Marilyn asks: I am 41-year-old woman who enjoys 5Ks and half marathons. My doctor just told me that I have arthritis in my left knee. He told me to run less if I still want to walk when I am 50. He advises I swim or bike or do other exercises that have less impact on my knees. I have a half marathon planned for January and am sad about this news. What should I do?

This is a great question that affects many runners who are in their 40s and beyond. I am going to assume that you have osteoarthritis localized to your knees as you do not mention other joints.

Osteoarthritis of the knees has been linked to heredity, obesity, and joint trauma. If your parents, grandparents, uncles, aunts, brothers, or sisters have had knee arthritis, there is a good chance you will, too. If you have a BMI over 30, you are at risk. If you have had an ACL tear or knee fracture through the articular (bone end) surfaces, you will have arthritis in the knee joint at some time in your life.

There is not a lot of evidence to show that running either causes or makes the arthritis worse. In fact, a recent study found that runners have less knee pain.

Your physician must feel that you have advanced changes in your knee to suggest that you will not be able to walk in nine years. If you are already or nearly bone on bone in your knee joint, you may be a candidate for a knee replacement. Replacing a joint is a matter of timing: You want to do the surgery before you start to lose too much strength, but you want to try to get the most out of your own parts.

I would let pain be your guide. If it hurts, you will have to find another exercise substitute for running. Something that does not put high loads on the joint will help. The activities your physician suggested are good, and I would add elliptical, skating (if you are reasonably skilled), rowing, kayaking, and Nordic skiing to the list. Walking is also a fine standby exercise that helps you protect your heart, maintain your body weight, and maintain your balance as you age.

Weight is an issue for some people and the multiplier effect is clearly important for managing knee pain. Ten pounds of extra weight in the trunk is like 30 pounds through the low back, 50 pounds through the hips, and 70 pounds through the knee joints. A small amount of weight loss can relieve knee pain. Weight loss is best achieved with a combination of exercise and reduced food portions.

Strength and motor control of the pelvis and femur muscles will keep the knee joint aligned and can also reduce your pain. Think of it like tires on a car that are out of alignment and balance: The tires wear faster and unevenly, reducing tire life. A physical therapist or personal trainer can help you with a strength program.

I have many patients and friends (some doctors) who choose to continue running with osteoarthritis of the knee. They back off when the pain is bad and ramp up when they feel good. Several of them are heading for knee replacements, but the running helps their stress and weight management, along with their fitness levels.

If the knee is not bone on bone, you may be a candidate for hyaluronic acid injections. If you are bone on bone, corticosteroid injections can give you some pain relief and postpone your knee replacement. You can discuss both options with your physician.