Q. Post-50 after a lifetime of beating on my body in all sorts of ways — a dozen 10Ks, 10-milers, half-marathons and nine of the 26-plus-milers later — my knees, or the cartilage in them anyway, seemed to disappear. I had a torn ACL repaired and both knees scoped. I bike more now and know that's part of the answer to the following couple of questions. Two and a half years since the operations, I have given up running, but the knees are aching again. Is there anything else I should be looking into? I've never been fond of being indoors when it comes to workouts, but maybe it's time to spend time on some sort of machine — or do I just admit I really am old now and camp out in front of ESPN?

Q. Post-50 after a lifetime of beating on my body in all sorts of ways — a dozen 10Ks, 10-milers, half-marathons and nine of the 26-plus-milers later — my knees, or the cartilage in them anyway, seemed to disappear. I had a torn ACL repaired and both knees scoped. I bike more now and know that's part of the answer to the following couple of questions. Two and a half years since the operations, I have given up running, but the knees are aching again. Is there anything else I should be looking into? I've never been fond of being indoors when it comes to workouts, but maybe it's time to spend time on some sort of machine — or do I just admit I really am old now and camp out in front of ESPN?

A. Don't get too cozy with your remote control just yet. Hunkering down and packing on pounds will only make your injured knees more peeved, says orthopedic surgeon Craig R. Faulks, of Washington Circle Orthopedic Associates, whose practice handles mainly middle-aged athletes. Resting on your duff will also weaken the muscles around your knees, making it even tougher to get around. Time to get up and change the picture.

If you've hung up your running shoes but are still hearing protests from your knees, consider walking them over to a doc. Pain is the body's way of yelling, "Hey, you." It might be time to listen.

Once you get an all-clear, you can start to map out your post-running, low-impact exercise life together. Biking is a smart move, but don't forget about swimming — your timing's perfect for splashing down at an outdoor pool. Rowing sometimes gets a bad rep for being rough on the joints, but if your stroke is right (to avoid over-compression, don't move your knees past your feet ), it could become your new athletic obsession. Worried your stroke's not up to snuff? Make a video of yourself, upload it to YouTube and send it to Concept2 (www.concept2.com). Someone will critique you for free.

Angela Hart, rowing instructor at Gold's Gyms in Virginia, says she gets inundated with runners — including ultramarathoners — looking to cross-train. "You get the same flow of endorphins," she says. But you can do it without the same ouch factor.

It might be just what you knee-d. (Sorry, I know you're already in pain, but I couldn't help myself.)