A single contraction is termed a muscle twitch. It is divided into a latent period, a period of contraction, and a relaxation period. When a muscle receives a rapid series of stimuli, it cannot relax between contractions. Maintenance of muscle tone requires receptors called muscle spindles. When a nervous impulse is received, all muscle fibers within the motor neuron contract together in a motor unit.
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.