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 gradual muscle weakness, difficulty climbing stairs, lifting objects, or reaching overhead. Diagnosis typically involves blood tests for muscle enzymes, electromyography (EMG), and sometimes a muscle biopsy. Treatment often includes corticosteroids and immunosuppressive medications to reduce inflammation and improve muscle strength.