According to Healthline, muscle rigidity is a condition in which the muscles are continually tense or contracted. Muscle rigidity is often a symptom of neurological problems or medicinal side effects.
Healthline notes that muscle tension is the constant tonic contraction of the skeletal muscles, resulting in firm, tense-feeling muscles that may be painful. Muscle rigidity most often occurs in the flexor muscles, but it can also occur in the extensor muscles. A common sign of muscle rigidity is muscular resistance to even small, passive movements, making it difficult for the affected person to move the limb at all.
According to Wikipedia, one of the most common pathological forms of muscle rigidity is hypokinesia. Hypokinesia is a category of neuromuscular disorders that become obvious when there is either full or partial loss of muscular control due to damage in the basal ganglia. Parkinson's disease, Tourette's syndrome and Huntington's disease are common disorders of the basal ganglia that result in severe muscle rigidity. Healthline explains that treatment for muscle rigidity depends entirely upon the underlying cause, which must first be determined by a CT scan or MRI. Other potential causes include a seizure, an injury of the spinal cord or a severe reaction to a prescription medication.