Cardiac autonomic control in elite juvenile cyclists

Abstract

Introduction: Frequency domain analysis of heart rate variability (HRV) may potentially identify the dominance exhibited by different branches of the autonomic nervous system. Autonomic contribution to cardio-deceleration following exercise has been studied in adults; however, no data are available for endurance trained juvenile athletes. Aim: The aim of this study was to evaluate the association between the increase in heart rate following exercise and any change in HRV frequency components in trained juveniles. Methods: A 6 min resting ECG (lead 2) was recorded from trained juvenile cyclists (23 male, 7 female, mean age 14.7 years), before (Pre) and after (Post) incremental exercise to volitional exhaustion on a stationary cycle ergometer. Each subject performed a progressive ramp protocol where power increased by 20W min-1, with initial power set at 60 W for females and 100 W for males. Oxygen uptake (VO2) was measured using breath-by-breath techniques. ECG was analysed in both time and frequency domains using commercially available software. Results: Mean VO2 max was 75.5 ml Kg-1 min-1, and mean power at VO2 max was 440 W. The R-R interval SD was lower following exercise (Pre: 86ms vs. Post: 36ms, P<0.01). Normalised high frequency HRV was lower (Pre: 36.5 vs. Post: 18.7, P<0.01) and normalised low frequency HRV was higher (Pre: 58.5 vs. Post: 75.0, P<0.01) following exercise. Conclusions: In elite juvenile athletes there were no associations between exercise-induced changes in high frequency variability and heart rate (R2=0.073). This suggests that in elite juveniles, the heart may be less sensitive to vagal influences- a suggestion further supported by the relatively high pre-exercise resting heart rates with normal high frequency variability.