Rigidity is stiffness and resistance to limb movement caused by increased muscle tone, an excessive and continuous contraction of muscles.[6] In parkinsonism the rigidity can be uniform (lead-pipe rigidity) or ratchety (cogwheel rigidity).[6][7][16][17] The combination of tremor and increased tone is considered to be at the origin of cogwheel rigidity.[18] Rigidity may be associated with joint pain; such pain being a frequent initial manifestation of the disease.[6] In early stages of Parkinson's disease, rigidity is often asymmetrical and it tends to affect the neck and shoulder muscles prior to the muscles of the face and extremities.[19] With the progression of the disease, rigidity typically affects the whole body and reduces the ability to move.