Human adaptations to endurance training : their influence on the ability to sustain submaximum exercise
The purpose of the study was to examine the proposition that adaptations of skeletal muscle are more important determinants of the ability to sustain submaximum exercise than maximum oxygen uptake (V0₂ max). The metabolic response to exercise was compared in 2 groups of runners with differing V0₂ max values. Trained female runners, despite their lower V0₂ max values, showed lower respiratory exchange ratio values (R) and higher post-exercise concentrations of plasma fat metabolites than active male subjects. This was so when subjects ran at a common absolute speed and when each individual ran at a speed equivalent to 60% V0₂ max. Six weeks of training on the cycle ergometer resulted in a 16% increase in V0₂ max but a 250% increase in endurance performance, i.e. time to exhaustion at 80% V0₂ max. During standard, submaximum exercise oxygen uptake, ventilation, heart rate, R values and blood lactate concentration were lower post-training than pre-training. The concentrations of plasma fat metabolites during and after exercises were not influenced by training. Training one leg and not the other resulted in 20% and 10% increase in V0₂ max but 340% and 150% increases in endurance for the trained leg (TL) and the untrained leg (UTL) respectively. The increases in both variables were significantly greater for the TL than for the UTL. Changes in submaximum V0₂, ventilation, R values and blood lactate concentration were mainly restricted to exercise with the TL but the reduction in heart rate was similar for each leg. Plasma FFA, glycerol and citrate concentrations during exercise were unchanged for either leg. The role of citrate in the training-induced interaction of fat and carbohydrate metabolism was investigated. Artificial elevation of plasma citrate concentrations by ingestion of citrate did not decrease blood lactate concentrations during exercise. After short-term fasting citrate accumulated in the myocardium but not in skeletal muscles in the rat. Plasma citrate concentration was a poor predictor of muscle citrate concentration in these circumstances. When subjects were exhausted after single-leg exercise at 80% V0₂ max muscle glycogen concentration decreased by 70% and muscle lactate concentration was increased 10-fold. Training halved the rate of glycogen depletion, decreased muscle lactate concentration and resulted in a decrease in R values only during exercise with the TL. The trend towards an increase in muscle citrate concentration was restricted to trained muscle. The experiments showed that absolute values of V0₂ max could be dissociated from the metabolic characteristics of endurance training and that V0₂ max is an insensitive indicator of training improvements in submaximum endurance. Peripheral adaptations are the dominant influence on the ability to sustain submaximum exercise but training only one leg can cause some systemic change which influences both V0₂ max and endurance for the UTL. The mechanism of the training-induced interaction of fat and carbohydrate metabolism remains unclear.