Polymyositis is an inflammatory muscle disease that causes weakness in the skeletal muscles, which are responsible for movement. It primarily affects the muscles closest to the trunk, such as those in the hips, thighs, and shoulders. The exact cause is unknown, but it is believed to involve an autoimmune response, where the body's immune system mistakenly attacks its own muscle fibers. Symptoms of polymyositis include muscle weakness, fatigue, and difficulty swallowing or breathing in severe cases. Diagnosis typically involves blood tests, muscle biopsies, and imaging studies. Treatment often includes corticosteroids and other immunosuppressive medications to reduce inflammation and improve muscle strength.