In the last two months I have run a marathon and done a lot of telemarking - now my knees hurt. How can I tell the difference between just mild pain that will go away with rest, and pain that really means I've done something bad to my knees?

— Beth Bandy, Marblehead, Massachusetts

As athletic, outdoorsy types, we get used to a little discomfort now and then, but figuring out when you've crossed the line can be tricky. Joint pain is typically a result of some inflammation or swelling in or around the joint. This process is part of a normal response to stress: Think of it as a busy remodeling project-- lots of fluids running around doing their best to repair the damage to the joint. Most of the time the changes are so slight you don't notice, and in 24 to 48 hours you're back at it and the remodeling project is complete. Problems develop when the repair cycle isn't quite complete and you go out and stress the joint again, which brings in more reconstruction materials, more inflammation, and more discomfort.

If the onset of your pain was gradual, not a result of a twisting fall on your skis or a sudden stabbing pain while running, it can be classified as chronic in nature, and the best remedy is rest, ice and (if necessary) an anti-inflammatory such as Ibuprofen. One of the most common causes of chronic pain is from training errors: basically doing too much exercise. Rest typically takes care of the problem, but you'll want to recognize that there's a limit to how much your knees can handle and try to err on the conservative side of the activity scale. Also, be sure to check your gear, especially your running shoes, which usually have a life span of about 300 to 450 miles. If your shoes have had it, they may not be doing their job, which means extra work for your knees. If, after a reasonable period of rest-- say two weeks-- you still have the same level of pain, that could indicate something more serious. In any case, if you have any doubts, see a physician or physical therapist right away, have an examination, and heed their advice on how to get back on track.