The following is an excerpt from my book Athletic Development ? The Art & Science of Functional Sports Conditioning, Chapter Seven ? Energy and Work Capacity. I thought it might help to clarify the use of the energy systems in training.
The next point in regard to the development of work capacity is the concept of training the energy systems. When the concept of training the energy systems was first articulated in the book ?Interval Training? by Fox and Mathews in 1974 this was major breakthrough in training. It was presented in such a manner that concepts and ideas that had been the exclusive domain of the scientist in the lab were articulated in terms and a context that coaches and practitioners could apply. Unfortunately as the concepts gained in popularity it seems we have deviated from some simple scientific facts.
Conceptually, the energy systems are intensity dependent, not time dependent. Somehow the misconception has arisen that the energy systems function like a set of timed switches that sequentially turn on as the duration of exercise increases. In actual fact all three energy systems are ?turned on? at the beginning of exercise. Essentially the proportionate contribution of each system will vary with the intensity and duration of the effort. ATP is necessary for movement; it is manufactured aerobically or anaerobically depending on the intensity of the exercise. Furthermore, the energy systems must interact with the other systems of the body to ensure that the output is an efficient, smooth, and coordinated action. We always train the energy systems, but it will not be an up front objective. Consideration is always given to the dominate energy system demands of the various training activities relative to the demands of the sport. This is to guide us, but it is not the sole objective in a workout.