Characterizing oxygen uptake (VO2) kinetics yields valuable information regarding both a) the effectiveness of the coupling of O2 delivery to tissue metabolic demands, and b) the ability of the muscle itself to utilize O2 for oxidative phosphorylation. During moderate exercise VO2 reaches a new steady state within 3 min in normal subjects, with little or no sustained rise in blood lactate. The steady state VO2 increases linearly with work rate. The time constant for VO2 in Phase 2 (after first 15-20 s) is constant across work intensities, and appears to reflect muscle oxygen utilization kinetics. However, when heavier exercise is performed, which elevates blood lactate throughout the exercise, the VO2 response becomes more complex. The predominant, Phase 2 response continues to rise exponentially with about the same time constant as for moderate exercise, and the amplitude continues to be linearly related to work rate. However, an additional, slowly developing rise in VO2 is also usually observed, beginning 100-200 s into exercise. This additional VO2 delays attainment of a steady state, increases the overall O2 "cost" of the exercise, and is statistically associated with the rate and magnitude of increase in blood lactate. Interestingly, in children, neither the slow component nor blood lactate rise as much during heavy exercise.