Skeletal muscles are stimulated to contract by motor neurons of the nervous system. At the neuromuscular junction (NMJ), the point where a branch of a neuron meets a muscle cell, the neurotransmitter acetylcholine is released, prompting contraction of the cell. Two special proteins in the cell, actin and myosin, interact to produce the contraction. ATP (the cell's energy compound) and calcium are needed for this response.

Most skeletal muscles contract rapidly to produce movement and then relax rapidly unless stimulation continues. Sometimes muscles are kept in a steady partially contracted state, to maintain posture, for example. This state of firmness is called tonus, or muscle tone.

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