Spasticity has been defined as an increase in muscle tone due to hyperexcitability of the stretch reflex and is characterized by a velocity-dependent increase in tonic stretch reflexes. This means that when moving the child's arm or leg, resistance increases as the speed of the movement is increased. In some cases, the rapid increase in resistance leads to a "catch" as the limb is moved, with a subsequent "release" of the resistance once the limb stops moving.

Spasticity does not necessarily interfere with the child's attempts at voluntary movement. It is essentially a property of passive  movement. Therefore, spasticity is triggered by the interaction between the child and the environment. Impairment in voluntary movement is more often caused by symptoms associated with spasticity or the upper motor neuron syndrome (such as weakness, dystonia, or ataxia), rather than by the spasticity itself.

      Definitions:

Both rigidity and spasticity cause an increased tone in muscles, but spasticity is typically only present during stretching of the muscle, and rigidity is present even when the muscle is at rest.

Spasticity: is a velocity-dependent resistance to movement felt by an examiner stretching a muscle group across a joint, resulting from the hyperexcitability of the stretch reflex. In spasticity, there is a catch and release, although the release may not be evident in severe cases.

Rigidity: is the resistance to stretch that is not velocity dependent—the examiner feels just about the same resistance to stretch irrespective of the velocity a muscle group is been stretched .

  Characteristics:

Spasticity, is associated with two other characteristic signs: the clasp-knife phenomenon and clonus. When first stretched, a spastic muscle provides a high level of resistance to the stretch and then suddenly yields, much like the blade of a pocket knife (clasp knife). Hyperactivity of the stretch reflex loop is the reason for the increased resistance to stretch in the clasp-knife phenomenon. The physiological basis for the inhibition that causes the sudden collapse of the stretch reflex (and loss of muscle tone) is thought to involve the activation of the Golgi tendon organs. the initial movement of the limb evokes a stretch reflex in the stretched muscles. The reflex contraction causes resistance to movement and the movement slows or stops. With continued application of force, a point is reached where the resistance gives way, the extensor muscles now lengthening easily. The loss of resistance is due to inhibition of extensor motor neurons.

Clonus refers to a rhythmic pattern of contractions (3–7 per second) due to the alternate stretching and unloading of the muscle spindles in a spastic muscle.