(HealthDay News) -- Rheumatoid arthritis is triggered by an overactive immune system that attacks the body's own joints, causing flares of pain and stiffness.

The American College of Rheumatology says about 13 million Americans have the disease. The ACR recommends regular exercise, except during painful flares.

Here's more of its expert advice:

• When joints are painful or you feel tired, stick to gentle range-of-motion exercises to help keep the joints flexible.
• When you are feeling well, focus on muscle-strengthening and low-impact cardiovascular exercises.
• Work with a physical or occupational therapist to determine which exercises and intensity are best for you.
• Feelings of anxiety, depression or isolation are common. Your health care provider can help you work through these feelings, which tend to subside when you're feeling better.