Polymyositis is an inflammatory disease primarily affecting the large muscle groups. The resultant weakness is noted with activities such as arising from a chair , climbing stairs and reaching or working overhead. Dermatomyositis is a similar disease, but also causes a distinct rash on the back of the hands and arms, the front of the legs, and the upper chest and shoulder region. The inflammation of these disorders can occasionally involve other organ systems, including the lungs, and may associate with Raynaud's phenomenon.