This is common in people who have cerebral palsy, brain injuries or a spinal cord injury, but it can happen to anybody e.g. having a stroke.One factor that is thought to be related to spasticity is the stretch reflex. This reflex is important in coordinating normal movements in which muscles are contracted and relaxed and in keeping the muscle from stretching too far. Although the result of spasticity is problems with the muscles, spasticity is actually caused by an injury to a part of the central nervous system (the brain or spinal cord) that controls voluntary movements. The damage causes a change in the balance of signals between the nervous system and the muscles. This imbalance leads to increased activity (excitability) in the muscles. Receptors in the muscles receive messages from the nervous system, which sense the amount of stretch in the muscle and sends that signal to the brain. The brain responds by sending a message back to reverse the stretch by contracting or shortening.Overall, a defining feature of spasticity is that the increased resistance to passive stretch is velocity-dependent. Lance (1980) describes it this way:  "...a motor disorder, characterised by a velocity-dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks, resulting from hyper-excitability of the stretch reflex as one component of the upper motor neurone (UMN) syndrome".Spasticity is found in conditions where the brain and/or spinal cord are damaged or fail to develop normally; these include cerebral palsy, multiple sclerosis, spinal cord injury and acquired brain injury including stroke.
Damage to the CNS as a result of stroke or spinal cord injury, alter the [net inhibition] of peripheral nerves in the affected region. This change in input to bodily structures tends to favor excitation and therefore increase nerve excitability. CNS damage also causes nerve cell membranes to rest in a more [depolarized] state. The combination of decreased inhibition and an increased depolarized state of cell membranes, decreases action potential threshold for nerve signal conduction, and thus increases activity of structures innervated by the affected nerves (spasticity). Muscles affected in this way have many other potential features of altered performance in addition to spasticity, including muscle weakness; decreased movement control; clonus (a series of involuntary rapid muscle contractions often symptomatic of muscle over-exertion and/or muscle fatigue); exaggerated deep tendon reflexes; and decreased endurance. Spasticity and clonus
Clonus (i.e. involuntary, rhythmic, muscular contractions and relaxations) tends to co-exist with spasticity in many cases of stroke and spinal cord injury likely due to their common physiological origins. Some consider clonus as simply an extended outcome of spasticity. Although closely linked, clonus is not seen in all patients with spasticity. Clonus tends to not be present with spasticity in patients with significantly increased muscle tone, as the muscles are constantly active and therefore not engaging in the characteristic on/off cycle of clonus. Clonus results due to an increased motor neuron excitation (decreased action potential threshold) and is common in muscles with long conduction delays, such as the long reflex tracts found in distal muscle groups.