [70.9 (64.6, 74.4) versus 47.7 (41.0, 51.9) mL (kg min)(-1); p = 0.01]. No statistical difference was found in heart rate variability in the group of athletes (p = 0.17-0.97), except for trend toward having lower coefficient of variation and low-frequency absolute power indices both in supine position (p = 0.06).
CONCLUSIONS: Bradycardia and higher oxygen uptake were found in association with unaltered cardiac autonomic modulation in elite mountain bikers athletes in supine and orthostatic positions, compared to active non-athletes. This bradycardia was not dependent on distinctive resting autonomic modulation. Intrinsic adaptation of sinus node and/or a peculiar state of autonomic adaptation to this exercise can be possible mechanisms.