Exercise is crucial if you have arthritis. But knowing just how much activity to do when you’re hurting can be tricky. After all, research has shown that moderate activity can help prevent the progression of arthritis and improve overall function. But while mild muscle soreness after a workout is normal, sharp pain during or immediately after can signal injury. And sometimes simply the fear of pain can keep you from wanting to do any kind of exercise at all. Here’s how to determine when it’s OK to work through exercise pain – and when it’s not.