Energy Expenditure at Rest and During Exercise

The immediate source for muscle contraction is delivered from splitting of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and a ‘free’ phosphate ion (P). This reaction is very fast and does not normally limit energy turnover andmuscle performance. However, the ATP stores in the muscles are very limited. The whole ATP pool would be emptied in only a few seconds of muscle contraction. Therefore ATP has to be continuously regenerated through other energy systems. These supporting energy systems are very effective and can keep the ATP concentration unchanged or only marginally lowered, even during heavy exercise. In essence, there are four such systems, with different speeds of reaction and capacities, which release energy for active phosphorylation of ADP to restore the ATP pool. They are (1) creative phosphate, (2) lactacid anaerobic metabolism, (3) aerobic metabolism and (4) adenylate kinase reactions.