Rushall notes, 1990.

As the duration of exercise at any intensity increases, fat becomes an increasingly more important energy source. Thus, as an extended performance progresses, in order to fuel the activity the contribution of fat keeps increasing, that is, the body gradually shifts to fat oxidation. That shift can be accelerated by an appropriate aerobic oriented warm-up.

At the start of a performance, the body may be using more carbohydrate than fat but by the end it will largely be using fat as the primary fuel source. This change is mainly due to a slowing of the rate of muscle glycogen breakdown.

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