WORK EFFICIENCY IN HEAVY EXERCISE IS MORE COMPLEX THAN HYPOTHESIZED
Cannon, D. T., Bowen, T. S., Murgatroyd, S. R., Bimson, W. E., Kemp, G. J., & Rossiter, H. B. (2012). ATP turnover and the coupling of mitochondrial oxidative phosphorylation during dynamic exercise in humans. Presentation 2206 at the 59th Annual Meeting of the American College of Sports Medicine, San Francisco, California; May 29-June 2, 2012.
"During constant work rate exercise above the lactate threshold, the kinetics of oxygen uptake (VO2) is supplemented by the VO2 slow-component, reflecting reduced work efficiency. The majority of the VO2 slow-component originates from the active locomotor muscles, yet the intracellular source of inefficiency is less clear. An increase in the ATP cost of power production has been postulated, rather than a change in the O2 cost of ATP resynthesis."
This study determined the ATP turnover rate during sub- and supra-lactate threshold constant work rate exercise in Ss (N = 7). Ss completed a series of prone, knee-extension, rest-exercise-rest protocols using a computer-controlled ergometer in a 3T superconducting magnet. Moderate work (sub-lactate threshold) and heavy work (supra-lactate threshold) constant work rate exercise was completed for three and eight minutes allowing total ATP turnover to be estimated at cessation from the dynamics of phosphocreatine and proton handling. Pulmonary VO2 was measured breath-by-breath using a mass spectrometer and turbine.
During moderate work there was no discernable VO2 slow-component. Conversely, during heavy work the VO2 slow-component was significantly larger. Similarly, the difference in phosphocreatine between three and eight minutes of moderate work was minimal, whereas the difference was larger during heavy work. Total ATP was constant during moderate work, but increased between three and eight minutes of heavy work. However, total ATP turnover was not related to the VO2 slow-component during heavy work. Three of seven Ss exhibited a non-linear phosphocreatine to oxygen uptake relationship.
Implication. The rate of ATP turnover increased between three and eight minutes of heavy exercise but not in moderate exercise. The poor correlation between total ATP turnover and the VO2 slow-component suggests that reduced work efficiency in heavy exercise is not wholly consequent to augmented power production, but that reductions in O2 cost of ATP resynthesis may also contribute.
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