I'm putting a lot of stress on my knees at Crossfit. A friend of mine warned me about Crossfit several months ago; he said he tried it and "blew his knee out." I talked to Dr. Mary about it, and she said, to the contrary, exercise will strengthen the knees.
So I googled the question, and got this.
The following is, furthermore, from the Livestrong website:
The long-held belief was that exercise aggravated joint cartilage loss
due to the progressive nature of osteoarthritis. In 2005, Swedish
researchers Leif Dahlberg and Ewa Roos utilized MRI technology to find
evidence that exercise improved joint symptoms and function, as well as
knee cartilage quality, in osteoarthritis sufferers. An exercise group
enrolled in a supervised program of aerobic and weight bearing exercises
three times weekly reported improved functional performance and gains
in physical activity.
Pretty good article here, too.
At my box, the coaches are careful about the knees. For example, in doing the squat, whether by itself or as part of another exercise, one keeps the front of the knee behind the front of the foot, the lumbar curve is flexed, one's chest is high, one's "butt" (a technical term used in Crossfit that my mom would never let me use) is sticking out, and the weight is back on the heels. The coaches say that this should keep the knees from getting injured.