Cardiac vagal activity following three intensities of exercise in humans

Cardioprotective benefits of exercising at vigorous intensities are known, but reservations remain in prescribing such activity to the untrained population, due to a perceived risk of cardiac events. Few studies have investigated the recovery of the autonomic nervous system (ANS) after a single exercise bout, especially following vigorous exercise in healthy, young but untrained individuals. In this study, the recovery of the ANS, in particular indices of vagal activity were measured postexercise, at three intensities similar to current international recommendations for health. Thirteen individuals (six females, 22.2 +/- 3.1 years) performed three 20-min constant load tests lying supine on a modified bicycle ergometer at the following intensities: moderate (2 mmol l(-1) blood lactate concentration, BLC); hard (3 mmol l(-1)BLC); and vigorous (4 mmol l(-1)BLC) as derived from a maximal test. ECG data were collected during 5-min epochs at baseline then at: 5, 15, 30, 45 and 65-min postexercise. Heart rate variability (HRV) analysis was performed to obtain R-R interval, standard time [root mean square of successive differences (RMSSD)] and frequency measures [natural logarithm of high (lnHF) and low frequency (lnHF)]. RMSSD, lnHF, lnLF and total power were reduced 5-min postexercise following all three intensities (P<0.01). Decreases persisted up to 15-min postexercise following hard and vigorous exercise only (P<0.01). In untrained young adults, parasympathetic reactivation is reduced up to 5-min postexercise regardless of intensity, returning to baseline by 30 min even after vigorous exercise. In this population, the benefits of exercise outweigh any risks of cardiac events that may be evoked by a reduction in the influence of vagal activity.