Strengthening the Knee: A Biomechanical Approach

Anterior knee pain is so prevalent, almost everyone knows someone dealing with it.  Most likely it is keeping them from doing the activities they love; things like running, going to the gym, playing basketball, or lifting up their grandchild. Anterior knee pain due to abnormal forces going through the patellofemoral joint is estimated to affect 1.5-7.3% of the general population. That’s about 25% of people with knee pain. Patellofemoral pain syndrome affects women more than men, due to their anatomical and biomechanical differences, and it has the highest rate of incidence in the 50-59 year old age group.

Knee Injury

It is extremely important that anyone dealing with patellofemoral pain have strong muscles surrounding the knee, and one of the most important is the quadriceps muscle.  But here in lies the challenge because for many people it is painful to perform quadriceps strengthening exercises at the gym like leg extensions or squats. The reason these exercises can be painful for people with patellofemoral pain is that contraction of the quadriceps loads the patellofemoral joint.

The patellofemoral joint consists of the patella or knee cap which sits in a groove called the trochlea on the femur or thigh bone. The patella is a sesamoid bone, which means it is not connected to the joint by ligaments, but it just sits within the quadriceps tendon. Because the patella is located within the patellar tendon, it can move up and down or side to side within the trochlear groove depending on the degree of quadriceps contraction, knee angle, and how the femur is moving underneath the patella. The gluteal muscles control the side to side and rotational movements of the femur and can help keep the femur in a good spot so the patella sits congruently in the trochlear groove.

Patellofemoral Joint

Patellofemoral knee pain is due to an increase loading of the joint, which can be due to several factors. One such factor is abnormal knee alignment due to weak gluteal muscles or poor motor control. If the femur is not in a position where the patella sits congruently in the groove, then the surface area of contact will decrease leading to an increase load on that area of the joint.

This concept can be represented by taking a hard cover book, placing it on your leg, and pushing as hard as you can. The force is evenly distributed over a large area and it doesn’t cause much discomfort. Now if you grab a pen and pushed it as hard as you could into your leg, mostly likely you’d end up with a puncture wound and it would hurt a lot. The pressure you applied was the same in each case, but the area of contact changed which led to two very different feelings. The same applies with your patellofemoral joint.

It can be quite difficult to learn how to keep your knee in good alignment, but physical therapists train clients how to do this every day. If you are suffering from anterior knee pain, it is highly recommended to seek out professional care from a physical therapist in order to learn how to strengthen your gluteal muscles and keep your knee in good alignment with daily/recreational activities.

Another factor that can lead to increased loading of the patellofemoral joint is contraction of the quadriceps muscle. This contraction leads to compression of the patella into the trochlear groove causing pain when the underlying cartilage is irritated and the force becomes too great. However, it is extremely important to have strong quadriceps in order to maintain a congruent relationship of the patella in the trochlear groove as well as allow for a stable knee during squatting and recreational activities.

Knee Pain
So how can you strengthen the quadriceps without over-aggravating the knee?

An excellent article published by Chris Powers in the Journal of Orthopedic and Sports Physical Therapy looked at weight bearing and non-weight bearing quadriceps strengthening exercises to see how they loaded the patellofemoral joint.  The article found advantages to both weight bearing exercises like squats and non-weight bearing exercises such as knee extensions. Weight bearing squats are more functional and they allow for contraction of multiple agonist and antagonist muscle groups, while non-weight bearing knee extensions required less co-contraction of muscles and thus isolated the quadriceps better.

When looking at patellofemoral joint loading, the article found that the squat placed higher stress on the joint than knee extensions at 90, 75, and 60 degrees of knee flexion; however, knee extensions placed higher stress on the joint at 30, 15, and 0 degrees of knee extension. Thus, it was recommended when strengthening the quadriceps for patients with patellofemoral pain syndrome, they should perform squats from 0-45 degrees of knee flexion, and then perform leg extensions from 90-45 degrees of knee flexion.

Knee Flexion

The reason for this increase in joint loading as the leg gets straighter for knee extensions is that the patella does less surface area contact when the leg is straight, and it increases as the knee bends up to 90 degrees due to more congruency of the joint. The long lever arm and low surface area contact means patellofemoral joint loads are high from 45-0 of knee extensions and low for squats. Although the surface area increases from 45-90 degrees during a squat, the direction of pull and force of the quadriceps leads to increased joint loads for squats between 45-90 degrees.

If you or anyone you know is suffering from anterior knee pain during daily tasks or recreational activities, maybe it is time to have a physical therapist evaluate your knee in order to determine if the pain is due to patellofemoral pain syndrome. Here at competitive edge physical therapy, we take a very data driven and biomechanical approach to treating clients and athletes with this problem. Using EMG biofeedback, we can visually see how well the muscles are contracting during exercise and know the best way to strengthen without causing pain. If you want to learn more about how physical therapy can help improve your knee pain and get you back out on the field, feel free to email, call, or visit our clinic in San Jose, CA.

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