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.