Houston Methodist. Leading Medicine

Knee News

It's Patellar Time

Pain under the kneecap is the most common knee problem in children and young adults. This condition is called patellofemoral pain syndrome (PFPS). The exact cause is unknown. For some reason, the kneecap is pulled slightly to the outside of the knee. This makes it move up and down unevenly over the leg bone and can cause pain.

Scientists are looking for a muscular cause of this problem. The quadriceps muscle in front of the thigh attaches around the kneecap. This muscle is made up of four parts divided into two main sections. These two sections are called the vastus medialis oblique (VMO) and the vastus lateralis (VL).

The VMO pulls the kneecap in, while the VL pulls it out. A balance between these two muscles keeps the kneecap in the middle. When there is a problem with the kneecap tracking properly, it is usually being pulled toward the outside. Exercise to strengthen the VMO and pull the kneecap more toward the inside has been tried.

However, there hasn't been enough proof to show that this really works. Researchers are using computers to help study this problem. With a custom interactive graphics program, it is possible to find out which part of the quadriceps muscle fires first. Computer analysis also shows how strongly each section of the quadriceps muscle is contracting. The results from normal knees are compared to knees with PFPS.

It looks like the timing of the muscles firing is the same between the two groups. The difference is the number of muscle fibers that fire at one time. For people without PFPS, an equal number of VMO and VL fibers contract at the same time.

For patients with PFPS, both sides of the quadriceps muscle contract more than in the normal group. There is also a difference in how these two muscles fire in the patients with PFPS. More of the vastus lateralis muscle is activated. This pulls the kneecap out to the side.

These findings give proof that patellofemoral pain is linked to a control problem within the quadriceps muscle. The next question to answer is: Can rehabilitation or exercise make a difference in the way the muscles are activated?


Tammy M. Owings, MS, and Mark D. Grabiner, PhD. Motor Control of the Vastus Medialis Oblique and Vastus Lateralis Muscles Is Disrupted During Eccentric Contractions in Subjects With Patellofemoral Pain. In The American Journal of Sports Medicine. July/August 2002. Vol. 30. No. 4. Pp. 483-487.

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