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One of the main principles in physiology and biochemistry is that of ATP homeostasis. Amazingly the level of ATP can be maintained in working muscle because high ATP turnover rates yield byproducts, such as ADP, AMP, and Pi that stimulate restoration of ATP to set-point levels. Muscle cells with high capacities for ATP use are powerful, but fatigue rapidly if ATP level cannot be maintained. Cells with high capacities to restore ATP after use possess excellent endurance because ATP use and restoration are balanced and [ATP] maintained.
Processes of food and energy substrate catabolism in cells are usually linked to the process of ATP restitution. Approximately 50% of the potential chemical energy released from foodstuffs is captured in the common chemical intermediate, ATP.

ATP, together with its storage form, creatine phosphate (CP), then serves as the immediate cellular energy source on which endergonic processes depend. ATP and CP not only supply immediate cellular energy sources, but their relative levels also stimulate or inhibit processes of energy metabolism. At rest, normally high levels of ATP and CP inhibit energy metabolism. When exercise starts, however, the utilization and decreased levels of ATP and CP, and the increased levels of ADP, AMP, and Pi stimulate processes of energy metabolism. Enzymes interact with products of energy metabolism to regulate the rate at which specific processes proceed. Muscles utilize three different systems of energy release during exercise, each of which differs in mechanism, capacity, and endurance. Consequently, the rate and capacity for muscular power output is determined by the ability of these three systems that maintain cell ATP homeostasis.

Athletic activities can be classified as one of three groups: power, speed, and endurance events (see Table 1).

Examples of these groups are the shot put, the 400-m sprint, and the marathon run, respectively. Success in each of these events depends largely on energetics and the biochemical mechanisms supporting ATP homeostasis. Skeletal muscle has three energy systems, each of which is used in these three types of activities. In power events, where the activity lasts a few seconds or less, the muscle has several immediate energy sources (Figure 1).

Figure 1- Energy Sources for muscle as a function of Activity duration

For rapid, forceful exercises lasting from a few seconds to approximately 1 minute, muscle depends mainly on nonoxidative, or glycolytic, energy sources, as well as on immediate sources. For activities lasting 2 minutes or more, oxidative mechanisms become increasingly important. Before describing these three basic muscle energy sources, we need to describe the chemical-mechanical energy transduction of muscle contraction, as depicted in Equation 1.

Equation 1

In this reaction, actin and myosin are the two contractile proteins of muscle, and Ca2 is the calcium ion whose presence triggers the combination of actin and myosin. Inorganic phosphate (Pi) is also produced by the reaction.

Source: McGraw Hill, Brooks, Fahey, Baldwin – Exercise Physiology, Human Bioenergetics and Its applications – Fourth Ed(book)