Over the years, I've spent a great deal of time trying to figure out exactly what knee conditions affect which muscles. When I initially started my investigation, I expected the research to show, for example, that arthritis caused problems mainly for one or two muscles, while a torn ligament took its toll on a completely different muscle.
What I found, however, was rather surprising as it quickly became apparent that for just about every single knee condition I was checking out (such as arthritis, meniscus tears, etc.) the SAME muscle kept cropping up over and over as being the one most affected. And just what muscle was it?
The quadriceps muscle. Its a large one, sits on the top of your thigh, and plays a HUGE role in keeping your knee stable. Perhaps the most interesting things about it was that when you have knee problems, it just simply stops working the way it should. And this, of course, creates further problems...
So why does your quadriceps muscle stop working properly when you have knee problems? Well, one reason is having swelling in the knee joint.
Experiencing swelling in your knee has been shown to be a major reason why your quadriceps muscle might not be working like it should. Let me explain.
If you were to lie flat on your back as I injected a harmless salt water solution known as saline into your knee, you would find that after a certain amount of this fluid entered your knee joint, you would no longer be able to lift your leg off the table. Then, as I removed the saline a little at a time, you would slowly start to regain control of your leg once again and be able to lift it. Here's what some of the studies that have looked into this unusual effect of having extra fluid in the knee joint have found...
-one study took patients with chronic (long-term) knee swelling, removed the excess fluid using a needle, and measured an immediate increase in quadriceps strength (Fahrer 1988)
-another study demonstrated that even if you have as little as 20 milliliters of fluid in your knee (which is just over one tablespoon), it's enough to decrease the strength of your quadriceps muscle (Spencer 1984)
As you can see, it takes a whole lot less than a big ol' swollen knee to prevent your quadriceps muscle from working properly. According to the research, having just over at tablespoon of fluid in the knee- probably too small an amount for most people to visibly notice- is sufficient to trigger a problem with your quadriceps. You can find out more about correcting problems with the quadriceps muscle in Treat Your Own Knee Arthritis.