Creaky knees, achy joints: Working out in your 40s and 50s isn't the same as when you were in your 20s.

Not only is your body more likely to balk at things you did easily in your youth, but it's more prone to injury as well. Case in point: A recent study conducted at the University of California, San Francisco, found that people between the ages of 45 and 55 who participated in high-impact activities, like running, for more than an hour a day at least three times a week were more likely to experience knee problems, including osteoarthritis.

Don't think, however, that this gives you permission to take up residency on the couch.

Those joints need physical activity, no matter your age. "Exercise helps maintain range of motion in the joints and stimulates the production of joint lubrication, which decreases friction and wear on your joints," says Dr. Bal M. Rajagopalan, a double-board-certified (American and Canadian) orthopedic surgeon in Beverly Hills, Calif., who specializes in hip and knee surgery.

Exercise also helps strengthen muscles around the joints, which will ultimately protect you in the long run.

So how do you avoid achy joints and muscles and still keep your body safely in shape as you age?

Listen to your body.

Your body knows best, which is why you need to pay attention to it. If you feel sore a day or two after working out, that shouldn't raise any red flags. "But if you have acute pain when you're exercising, your body's telling you something's not right," says Rajagopalan. Stop the activity and see a doctor if the pain persists.

Cross-train.

The best way to keep doing the activities you love is to vary what you do. "I never tell people they have to quit their favorite activities just because they're getting older," says Rajagopalan. Of course, if you're having pain during a specific activity, that's a different story, he adds. But if your joints aren't protesting and you love playing tennis, for example, try alternating with hiking or yoga. "By cross-training, you work different muscles and avoid putting repetitive stress on the body, which can lead to injury," he says.

Vary your impact.

High-impact activities, like running and playing tennis, place a lot of stress on your body -- especially your joints -- whereas low-impact activities like walking, cycling and Pilates place little stress on the body. In fact, people in the above study who did light exercise, like walking, had the healthiest knee cartilage. So if you're always doing high-impact activities, then give your body a break and swap in some low-impact activities a few times a week.

Modify your routine.

Tough as it is to accept, there may come a day when injury forces you to give up an activity, perhaps even permanently. But don't despair. This doesn't mean you're completely sidelined. Work with your doctor to come up with alternatives, advises Rajagopalan. For instance, if running is taking too big a toll on your knees, then try taking it down a notch and walk or use an elliptical machine instead. By finding similar, more joint-friendly activities, you can respect your body, avoid achy joints and still enjoy the benefits of a healthy, active lifestyle at every age.

Copyright © 2011 Studio One Networks. All rights reserved.