Physiology 101

In physiology, medicine, and anatomy, muscle tone (aka residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles. It helps maintain posture, and it declines during REM sleep. It is not to be confused with the concept of toning in physical exercise.
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.
The presence of near-continuous innervation makes it clear that tonus describes a "default" or "steady state" condition. There is, for the most part, no actual "rest state" insofar as activation is concerned.
In terms of skeletal muscle, both the extensor and flexor muscles, under normal enervation maintain a constant tone while "at rest" that maintains a normal posture.
Cardiac muscle and smooth muscle, although not directly connected to the skeleton, also have tonus in the sense that although their contractions are not matched with those of antagonist muscles; their non-contractive state is characterized by (sometimes random) enervation.

Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. Another form of hypertonia is Paratonia, which is associated with dementia.

Paratonia or gegenhalten is classified as a form of hypertonia with an involuntary variable resistance (i.e reduced ability of a muscle to stretch) during passive movement (i.e a movement without effort). The disease develops during a period of dementia and the degree of effect is dependent upon the disease's progress. Paratonia was classified very recently by experts, and is unrelated to the much more typical spasticity associated with spastic diplegia and simlilar forms of cerebral palsy.
Clasp-knife response refers to a stretch reflex with a rapid decrease in resistance when attempting to flex a joint, usually during a neurological examination. It is one of the characteristic responses of a upper motor neuron lesion.
It gets its name from the resemblance between the motion of the limb and the sudden closing of a claspknife after sufficient pressure is applied.

Although seemingly a stretch reflex when flexing a joint, force from the muscle during the attempt to flex a joint is actually thought to be caused by the tendon reflex of the antagonistic muscle of that joint, which is an extensor muscle that becomes stretched. In upper motor neuron lesions, muscle tonus may increase and resistance of muscle to stretch increases. However, if sufficient force is applied, limb resistance suddenly decreases.