Effects Of Caffeine On Heart Rate And Heart Rate Variability At Rest And During Exercise

PURPOSE: The aim of this study was to investigate the effects of caffeine on heart rate and heart rate variability (HRV) at rest and during submaximal incremental exercise. METHODS: Using a balanced, double-blind, randomized, crossover design, 16 male cyclists (Means ± standard deviation for age, height, body mass, and V̇O2max were: 37 ± 9 years, 1.79 ± 0.08 m, 73.7 ± 8.1 kg, and 4.44 ± 0.67 L·min-1 respectively) completed three trials in an air-conditioned laboratory. In Trial 1, cyclists completed two incremental cycling tests to establish the V̇O2-power output relationship and V̇O2max. In trials 2 and 3, cyclists were evaluated for heart rate and HRV at rest after which they ingested a capsule containing 5 mg·kg-1 of caffeine or placebo (maltodextrin). 35 minutes post-supplementation, additional resting heart rate and HRV readings were taken after which cyclists completed a submaximal incremental cycling test (6 min stages) at 40, 50, 60, 70, and 80% of V̇O2max; with HR and HRV measurements taken in the last 5 minutes of each increment. HRV was determined from the root mean square of successive differences between R-R intervals. RESULTS: There were significant supplement × exercise intensity interactions on heart rate (p = 0.019) and HRV (p = 0.023), with post hoc tests on the latter showing that caffeine increased HRV at 40, 50, and 60% of V̇O2max by 3.6 ± 4.9, 2.6 ± 2.8, and 0.6 ± 1.7 ms, respectively. There was a supplement × time interaction effect on resting HRV (p < 0.001), but not on heart rate (p = 0.351). CONCLUSIONS: The results of this study support the suggestion that caffeine increases the parasympathetic modulation of heart rate.