Background. Whereas aerobic capacity declines with age, major factors responsible for such decline have been poorly defined by past studies. Methods. Participants were relatively healthy older individuals (339 women, 253 men) in whom demographic information and cardiopulmonary physiological measurements were obtained at baseline and biannually for three additional measurements. Results. The study identified progressive declines in both forced expiratory volume in 1 second (FEV 1 ) and in

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... exercise heart rate as two variables that accounted primarily for the longitudinal decline of aerobic performance in this cohort of relatively fit older persons who achieved high respiratory exchange ratios (RER; mean ¼ 1.08 for women, 1.12 for men). Whereas women achieved a peak oxygen consumption (VO 2peak ) only 77% that of men, oxygen uptake became similar to men (to 95%) when measured per kilogram of lean body mass rather than per kilogram of total body mass. During the 6 years of the study (four time points) aerobic capacity declined in both sexes, however, less steeply for women than for men (18% vs 24% per decade, respectively). The rate of decline was independent of baseline variables such as body composition (e.g., lean body mass, lean/fat ratio), smoking status, medications, or concomitant health conditions, even though these variables strongly influenced baseline aerobic performance. Inclusion of FEV 1 and maximal exercise heart rate into the statistical models, however, accounted for most of the longitudinal decline of aerobic performance. When adjusted for these two variables, aerobic capacity declined 9.7% and 10.4% per decade in women and men, respectively. Conclusions. Our findings emphasize the primary importance of declining FEV 1 and declining maximal exercise heart rate in accounting for the "aging effect" on aerobic capacity. Thus, when comparing longitudinal studies, all estimates of aerobic decline should be interpreted with respect to the specific variables included in the models, which also need to include FEV 1 and maximal exercise heart rate.