Cardiac parasympathetic regulation: respective associations with cardiorespiratory fitness and training load

The objective of this study was to establish the separate associations between parasympathetic modulations of the heart [ evaluated through heart rate ( HR) variability ( HRV) indexes and postexercise HR recovery ( HRR) indexes] with cardiorespiratory fitness and training load. We have measured cardiorespiratory fitness through peak oxygen consumption ( VO2 max) and estimated weekly training load with the Baecke sport score in 55 middle-aged individuals ( 30.8 +/- 1.8 yr, body mass index 24.5 +/- 0.4 kg/m(2)). HRV indexes were analyzed at rest under controlled breathing, and HRR was estimated from HR curve fitting after maximal exercise or from measurements of the number of beats recovered at 60 s after exercise. Multiple linear regressions were used to investigate the separate relationships between vagal-related HRV indexes and VO2 max and Baecke scores. On the basis of their VO2 max and Baecke scores, subjects were classified as fit or unfit and as low trained ( LT) or moderately trained ( MT), which yielded four groups: Unfit(LT), Unfit(MT), Fit(LT), and Fit(MT). Vagal-related HRV indexes were positively correlated with VO2 max ( P < 0.05) but not with Baecke scores. In contrast, HRR indexes were related to Baecke scores ( P < 0.05) but not with VO2 max. Fit(LT) and Fit(MT) had significantly higher ( P < 0.05) normalized vagal-related HRV indexes than Unfit(LT) and Unfit(MT), but HRR did not change. Moderate training was associated with significantly lower HRR indexes both in Unfit(MT) and Fit(MT) compared with Unfit(LT) and Fit(LT), but there was no difference in vagal-related HRV indexes. These results indicate that vagal-related HRV indexes are related more to cardiorespiratory fitnes