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Figure 1 Oxygen consumption (VO₂), cardiac output, and its components. On this and subsequent figures, mean values ± SEM of individual cardiac parameters are shown before exercise, every 10 min during exercise through 40 min, and at end-exercise for subjects younger versus those older than the median age of 50 years. The p value shown is derived from repeated measures analysis of variance comparing all time points between 10 min and 40 min of exercise, achieved by 19 of 20 younger subjects and 17 of 20 from the older group. Oxygen consumption remained constant in both age groups across time but was higher in younger subjects (A). An increase in cardiac index (CI) (B) occurred in each group, with a trend toward a larger increase in younger subjects. Stroke volume index (SVI) (C) increased modestly and to a similar extent in both groups between 10 and 40 min; heart rate (HR) (D) was consistently higher in the young.