Hypertonia Rigidity Rigidity HP:0002063 Rigidity is often a manifestation of basal ganglia diseases. UMLS:C0026837 Muscle rigidity SNOMEDCT_US:16046003 Continuous involuntary sustained muscle contraction. When an affected muscle is passively stretched, the degree of resistance remains constant regardless of the rate at which the muscle is stretched. This feature helps to distinguish rigidity from muscle spasticity.