Patellofemoral pain syndrome (PFPS) is the most common form of knee pain treated in physical therapy clinics. I made this series to help people better understand this condition, why it occurs and how to fix it.
In case you missed it:
Today we’ll be discussing how PFPS occurs and the factors that influence getting into pain (and eventually back out of it). PFPS is largely described as an issue of overuse (4, 11). If we get too much stress within the patellofemoral (PF) joint we can create some tissue irritation and as a result end up in pain. They key point here is the issue of “too much” stress. Again remember that the knee is designed to handle stress and that in general movement and exercise is good.
As described, certain activities place more stress on the PF joint then others. Activities like running and deep squatting place larger forces on the PF joint. Because of this PFPS is a common injury seen in runners and weightlifters.
Also remember how certain biomechanics of the hip, knee, foot and ankle all play a role in knee pain. If biomechanics are off, we run into decreased surface area in the PF joint to dissipate force as well as more stress on the lateral (or outside) portion of the joint. Make sure you check out part 2 again for a refresher on these concepts. The third aspect that is important to understand is the “capacity” of the knee joint.
Scott Dye is a famous knee researcher who first discussed the topic of “tissue homeostasis” and capacity of the PF joint. Wikipedia defines homeostasis as:
Homeostasis can be defined as the stable condition of an organism and of its internal environment; or as the maintenance or regulation of the stable condition, or its equilibrium; or simply as the balance of bodily functions.
Basically the PF joint is attempting to maintain it’s homeostasis like any other part of the body. Scott believes that a single insult or a period of prolonged loading to the PF joint can alter the joint’s homeostasis and create knee pain (11). It sounds like overuse of the knee joint fits the bill in this situation.
What’s interesting about the PF joint is that this initial insult can be prolonged indefinitely if not intervened upon (11). In other words, pain tends to stick around forever unless we do something about it (keep in mind that rest tends not to fix the issue) (13).
Scott described the knee’s ability to handle stress (the capacity of the PF joint) as the joint’s “envelope of function” (11). Basically, the knee joint can handle a certain amount of stress prior to it becoming injured and or painful. If we exceed this capacity then we end up with knee pain.
Let’s look at 2 examples below. One is a beginner lifter who just became interested in weight training and wants to start a beginner weightlifting program. In the image below the area in grey represents what the lifter’s knee is capable of handling before an injury occurs. The area in the white represents what the knee is not quite capable of and can cause injury:
As you can see this individual can perform stair climbing, squatting and running without any pain. All of these activities are within their envelope of function. However, they’re not quite ready to start a beginner lifting program. That’s right above their envelope of capacity. Put them into an elite training program and they’re gonna have some pretty angry knees.
What’s very interesting about the PF joint is that it seems we can increase the capacity of the knee joint with training over time. The greater the capacity of the joint, the greater tolerance we have before experiencing pain. This is potentially why you can have athletes who can perform a large volume of running or loaded deep squats throughout a training week and never experience any pain. They’ve built their capacity for it over time. If you give the same training load to a coach potato then you’ve probably just injured someone.
For the athlete depicted in the graph above they will require a period of training to increase their capacity prior to beginning a true program. Let’s imagine that we take the same lifter and train them over the course of 10 years. Now they’re an elite weightlifter. Their envelope of function has improved.
Our weightlifter has been building the capacity of the knee joint over the course of years and they are now able to handle much more volume, intensity and frequency of loading to the PF joint then when they started. Even though the stress on the PF joint is far higher in the elite lifter’s program their capacity has risen to match this stress. Therefore they continue to train within their envelope of function and maintain the homeostasis of the joint.
Now, what happens when 2 elite athletes run the same race and only 1 gets hurt? Well this can be simplified with the images below. Look at running athlete “A” below:
They have enough capacity to handle races up to marathon distance without getting injured. Let’s contrast this to athlete “B”
In this example our running athlete can handle most running distances below a marathon. Theoretically, once they decide to try a marathon they would end up in pain. This would be the patient who comes in with knee pain following a marathon that had no pain prior to the race (Let’s also theorize she did not train above 20 miles in her training prior to the marathon). The race exceeded her capacity and resulted in knee pain.
So to recap:
Now as you might have guessed, PF joint capacity is going to be influenced by a variety of factors. In the next article we go over what these factors are and how they can help you to build bulletproof knees.
Knee Pain for Life,
Dan Pope DPT, OCS, CSCS, CF L1
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