Endurance exercise training results in profound adaptations of the cardiorespiratory and neuromuscular systems that enhance the delivery of oxygen from the atmosphere to the mitochondria and enable a tighter regulation of muscle metabolism. These adaptations effect an improvement in endurance performance that is manifest as a rightward shift in the 'velocity-time curve'. This shift enables athletes to exercise for longer at a given absolute exercise intensity, or to exercise at a higher exercise intensity for a given duration. There are 4 key parameters of aerobic fitness that affect the nature of the velocity-time curve that can be measured in the human athlete. These are the maximal oxygen uptake (VO2max), exercise economy, the lactate/ventilatory threshold and oxygen uptake kinetics. Other parameters that may help determine endurance performance, and that are related to the other 4 parameters, are the velocity at VO2max (V-VO2max) and the maximal lactate steady state or critical power. This review considers the effect of endurance training on the key parameters of aerobic (endurance) fitness and attempts to relate these changes to the adaptations seen in the body's physiological systems with training. The importance of improvements in the aerobic fitness parameters to the enhancement of endurance performance is highlighted, as are the training methods that may be considered optimal for facilitating such improvements.

We tested the hypothesis that heavy-exercise phase II oxygen uptake (VO(2)) kinetics could be speeded by prior heavy exercise. Ten subjects performed four protocols involving 6-min exercise bouts on a cycle ergometer separated by 6 min of recovery: 1) moderate followed by moderate exercise; 2) moderate followed by heavy exercise; 3) heavy followed by moderate exercise; and 4) heavy followed by heavy exercise. The VO(2) responses were modeled using two (moderate exercise) or three (heavy exercise) independent exponential terms. Neither moderate- nor heavy-intensity exercise had an effect on the VO(2) kinetic response to subsequent moderate exercise. Although heavy-intensity exercise significantly reduced the mean response time in the second heavy exercise bout (from 65.2 +/- 4.1 to 47.0 +/- 3.1 s; P < 0.05), it had no significant effect on either the amplitude or the time constant (from 23.9 +/- 1.9 to 25.3 +/- 2.9 s) of the VO(2) response in phase II. Instead, this "speeding" was due to a significant reduction in the amplitude of the VO(2) slow component. These results suggest phase II VO(2) kinetics are not speeded by prior heavy exercise.

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