Pulmonary oxygen uptake ([Formula: see text]) slowly increases during exercise above the anaerobic threshold, and this increase is called the slow component of [Formula: see text]. The mechanism of the increase in [Formula: see text] is assumed to be due to increasing energy cost associated with increasingly inefficient muscle contraction. We hypothesized that the increase in [Formula: see text] would be accompanied by a constant or increasing rate of accumulation of blood lactate, indicating sustained anaerobic metabolism while [Formula: see text] increased. Ten male subjects performed cycle ergometry for 3, 6, and 9 min at a power output representing 60% of the difference between lactate threshold and maximal [Formula: see text] while [Formula: see text] and blood lactate accumulation were measured. Blood lactate accumulation decreased over time, providing the energy equivalent of (mean ± SD) 1586 ± 265, 855 ± 287, and 431 ± 392 ml of [Formula: see text] during 0-3, 3-6, and 6-9 min of exercise, respectively. As duration progressed, [Formula: see text] supplied 86.3 ± 2.0, 93.6 ± 1.9, and 96.8 ± 2.9% of total energy from 0 to 3, 3 to 6, and 6 to 9 min, respectively, while anaerobic contribution decreased. There was no change in total energy cost after 3 min, except that required by ventilatory muscles for the progressive increase in ventilation. The slow component of [Formula: see text] is accompanied by decreasing anaerobic energy contribution beyond 3 min during heavy exercise.

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