Whenever a human exercises, he/she utilizes energy, that is released by ATP hydrolysis. ATP resynthesis is sustained by energy metabolism, which consists of three major biochemical pathways: aerobic (oxygen consumption), anaerobic lactic (muscle lactate accumulation in absence of oxygen consumption), and anaerobic alactic (phosphocreatine hydrolysis) metabolism. The maximal power and capacity of these metabolisms are discussed, as are the factors that limit them. Moreover, the metabolic and cardio-pulmonary mechanisms of the dynamic response to exercise are discussed. The way power and capacity of the three major energy metabolisms are affected under a number of different conditions, such as training, hypoxia, microgravity, is described.