Spasticity, a neurological problem secondary to an up permotor neuron lesion, has a significant effect on skeletal muscle. The upper motor neuron lesions may be secondary to a cerebral vascular accident, head injury, spinal cord injury, or degenerative diseases such as multiple sclerosis, or perinatal brain injuries such as cerebral palsy. Function alability in these patients can be severely compromised but the basic mechanisms underlying these deficits are not clearly understood. In this review we evaluate the current evidence in the literature that suggests that skeletal muscle tissue itself is altered in spastic conditions. Experimental studies were evaluated that included a variety of methods encompassing joint mechanics, tissue mechanics, and muscle morphology.