Muscular exercise and fatigue.

The development of muscular fatigue during exercise is a common phenomenon, and several forms depend on the precise type of exercise performed. The causes are still not clearly established, although the involvement of electrical and metabolic factors have been demonstrated. Several techniques which allow for the analysis of muscle function in terms of electrical activation and energy metabolism are (a) a needle biopsy of muscle for histochemical and metabolic studies, (b) magnetic resonance spectroscopy for the non-invasive study of muscle energy metabolism and pH, (c) electromyographic analysis of the electrical characteristics of muscle, and (d) percutaneous electrical stimulation of muscle for the force-frequency and relaxation characteristics of muscle. Endurance training increases the capacity to sustain exercise possibly by altering muscle energy metabolism and contractile properties. Fatigue is a self-protective mechanism against the damage of contractile machinery of muscle as, for example, with the development of rigor, which occurs if the energy stores are depleted. To illustrate the roles of energy supply and electrical properties in muscle in fatigue, the 'catastrophe theory' used in engineering has been applied. This may explain abrupt changes of function of individual muscle cells, while for the muscle as a whole, fatigue may be manifested as a more gradual loss of force.