Medical Studies About Bad Knees: Approach With Caution

Recently I came across an essay by an accomplished doctor about knee pain. I read through it with interest. Toward the end, he noted this Mayo Clinic study as evidence that strong quadriceps muscles prevent knee cartilage from deteriorating.

Now, let me get something out of the way: I do believe that having strong quads is helpful for knee pain sufferers. So what follows is written partly in a spirit of mischievous contrariness. But that’s okay, because today’s message is really about how to evaluate medical findings.

First, did you take a look at the abstract I linked to? Go ahead. I’ll wait. ;)

Now, with regard to preventing knee cartilage loss, what’s worth noting about this study?

The claim is very modest.

Let’s look at that section of the conclusion:

Greater quadriceps strength had no influence on cartilage loss at the tibiofemoral joint, including in malaligned knees. We report for the first time that greater quadriceps strength protected against cartilage loss at the lateral compartment of the patellofemoral joint, a finding that requires confirmation.

The claim isn’t that stronger quads protect against cartilage loss all across the knee joint. Or even that they protect against loss all across the patellofemoral part (where the femur and patella meet). Rather, benefits are restricted to the “lateral compartment” (the outer one, in other words) of the patellofemoral joint. And this finding “requires confirmation.”

At this point, I should backtrack a little: That’s really nothing to be alarmed about. Medical researchers write with an abundance of caution and interpret findings conservatively. That’s a good thing. Still, it shows the study didn’t prove that much (though, again, I do believe that quad strength does help protect joints and would expect the findings to be confirmed by other studies and perhaps expanded, in terms of the benefits).

The claim may not even be proven.

This is where I wish I had the full study to examine, but it’s behind a paywall. Oh well.

Anyway, consider what appears to be going on here, reduced to simple terms: An inverse correlation has been observed between two variables. When A (quad strength) is high, B (cartilage loss) is low.

So A inversely influences B.

Or does it?

Notice the assumption of cause and effect here: a higher A (quad strength) causes less B (cartilage loss). But what if cause and effect should be flipped around? Namely, more B contributes to less A?

Well, how would that work, you may be wondering. Consider this: The process of cartilage loss, especially in those with knee osteoarthritis (the members of this study), is generally inflammatory/painful, as fragments of cartilage migrate out to the nerve-rich synovium. So cartilage loss causes knee discomfort, knee discomfort discourages the sufferer from moving/exercising, and the inactivity leads to weaker quads.

Ergo, cartilage loss causes weaker quads! And so, by a little logic jiu jitsu, the conclusion is stood on its head.

Or consider another possibility: there is a larger force at work here, which causes both weaker quads and more cartilage loss. Let’s say it’s the disease process of osteoarthritis. Let’s suppose inflammatory factors associated with the disease cause more cartilage loss, and at the same time the pain of osteoarthritis results in less movement, and thus weaker quads.

In such a scenario, it wouldn’t make sense to speak of more A (strong quads) leading to less B (cartilage loss), or even the opposite, because they are both at the mercy of the intensity of the disease.

The claim isn't prescriptive.

A naive reader, after perusing this abstract, may conclude, “Oh, because I’ve got osteoarthritis, and strong quads protect against cartilage loss, I should build up my quads!” But the conclusion is observational (more A leads to less B), not prescriptive (if you have osteoarthritis, you need to exercise so that you’ll have more A).

In fact, that same point is basically made by this article that summarizes the study (my bold):

Although the study did not involve exercise training to strengthen the quadriceps, there have been several short-term studies that show that improving quadriceps strength has a beneficial effect on knee pain and function.

Ah, but you may be thinking, that even though the Mayo Clinic study doesn’t settle the issue, “there have been several short-term studies that show that improving quadriceps strength has a beneficial effect on knee pain and function.” So surely they settle it, right?

Maybe not. Because look at this line from the Mayo Clinic study:

While our findings suggest that maintaining strong quadriceps is of benefit to those with knee OA, further work is needed to determine the type and frequency of exercise regimen that will be both safe and effective.

This is the crux of the matter. Even if you’re convinced that strong quads protect weak knees, what’s the most effective way of gaining (or maintaining) quad strength when your knees hurt most of the time? What if, paradoxically, it’s better not to worry about quad strength for a while and strengthen your joints instead? For example, what if, in those studies showing improved quad strength benefiting knee pain and function, the subjects have a 60 percent success rate, while if they focused on improving joint strength (what I did), they might have a 90 percent success rate?

Some kind of exercise/motion is beneficial for bad knees, I’m pretty sure, but exactly what kind needs more rigorous scientific investigation. In the meantime, remember to read scientific studies carefully, and make sure you understand what is really being proven -- and what isn’t.