Rigidity can be defined as increased resistance when stretching a muscle passively and is commonly associated with bradykinesias. Rigidity is one of the symptoms that has important implications in the quality of life of all patients; therefore, its assessment and treatment is important in every single case.
Rigidity, while seldom the main symptom early in PD, is experienced as stiffness of the arms or legs beyond what would result from normal aging or arthritic changes. Like other motor symptoms, rigidity often presents unilaterally and continues to manifest asymetrically, contributing to the postural deformities that are seen in many PD patients. Treatment with levodopa and dopamine agonists works well with rigidity.
Ebadi M, Pfeiffer, R; Parkinsons's Disease; CRC Press; 2005; P:173-174