Polymyositis (pol-e-my-o-SY-tis) is an uncommon inflammatory disease that causes muscle weakness affecting both sides of your body. Having this condition can make it difficult to climb stairs, rise from a seated position, lift objects or reach overhead.
Polymyositis most commonly affects adults in their 30s, 40s, or 50s. Women are affected more often than men. Signs and symptoms usually develop gradually, over weeks or months.