Myositis is inflammation of your skeletal muscles, which are also called the voluntary muscles. These are the muscles you consciously control that help you move your body. An injury, infection or autoimmune disease can cause myositis.
The diseases dermatomyositis and polymyositis both involve myositis. Polymyositis causes muscle weakness, usually in the muscles closest to the trunk of your body. Dermatomyositis causes muscle weakness, plus a skin rash. Both diseases are usually treated with prednisone, a steroid medicine, and sometimes other medicines.