The sceletal muscles are tied to the bones and serve to move them. The energy required for work originates from the chemical reactions that occur in the muscle cells during the contraction. In the muscles there are stocks of energy source ATP, and energy is also stored in other forms, ie energy rich molecules. The energy stored in them is used in resynthesis of ATP, and thus exploited in muscular work. This paper explains the processes of energy production which occur during the muscular work by aerobic and anaerobic metabolic pathways, and highlightes differences in these processes with regard to the rate and amount of energy released. According to the dominance of physiological processes of the organism, the sports are divided into anaerobic, aerobic and mixed or anaerobic-aerobic. The body uses energy systems during exercise depending on the intensity and duration of the activity. Nutrients that metabolize energy to the body are carbohydrates, fats and proteins. The main mechanisms of metabolic processes of obtaining energy from nutrients are explained in this paper.