The VMO, or vastus medialis obliquus, is the large tear drop shaped thigh 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 it is the only one that attaches directly to the inner (medial), upper margin of the patella.
For this reason, it pulls inward on the kneecap and opposes the slightly outer (lateral) pull of the other three muscles. 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 behind 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 (lateral side), and often because the VMO is weak and can’t counteract the pull of the other three quadriceps. Usually, this is accompanied by tightening or contracture of the soft tissues on the outer side (lateral side) of the kneecap, which makes it much harder to treat and correct 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, these factors can result in degenerative softening of the cartilage under the kneecap, a disease called 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 within the joint is also a source of knee pain and instability.
In every case, strengthening the VMO is critical to relieving knee pain and restoring knee joint stability. So, what exercises are best for doing that? Here again, we take our lead from the functional anatomy. Since the VMO muscle fully 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, to strengthen it.
Isometric exercises, technically defined as muscle contraction without shortening, those done with the knee fully straight, can increase VMO tone and strengthen its 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, but without any movement of the joint.
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, while maintaining the contraction, then 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 overt movement of the joint. This type of exercise can utilize the resistance of bodyweight only, or additional 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 (a “closed chain” exercise). This is especially true if you have kneecap disease, like chondromalacia patella, or patellofemoral 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 subjectively 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.