Spasticity is a disorder that causes stiffness in muscles. It is one of the most commonly occurring disorders due to cerebral palsy. In this, the part of the brain controlling voluntary movements is affected such that it causes the muscles to become extensively stiff and when you may try to move or relax your muscles, their stiffness aggregates. This forces the affected individual to keep his affected limb in one position, which sometimes results in weakness of the muscle. For instance, if spasticity occurs in the arms, the muscles of the arm tighten, pulling the elbows toward the body, and hands and wrists toward the chin. The hands themselves form tight fists. This constant tightened state may in turn weaken the muscles, stretching them to the point where some of their functionality is lost.
Other than cerebral palsy, some abnormalities like traumatic brain injury, stroke, spinal cord injury and multiple sclerosis can also cause spasticity.