SACRAMENTO, Calif. - The joint won't be jumping if your knees aren't up to it. There won't be much running, walking, dancing, skipping or hopping, either.

For all these motions, you need your knees, but they can pay a painful price.

"Their main function is mobility, and there's an inverse relationship between mobility and stability," says Michael McKeough, a professor of physical therapy at California State University, Sacramento.

In other words, the more you move, the greater the risk of tweaking, twisting or otherwise stressing the knee joints that keep you going.

McKeough knows about this personally as well as professionally. A gymnastics injury in the 1960s led to surgical removal of the meniscus in both knees. The meniscus is a piece of cartilage that acts as a sort of pad between the thigh bone (femur) and the shin bone (tibia).

He was told to expect arthritis and having to use a cane. But, at 59, he has managed to stay active - biking at least 100 miles a week, skiing every year. And he does these comfortably.

"I've not had one day of knee pain," he says of his current regimen.

His secret: Keeping his leg muscles "really strong."

"You can increase stability with muscular strength," he explains. His favorite exercises are lunges and wall squats, which work the quadriceps.

The muscles that most influence knee function are the quadriceps and the hamstrings. Other muscles that get in on the act include the hip abductors and the hip adductors.

Imbalances of muscular strength and flexibility can lead to knee problems. McKeough says that, generally, men are more likely to suffer from overly tight hamstrings, while women are more likely to suffer from weak quadriceps.