Spasticity is a form of muscle overactivity. A spastic muscle is one in which a muscle resists being stretched out, and the resistance to stretch is greater the faster the muscle is moved. Spasticity is often used as an umbrella term for other forms of muscle overactivity that often occur at the same time in the same patient.
Spasticity occurs following damage to the neurons, or nerve cells, that send signals from the brain to the muscles to cause movement. These neurons, which run from the brain through the spinal cord, are called upper motor neurons, and damage to them produces an upper motor neuron syndrome. The upper motor neuron syndrome may be caused by stroke, traumatic brain injury, spinal cord injury, multiple sclerosis, or numerous other less common causes of damage to the motor neurons. Damage to the brain occurring prior to or shortly after birth is called cerebral palsy (CP), which is the most common cause of an upper motor neuron syndrome in children.
The other forms of muscle overactivity common in the upper motor neuron syndrome are:
Clonus, a relatively slow rhythmic contraction and relaxation of a muscle, typically occurring after a stimulus such as movement or while attempting to hold the muscle still. Clonus can be mild or severe in intensity.
Spasms, strong and sustained contractions of muscles, which are often painful.
Increased reflexes, in which the normal reflexes (such as knee extension in response to tapping) are greatly exaggerated.
Together, all these forms of muscle overactivity can cause significant disability in a patient, interfering with dressing, bathing, feeding, mobility, and other activities of daily living. The upper motor neuron syndrome also involves weakness and loss of dexterity, which may be even more disabling to the patient, and may be much less amenable to treatment.
Richard Robinson, The Gale Group Inc., Gale, Detroit,