Spasticity is a muscle control disorder that is characterized by tight or stiff muscles and an inability to control those muscles. In addition, reflexes may persist for too long and may be too strong (hyperactive reflexes). For example, an infant with a hyperactive grasp reflex may keep his or her hand in a tight fist. Spasticity is caused by an imbalance of signals from the central nervous system (brain and spinal cord) to the muscles. This imbalance is often found in people with cerebral palsy, traumatic brain injury, stroke, multiple sclerosis, and spinal cord injury.
Spasticity, if severe, can cause pain and difficulty with care and positioning, leading to skin irritation and even skin breakdown. It can also hamper walking due to stiffness. If spasticity persists for a long time, the tendons and ligaments can permanently shorten causing limbs to contract and become difficult to straighten out.

Spasticity tends to occur more often in the legs than in the arms. It also tends to be asymmetrical, meaning it is worse or happens more often on one side of the body than the other. There are several types of spasms, including:

• Extensor Spasms: These happen when a limb, usually a leg, stiffens and the person is unable to bend the joint. These cause the limb, usually a leg, to jerk away from the body. It usually affects the quadriceps (the large muscles on the front of the thigh), causing the lower leg to straighten.
• Flexor Spasms: This type of spasm causes a limb to contract, or bend, toward the body. This type of spasm almost always affects the legs, especially the hamstrings or hip flexors.
• Clonus: This is when muscles jerk or twitch repeatedly. The most common forms of clonus is when a person’s foot taps rapidly and repetitively on the floor or knee or ankle jerk repeatedly after stimulation (such as tapping at the joint), rather than the normal response of one tap or jerk.
• Adductor Spasms: These are more rare. These spasms cause a person’s legs to close together tightly, making it difficult to separate them.
• Stiffness: This can be thought of as mild spasticity. While not as dramatic as some of the forms of spasms described above, when muscles are slow to relax, it can cause problems walking or using the hands and fingers to perform delicate movements. In some cases, the stiffness may not pose a huge problem. In other cases, the spasticity can cause problems with mobility or be painful enough to interfere with daily life.

While the exact number of people affected by spasticity is not known with certainty, it is likely that it affects over half a million people in the United States alone and over 12 million worldwide.
For many people spasticity can be a mere annoyance that hinders smooth walking one day but is absent the next. It may just be that walking quickly or climbing stairs is harder than it was previously. Others may actually benefit from mild spasticity or stiffness, as it can counteract some degree of muscle weakness and make it easier to stand.
However, for some people, severe forms of spasticity or stiffness can cause a problem with mobility, as walking becomes difficult or impossible. Some spasms can be aggravated when moving from the bed into a wheelchair. Some extensor spasms can be so sudden and strong that the person can fall out of a chair or bed. Flexor spasms can cause limbs to be held in painful positions and lead to secondary joint pain.