COMPARISON OF 24-HOUR PARASYMPATHETIC ACTIVITY IN ENDURANCE-TRAINED AND UNTRAINED YOUNG MEN

Objectives. This study compares 24-h parasympathetic activity in aerobically trained and untrained healthy young men. Background. Higher values of parasympathetic nervous system activity are associated with a low mortality rate in patients after myocardial infarction, but it remains uncertain what therapeutic interventions can be used to increase parasympathetic activity. Although it is thought that exercise training can increase parasympathetic activity, studies have reported conflicting results, perhaps because this variable was measured for only brief intervals and usually inferred from changes in reflex responses induced by pharmacologic blockade. Methods. Parasympathetic activity was assessed noninvasively from 24-h ECG recordings by calculating high frequency (0.15 to 0.40 Hz) beat to beat heart period variability in eight endurance-trained men (maximal oxygen consumption greater-than-or-equal 55 ml/kg per min) and eight age-matched (mean = 29 yr) untrained men (maximal oxygen consumption less-than-or-equal-to 40 ml/kg per min). The data were analyzed separately for sleeping hours when parasympathetic activity is dominant and also for waking hours. Results. The geometric mean of high frequency power was greater in the trained than in the untrained men during the day (852 vs. 177 ms2, p < 0.005), during the night (1,874 vs. 427 ms2, p < 0.005) and over the entire 24 h (1,165 vs. 276 ms2, p < 0.001). Conclusions. Parasympathetic activity is substantially greater in trained than in untrained men, and this effect is present during both waking and sleeping hours. These data suggest that exercise training may increase parasympathetic activity over the entire day and may therefore prove to be a useful adjunct or alternative to drug therapy in lessening the derangements of autonomic balance found in many cardiovascular diseases.