Polymyositis & Dermatomyositis Support Group
Polymyositis (PM) is a type of chronic inflammation of the muscles (inflammatory myopathy) related to dermatomyositis and inclusion body myositis. The inflammation is predominantly of the endomysium in polymyositis, whereas dermatomyositis is characterized by primarily perimysial inflammation.
Exercise is also terrific for eliminating muscle stiffness. I ride a stationary bike. The bike exercise has improved my muscles in my legs.
If exercise makes your stiffness worse then it might be inflammation causing arthritis. Your doctor would be able to make a determination.