Isotonic contractions occur when a muscle tenses and shortens. Isometric contractions involve tensing, but not shortening, of a muscle. ATP can be supplied to muscle in three different ways. Aerobic cellular respiration can supply muscles with energy. A second anaerobic pathway, fermentation, provides energy without using oxygen.
Fermentation ends with lactic acid, which changes muscle pH, leading to fatigue.
If an activity is not too strenuous, the three pathways work together to supply energy. In people that train, the number of mitochondria per muscle cell increases. Oxygen debt occurs when cells have used up available supplies of oxygen. Repaying oxygen debt means replenishing creatine phosphate and converting lactic acid back to pyruvate. Exercise has been shown to prevent certain kinds of cancer as well as to increase bone mass.