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Oxygen consumption during constant-load exercise

J M Hagberg, J P Mullin, F J Nagle

    Journal of Applied Physiology: Respiratory, Environmental and Exercise Physiology
    |September 1, 1978
    PubMed
    Summary
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    Oxygen consumption (VO2) continues to rise during prolonged exercise above 60% VO2 max. This study found increased ventilation and temperature explain much of this sustained VO2 increase, not lactate.

    Area of Science:

    • Exercise Physiology
    • Human Physiology

    Background:

    • Oxygen consumption (VO2) typically stabilizes after an initial transient phase during exercise.
    • Previous research suggests VO2 may continue to rise during prolonged exercise exceeding 60% of VO2 max.

    Purpose of the Study:

    • To investigate the underlying physiological mechanisms responsible for the sustained increase in VO2 during prolonged exercise at high intensities.
    • To evaluate the contributions of elevated body temperature and increased ventilation to this phenomenon.

    Main Methods:

    • Eighteen healthy subjects performed 20-minute cycling exercises at 65% and 80% of their VO2 max.
    • Continuous monitoring of VO2, ventilation (VE), and respiratory exchange ratio.
    • Intermittent measurements of blood lactate and rectal temperature.

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    Main Results:

    • VO2 significantly increased from minute 5 to 20 in 81% of trials at both intensities.
    • The magnitude of VO2 increase was similar across both work loads.
    • Elevated body temperature accounted for ~30% of the VO2 rise.
    • Increased ventilation cost explained 30% and 81% of the VO2 rise at 65% and 80% VO2 max, respectively.
    • Lactacid accumulation did not appear to explain the sustained VO2 increase.

    Conclusions:

    • The sustained rise in VO2 during prolonged high-intensity exercise is significantly influenced by increased thermoregulatory demands and the energetic cost of ventilation.
    • These factors, combined, can account for a substantial portion, and potentially exceed, the observed VO2 increase.
    • The lactacid hypothesis is not supported by these findings.