Spasticity by definition is an involuntary, velocity-dependent, increased resistance to stretching. This means that the amount of resistance to stretching is at least partly determined by the speed with which a spastic muscle is stretched. The faster one tries to move, the harder it becomes.
While spasticity presents as problems with the muscles, it is actually a caused by an injury to a part of the central nervous system which controls voluntary movements (either the brain or spinal cord). This damage can be caused by cerebral palsy, amoungst other conditions.
Spasticity varies in it's severity, from slight stiffness of the muscles to a shortening of the muscle which can in turn deform joints. There is plenty of research being done into ways to lessen the severity of spasticity, through physical therapy, drugs, and surgery. This is important research to those with cerebral palsy, since roughly 70% of people with cerebral palsy also have spasticity. Here is a sample of this research in 2004.
