How is spasticity described in terms of muscle tone?

Spasticity is characterized by a velocity-dependent increase in muscle tone, meaning that the faster a muscle is stretched, the greater the resistance encountered. This resistance is due to an increase in the excitability of the spinal reflexes. In conditions such as stroke or cerebral palsy, spasticity results from an imbalance of signals from the brain to the muscles, leading to hyperactivity in the stretch reflex.

The defining feature of spasticity is not just the presence of muscle tone but the way it changes in relation to the speed of a movement. When a limb is moved quickly, it encounters more resistance than when moved slowly. This phenomenon is crucial for understanding how spasticity affects movement and muscle control in individuals with neurological impairments, highlighting the importance of both velocity and muscle response in the clinical assessment and treatment of spasticity.