Hypertonia:

Increased (rigid) muscle tone, such as spasticity. High tone, or very stiff muscles, may cause deformity of the bones over time. The muscles can become shorter as a result of stiffness. This is called contracture. This is a common complication of spastic types of cerebral palsy.

Muscle tone that is too high, resulting in overly tense muscles.

(of muscular tissue) the state of being hypertonic