Adaptations in muscle metabolism to prolonged voluntary exercise and training.

In previous research we established using a short-term (5-7 days) training model that increases in muscle oxidative potential are not a prerequisite for the characteristic energy metabolic adaptations (lower lactate, glycogen depletion, and phosphocreatine hydrolysis) observed during prolonged exercise. To investigate whether increased muscle aerobic potential further potentiates the metabolic adaptive response, seven healthy male volunteers [maximal O2 uptake (VO2max) = 45.1 +/- 1.1 (SE) ml.kg-1.min-1] engaged in an 8-wk training program consisting of 2 h of cycle exercise at 62% of pretraining VO2max 5-6 times/wk. Analysis of tissue samples obtained from the vastus lateralis after 60 min of exercise revealed that by 4 wk of training muscle lactate concentration, phosphocreatine hydrolysis, and glycogen depletion were depressed (all P < 0.05). Further training for 4 wk had no additional effect (P < 0.05). The ratio of fructose 6-phosphate to fructose 1,6-phosphate, an index of phosphofructokinase activity, was not altered with training. Muscle oxidative potential as estimated from the maximal activity of succinic dehydrogenase increased by 31% by 4 wk of training (P < 0.05) before plateauing during the final 4 wk of training. The increase in VO2max of 15.6% (P < 0.05) noted with training was also primarily expressed during the initial 4 wk. O2 uptake during submaximal exercise was unchanged. Because the metabolic response was similar in magnitude to that previously observed with short-term training, we conclude that, at least for the conditions of this study, the development of increased muscle aerobic potential is of minimal consequence on the magnitude of the energy metabolic adaptations examined.