PHYSICAL-FITNESS AS A DETERMINANT OF VAGAL MODULATION

The association between increasing age and decreasing vagal modulation is well known. However, the importance of fitness as a determinant of the decline in va-al 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 narro w age range. We assessed fitness by measuring the maximal oxygen uptak e ((V) over dot O-2max) achieved during incremental bicycle exercise. Vagal modulation was assessed by calculating high frequency power (0.1 5-0.40 Hz) of the RR variability power spectrum from 24-h ECG recordin gs. We studied 37 healthy volunteers who were 22-44 yr old. In our sam ple, (V) over dot O-2max ranged from 25 to 70 mL . min(-1). kg(-1) (me an of 45 +/- 13). Age was not significantly related to high frequency power, but (V) over dot O-2max was highly correlated with high frequen cy power (r = 0.74, P = 0.0001), indicating that physical fitness is s trongly associated with vagal modulation. Thus, the decline in vagal m odulation often attributed to increasing age may, instead, be the resu lt of a decline in fitness.