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The VMO-The Key to Pain Prevention and Knee Stability

The VMO, or vastus medialis obliquus, is the large tear drop shaped muscle that lies just above and on the inner side of the kneecap, or patella. It is one of the four quadriceps muscles in the front of the thigh, but the only one that attaches directly to the inner, upper margin of the patella. For this reason, it pulls inward on the kneecap and opposes the slightly outer pull of the other three muscles (see Figure 1). The VMO is most active in the last 30 degrees of extension, meaning that it helps to lock the knee out fully straight. So, you can see that this one muscle has a strong influence on both the kneecap and knee stability, as a consequence of its anatomy.

Pain in the kneecap usually results from a direct blow or a fall, or it may result from maltracking, meaning that the kneecap is pulled off its normal path, usually to the outside, and often because the VMO is weak and can't counteract the pull of the other three quadriceps. Often, this is also accompanied by tightening or contracture of the soft tissues on the outside of the kneecap, which makes it much harder to correct and the problem. Some combination of these factors is usually involved in kneecap pain, which can also be accompanied by instability, or giving way of the knee. Over time, this can result in degenerative softening of the cartilage under the kneecap, or chondromalacia patellae.

The other major source of pain in the knee comes from within the joint, either from a twisting injury that may injure a meniscus, one of the cushions within the joint, or a sprain of one of the ligaments, or an effusion, or "water on the knee," which results from inflammation of the lining membrane of the joint, the synovium. And, in the older population, degenerative arthritis in the joint is also a source of pain and instability.

In every case, strengthening the VMO is critical to relieving pain and restoring stability.

So, what exercises are best for doing that? Here again, we take our lead from the functional anatomy. Since the VMO muscle straightens the knee, and is most active in the last 30 degrees of extension, we want to load the muscle within that range of motion.

Isometric exercises, technically defined as muscle contraction without shortening, those done with the knee fully straight, can increase VMO tone and strengthen the tendinous attachments. For example, quad setting is done by tensing the straight leg, contracting the quadriceps, as hard as you can, for six seconds. This is particularly important when you have a fresh injury, or in the first few days after knee surgery. It adds strength, without movement.

Similar in effect is straight leg raising. Lying on your back, with the opposite leg bent up (to straighten your spine, and relieve stress across the lower back), you tense your quads, and maintaining the contraction, slowly raise the straight leg up to about 45 degrees. Then, slowly lower it again. To start with, try for 3 sets of 15-20 repetitions. This, too, is an exercise that is very useful in the immediate aftermath of an injury or surgery.

Isotonic exercise is muscle contraction with shortening and what we normally think of as exercise, with movement of the joint. This can involve the resistance of bodyweight only, or external resistance, like free weights or an exercise machine. The simplest exercise is a quarter squat. With or without weights, with your back against the wall, simply squat down just a quarter of the way, approximating about 30 degrees of knee flexion, then stand straight and tense the quads strongly. Even full range movements, like Full Squats or Hindu Squats, can be converted to VMO strengthening exercises by that isometric tensing of the quads, with the knee fully straight, with each repetition. My personal favorite for working the VMO is the Hack Squat. In this exercise, you hold a barbell or two dumbbells behind your knees and leaning backward to put maximal stress on the VMO, do quarter squats. It's helpful to have a wooden block under your heels to help with balance.

Machine exercises are also useful, but leg press machines are probably better for your knee than leg extension machines, because the leg press loads your knee from above, and is more physiological. This is especially true if you have kneecap disease, like chondromalacia, or arthritis. Here again, you can handle very heavy weight eventually, because you're using it for a very short arc, just 30 degrees.

Whatever exercises you choose, you will begin to notice an improvement in your pain and your stability within about 10 days, and real improvement within 30 days. As the VMO becomes stronger, your patella tracking will improve and your knee will feel more stable. Although this regimen can't cure everything, it can go a long way toward making your knees feel better and stronger. Give it a try.