Oxygen Demand Increases During A Bout Of Severe Intensity Exercise

Historically, it has been assumed that the energy demand in severe intensity exercise, quantified as the oxygen demand, is constant. However, the results of some recent studies have suggested that the oxygen demand may actually increase over time, although the cause of this purported increase has yet to be elucidated. PURPOSE: The purpose of this study was to determine if the oxygen demand is constant during a bout of constant power, severe intensity, cycle ergometer exercise. METHODS: Eight women (age 22 ± 3 y, height 171 ± 8 cm, and weight 69 ± 13 kg) and 15 men (22 ± 2 y, 181 ± 9 cm, 78 ± 10 kg) volunteered to participate. Each of these 23 participants performed three constant power, severe intensity, cycle ergometer tests, which were terminated by the investigators after either 3 min, or 6 min, or 9 min, with the order of durations determined using randomization. The work rates for these tests were individually determined by the investigators to be tolerable for ~15-20 min. The oxygen demand (in mL/kg/min) during each 3-min segment (0-3 min, 3-6 min, and 6-9 min) was determined from the total oxygen cost for that 3 min (in mL/kg), which was calculated as the sum of (i) the aerobic energy contribution (accumulated oxygen uptake), (ii) the glycolytic energy contribution (which was based on the post-exercise blood lactate concentration), and (iii) the phosphocreatine energy contribution (which was based on the post-exercise VO2). The oxygen demands in each of the three 3-min segments were compared using ANOVA with post hoc comparisons, and a significance level of p < 0.05. RESULTS: Results of the ANOVA revealed a significant effect of duration on the oxygen demand. Oxygen demand was 39 ± 7 mL/kg/min in 0-3 min, 41 ± 7 mL/kg/min in 3-6 min (greater than the value in 0-3 min, p < 0.01), and 43 ± 9 mL/kg/min in 6-9 min (greater than the value in 0-3 min, p < 0.01, but not significantly different from the value in 3-6 min, p = 0.23). CONCLUSION: These results suggest that oxygen demand increases - that is, gross metabolic efficiency decreases - over time, during constant power, severe intensity exercise. This has implications for calculation of accumulated oxygen deficit, for the estimation of the energy cost of exercise, and for characterization of exercise intensities.