Coupling of muscle phosphorylation potential to glycolysis during work after short-term training.

To examine whether the metabolic adaptations to short-term training are expressed over a range of submaximal levels of mitochondrial respiration, seven untrained male subjects [maximal O2 uptake (VO2max) = 45.9 +/- 1.9 (SE) ml.kg-1.min-1] performed a progressive three-stage protocol of cycle exercise at 60% (20 min), 79% (20 min), and 92% (11 min) of pretraining VO2max before and after training. Training consisted of 5-6 days of cycling for 2 h/day at 65% VO2max. Muscle tissue rapidly obtained from the vastus lateralis by needle biopsy indicated that training blunted (P < 0.05) the increase in lactate observed at 60% (23.4 +/- 6.5 vs. 12.4 +/- 2.9 mmol/kg dry wt), 79% (48.9 +/- 5.1 vs. 25.6 +/- 5.2 mmol/kg dry wt), and 92% (68.3 +/- 6.4 vs. 41.5 +/- 6.5 mmol/kg dry wt) of VO2max. Training also resulted in a higher phosphocreatine and lower creatine and P(i) concentrations at both 79% (P < 0.05) and 92% (P < 0.05) of VO2max and higher muscle glycogen levels (P < 0.05). These changes were accompanied by small but significant reductions (P < 0.05) in O2 uptake at the two higher exercise intensities. Given that the lactate-to-pyruvate ratio and the calculated free ADP and AMP were also reduced (P < 0.05), it would appear that short-term training results in a tighter metabolic control over a range of mitochondrial respiratory rates.