Polymyositis is a rare disease of the muscles. It usually affects the muscles closest to the trunk of the body. However, it may affect muscles anywhere in the body. The muscles become inflamed or swollen. This causes pain. The disease is progressive and starts slowly. If untreated, the muscles gradually become weaker. The pain in the muscles also increases.

Front Muscles of Trunk
Trunk Core Muscles
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Polymyositis may be caused by a combination of genetic and environmental factors that trigger an abnormal immune response.