Muscle tone is a state of tension that is maintained continuously – minimally even when relaxed – and which increases in resistance to passive stretch. It helps to maintain posture and decreases during sleep. The stretch reflex is a muscle contraction in response to stretching within the muscle. This reflex, by definition extremely fast exists to allow the muscle to adapt to any kind of muscular tone, to avoid over-stretching , e.g. when need to adapt to a brutal change of the ground.
In spasticity, the muscle tone is abnormally increased (muscle hypertonia), and reflexes such as the stretch reflex may persist for too long and may be too strong (hyperactive reflexes). These phenomena cause an increased resistance to passive movement (e.g., if someone else tries to move the extremities of the person affected) characterized in the following ways:
- it increases with increasing speed of stretch and varies with the direction of joint movement and/or
- it increases above a patient-specific threshold speed or joint angle
The slower the speed and the smaller the angle when spasticity appears, the more severe it is.