Physical fitness as a determinant of vagal modulation

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Abstract
The association between increasing age and decreasing vagal modulation is well known. However, the importance of fitness as a determinant of the decline in vagal modulation with age is not established. To test the hypothesis that decreasing vagal modulation is largely a function of declining fitness, rather than increasing age, we studied a sample of healthy volunteers with a wide range of fitness levels, but a narrow age range. We assessed fitness by measuring the maximal oxygen uptake (˙VO2max) achieved during incremental bicycle exercise. Vagal modulation was assessed by calculating high frequency power (0.15-0.40 Hz) of the RR variability power spectrum from 24-h ECG recordings. We studied 37 healthy volunteers who were 22-44 yr old. In our sample, ˙VO2max ranged from 25 to 70 mL·min-1·kg-1 (mean of 45 ± 13). Age was not significantly related to high frequency power, but ˙VO2max was highly correlated with high frequency power (r = 0.74, P = 0.0001), indicating that physical fitness is strongly associated with vagal modulation. Thus, the decline in vagal modulation often attributed to increasing age may, instead, be the result of a decline in fitness.