Muscle tone (also called tonus of residual muscle tension) is a measure of a muscle's resistance to stretching while in a passive, resting state . It is also a measure of the continuous, partial contraction of muscles while in a passive state of activity. When external force is applied to a passive muscle, muscles will increase tension due to the partial contraction, reflexively responding to avoid stretching. Both extensor and flexor muscles act in the maintenance of tone. It helps maintain posture, and it declines during REM sleep.
Tone is not limited to skeletal muscles, but is also a property of cardiac and smooth muscles. However, the tone of such muscles is not characterized by matching agonist muscles that resist stretching, but rather the passive state typically exhibits enervation without active contraction.
Unconscious nerve impulses maintain the muscles in a partially contracted state. If a sudden pull or stretch occurs, the body responds by automatically increasing the muscle's tension, a reflex which helps guard against danger as well as helping to maintain balance. Such near-continuous innervation can be thought of as a "default" or "steady state" condition for muscles. There is, for the most part, no actual "rest state" insofar as activation is concerned.
Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. Another form of hypertonia is paratonia, which is associated with dementia. Hypotonia is seen in lower motor neuron disease like poliomyelitis. Hypotonia can present clinically as muscle flaccidity, where the limbs appear floppy, stretch reflex responses are decreased, and the limb's resistance to passive movement is also decreased.
Hypertonia is seen in upper motor neuron diseases like lesions in the pyramidal tract and extrapyramidal tract. Hypertonia can present clinically as either spasticity or rigidity. While spasticity is velocity-dependent resistance to passive stretch (i.e. passively moving an elbow quickly will elicit increased muscle tone, but passively moving elbow slowly may not elicit increased muscle tone), rigidity is velocity-independent resistance to passive stretch (i.e. there is uniform increased tone whether the elbow is passively moved quickly or slowly).
Spasticity can be in the form of the clasp-knife response, in which there is increased resistance only at the beginning or at the end of the movement. Rigidity can be of the leadpipe type, in which there is resistance throughout to passive movement, or it may be of cogwheel type, in which the resistance to passive movement is in a jerky manner.