Medicine & Science in Sports & Exercise:
Thursday Morning Poster Presentations: Posters displayed from 7:30 a.m.-12:30 p.m.: One-hour author presentation times are staggered from 8:30-9:30 a.m. and 9:30-10:30 a.m.: D-31 Free Communication/Poster - Supplements: THURSDAY, JUNE 1, 2006 2:00 PM - 5:00 PM ROOM: Hall B
Wayne State University, Detroit, MI.
PURPOSE: Heart rate variability (HRV) is a measure of the variability from one heart beat to the next; is vagally mediated; and has been found to change significantly during the aerobic-anaerobic transition of exercise. Ventilatory threshold (VT) and HRV both have a strong relationship with autonomic control and previous studies have suggested that a heart rate variability “threshold” (HRVT) exists which coincides with VT during graded exercise. Caffeine however, is a common stimulant that influences autonomic control, and may therefore influence HRV. Therefore, the purpose of this investigation was to determine the influence of caffeine on HRV during progressive exercise.
METHODS: Eleven subjects performed graded maximal cycle ergometry to volitional fatigue. The study was a randomized placebo controlled, double-blind design, where each subject served as his/her own control (caffeine vs. placebo). Metabolic gas and ventilatory parameters were measured breath-by-breath averaged over thirty seconds, and blood lactate was measured every three minutes. Lactate threshold and VT were measured according to Gaskill et al., 2001. Heart rate variability was determined using time domain indexes including standard deviation, coefficient of variation, and mean successive difference, which have been shown to correlate strongly with vagal tone (Hayano et al., 1991).
RESULTS: Caffeine consumption of 5mg/kg significantly (p <.05) increased blood lactate and reduced some indices of HRV (coefficient of variation and mean successive difference) at rest. However, during exercise the placebo and caffeine trials where no longer significantly different for lactate, VT, or HRV.
CONCLUSIONS: While caffeine has a significant impact at rest, it does not influence lactate threshold, VT, or HRV during vigorous exercise.