Knees can be a problem, even for those of you who do not exercise. They can give out when you least expect it, they can sound like you're grinding walnuts and they can even slide out of alignment.
The sliding kneecap is an interesting phenomenon, not just for the exercise students but also for the instructor. As an instructor, you may complete half of your aerobic portion of class when suddenly you see one of your students stop suddenly and calmly manipulate a kneecap back to its original position.
That sliding kneecap has a fancy name, either subluxation or dislocation depending on whether it has jumped the track totally or only in part.
Your knee gets its strong powerful movement from the extensor mechanism. This mechanism is a combination of the quadriceps muscle, kneecap and tibia. All working together, they can generate a force up to five times your body weight. And that little kneecap is only about one inch thick but can take a force of 1,000 to 2,000 pounds. The knee takes a large beating from sports activities, lifting, carrying weights, sudden starts and stops and off balance quick turns. Now, normally your kneecaps do not wobble side to side; they travel in a track with up and down movement. Keeping it stabilized is the muscular heads of the quad muscle plus the groove in the end of the femur, the knee joint capsule and the quadriceps tendon.
If the balance of your kneecap movement is off center, perhaps by having a shallow grove in the femur or having muscle imbalance, you may wind up with a sliding kneecap just from bending and straightening your legs.
When the kneecap jumps the track entirely, it is called subluxation. A partial is dislocation. These conditions can happen because of the overpowering of the outer structures of the knee, the capsule and the outer quadriceps.
Some people have naturally high riding kneecaps, they sit up too high on the leg just due to structure (tall thin). There are those small portions of people who have small kneecaps which inhibit the smooth riding of the kneecap. And also there are those people with a shallow groove for the kneecap to ride on.
You can test your own kneecap by sitting on the edge of a table and holding your kneecap while you straighten and bend your knee. You can feel if it doesn't travel a straight path, and if it moves more than a half inch either way, you have a loose kneecap. There are other more scientific ways to test for lose or sliding kneecaps. Check with your doctor if you have any doubt. It's better to spend a little time and money to prevent later damage. It might be as simple as wearing a knee brace or learning to strengthen the opposite muscle groups to even the pull on the kneecap. I watch in class for knees that bend in, or feet that do not hit the ground evenly. That can be a beginning indication of future problems.
Watch yourself in a mirror when you run and see how straight and even you pump those feet up and down. Stopping and relocating your kneecaps is a good conversation piece, but a doctor should advise you on your structural defect.
Jerry Vance is certified by the American Council on Exercise and teaches fitness at the Carson City Community Center and for the American Lung Association.