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We undertook this study to determine whether low intensity exercise (55 % O2,max) would significantly alter the metabolic and ventilatory responses observed during 10 min of subsequent moderate intensity exercise (75 % O2,max). By executing this work, we hoped to further our understanding of the mechanisms that limit mitochondrial ATP production at the onset of exercise. Seven healthy human subjects performed 10 min of moderate intensity exercise in the presence and absence of 10 min of low intensity exercise, which preceded the moderate intensity exercise by 3 min. Muscle biopsy samples were obtained from the vastus lateralis at pre-determined time points and oxygen consumption kinetics were determined at rest and during low and moderate intensity exercise. Following low intensity exercise and 3 min of passive recovery, muscle lactate and acetylcarnitine concentrations were elevated above basal levels, but O2 had returned to the resting rate. When moderate intensity exercise was preceded by low intensity exercise, there was a significant sparing of phosphocreatine (PCr, ~25 %, P < 0.05) and reductions in glucose 6-phosphate (G-6-P, ~50 %, P < 0.05) and lactate (~50 %, P < 0.05) accumulation during the first minute of moderate intensity exercise. No differences were observed after 10 min of moderate intensity exercise. The O2 on-kinetic response over the first minute of moderate intensity exercise was accelerated when preceded by low intensity exercise. Collectively, our results suggest the lag in the oxidative ATP delivery at the onset of moderate intensity exercise can be overcome by prior low intensity exercise. Furthermore, our findings support the view that this lag is at least in part attributable to a limitation in acetyl group delivery/availability at the onset of exercise, rather than delayed oxygen supply.
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