A single maximal muscle contraction utilizes only ATP. Thereafter, creatine phosphate splitting and glycolysis are maximally activated, peaking within about 3 to 6 seconds (Boobis, 1987; Hultman et al, 1987; Spriet, 1987). Next, the control mechanisms come into play so that the rates of creatine phosphate splitting and glycolysis become progressively inhibited and so that oxidative (mitochondrial) metabolism becomes increasingly important.
After 90 to 120 seconds, oxidative metabolism becomes the predominant energy source. Note that the rate of energy production from the breakdown of the phosphagens (ATP and creatine phosphate) and from glycolysis exceeds, by far,
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Maximum exercise for different durations stresses different metabolic pathways.
That from purely oxidative pathways. This explains why it is possible to do very intense exercise only for brief periods of time.
This information helps explain the different metabolic pathways that are activated by high-intensity interval training of different durations and the nature of the fatigue experienced during very high-intensity exercise of different durations.
Interval training is a form of supramaximal exercise in which the athlete alternates periods of high-intensity exercise with periods of either rest or mild exercise. During these rest periods the energy systems depleted during exercise have a chance to completely or partially replenish themselves.
Astrand and Rodahl (1977) conducted an interesting experiment in which three groups of subjects performed the same total amount of exercise using three different combinations of intervals. Each group followed an exercise:rest ratio of 1:2, but the durations of exercise periods were 10, 30, or 60 seconds, and corresponding rest periods were 20, 60, or 120 seconds, respectively (see Exercises 3.12, a andb).