It has been reported that maximal oxygen uptake (
VO
2 max) is linearly correlated with blood volume (BV) in young people and that there is a reduction in
VO
2 max with aging. To examine the involvement of BV in the reduction of
VO
2 max, we used an incremental cycle ergometer protocol in a semirecumbent position to determine the relationship between peak oxygen uptake (
VO
2 peak) and BV in older subjects (69.1 ± 1.0 years;
n = 22), then compared that relationship with that in young subjects (22.3 ± 0.5 years;
n = 31). In the present study,
VO
2 peak and BV were significantly lower in the older subjects, compared with those in the young subjects. A linear correlation was demonstrated between the
VO
2 peak and BV in both the older (
r = 0.705;
p < 0.001) and the young (
r = 0.681;
p < 0.001) subjects within the groups. However, an analysis of covariance with BV as a covariate revealed that
VO
2 peak at a given BV was smaller in the older subjects than in the young subjects (
p < 0.001), i.e., graphically, the regression line determined for the older subjects showed a downward shift. The decreased peak heart rate as a result of aging (153 ± 3 beats/min in the older vs. 189 ± 2 beats/min in the young subjects) contributed partly to this downward shift. These results suggest that the BV is an important determinant factor for
VO
2 peak, especially within an age group, and that the age-associated decline of
VO
2 peak is also, to a relatively larger degree, because of factors other than BV and heart rate.
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