"Any exercise is good," says women's sports medicine expert Lisa Callahan, MD, "but some exercises are easier on your knees than others." So if you've been having knee pain, it doesn't mean you have to stop exercising—it's just time to try a lower-impact activity. Here's a guide to choosing the right exercise for your knees.

High Impact
Kickboxing / Running / Dance that involves jumping, especially aerobic dance / Downhill skiing / Tennis / Handball / Basketball / Volleyball / Soccer / Tae-bo

Medium Impact
Hiking / Walking / Stair machine / Dance in which your feet stay on the ground—for example, ballroom dance

Low Or No Impact
Most weight training / Swimming / Bicycling / In-line skating / Low-impact aerobics / Water aerobics / Walking in a pool or water jogging / Cross-country skiing / Cross-country ski machine / Yoga / Pilates / Elliptical trainer / Upper-body ergometer (UBE)