Why That Study About Quad Strength Doesn’t Mean What You Think It Does

Last week I told The Odd Story of the HQ ratio, based mainly on this study.

This week we look at a different study (in the spirit of continuity, it made a cameo appearance last week).

What if you were watching TV and an announcer broke in and said:

News flash! The Pope is Catholic! Details at eleven.

Chances are good, you wouldn’t bother tuning in for that newscast.

A certain family of knee studies is pretty much the analogue of that "news." Here’s a specific example (and today’s topic): “Quadriceps Weakness Predicts Risk for Knee Joint Space Narrowing of Women in the MOST cohort.” (Knee joint space narrowing is significant because it signifies a loss of cartilage and accompanies the progression of osteoarthritis.)

This time, let’s start with the conclusion and work backward. That’s because you’ll probably think it means something that it doesn’t. And that part is important to get right from the outset.

Here’s the wording, which is fairly simple and straightforward (note: I’ve snipped an introductory phrase -- which I’ll return to, not to worry, for any careful readers):

Quadriceps weakness was associated with increased risk for tibiofemoral and whole knee joint-space narrowing.

Here’s how you probably translate that:

A patient who has knee pain should strengthen the quadriceps to prevent tibiofemoral and whole knee joint-space narrowing.

In a moment, we’ll look at why that’s a potentially dangerous misinterpretation. Before that, what is the family of knee studies that this belongs to?

Others that resemble it sound like this: Quadriceps Weakness Predicts Higher Levels of Knee Pain Among Osteoarthritis Patients. Quadriceps Weakness Predicts Worse Outcomes After Knee Surgery. Quadriceps Weakness Predicts Increase in Knee Cartilage Lesions. Etc.

So if quadriceps strength protects against all this bad stuff, why shouldn’t someone with chronic knee pain who has some of this bad stuff already, but doesn’t want it to get worse, strengthen the quads? What makes that a potentially dangerous misinterpretation?

The problem is, such studies don’t support that interpretation, if you just look at the raw data and the basic conclusions.

For example, the MOST cohort study essentially says this: If you start with two people, Jane with a quad strength of 80 “units” (I’m using this word as a shorthand; feel free to substitute in your favorite measurement) and Jill 40 units, Jill will be at increased risk of knee joint space narrowing.

The study is silent on whether, if both women have knee pain, Jane should try to strengthen her quads to say 120 units and Jill should try to reach 60 units, to lower their risk. Assuming the study implies this is potentially dangerous. Maybe quad strengthening exercises would damage their knees further!

(Brief digression: It’s probably time to capsulize the study. Here goes: a total of 3,856 knees were tracked over a 30-month period; subjects were 50 to 79 years old; all had knee osteoarthritis or risk factors for it.)

That greater quad strength, to some degree, protects knees seems like a yawner of a conclusion, like “discovering” the Pope is Catholic. When you suffer from constant pain though, the question isn’t do strong quads help -- it’s how do you get there, and more importantly, how do you heal your bad knees that hurt all the time. Unfortunately, there’s no such thing as a quad strength fairy who, with one tap of her magic wand, can bulk up your weak quads.

In the absence of such a fairy, you have to build up the muscles the old-fashioned way, through exercise. While such strengthening isn’t impossible when you have a tender, easily irritated joint, it’s much harder and much trickier. The worse your knee pain, the more difficult the task will be.

But listen: You’re chasing the wrong objective anyway! Physical therapist Doug Kelsey nailed this one: “Having stronger muscles is helpful but weak muscles are not the primary problem.”

It’s your bad knees that need fixing.

Two more things:

(1) What was the introductory phrase I left out?

The full sentence of the conclusion reads (my bold):

In women but not in men, quadriceps weakness was associated with increased risk for tibiofemoral and whole knee joint-space narrowing.

I omitted the women vs. men part because (1) I didn’t want to have to deal with that confusing aspect of the study yet (2) The reason the researchers cite for the difference -- namely, that men have higher strength and this may provide a greater reserve -- strikes me as being sensible. In other words, if the quad strength differences were large enough, you’d see the same effect in the male population too.

Still, including this omission only fortifies my side of the argument.

If quad strength was found to protect against joint space narrowing in women, but not in men, you might surmise that quad strength may not have that much of a protective effect after all. In fact, the MOST cohort researchers cite another study that “found no significant differences in mean quadriceps strength between women with and without tibiofemoral osteoarthritis worsening.”

So while I think it’s obvious that strong quads help shield knees from harmful impacts that lead to everything from knee pain to bone spurs, the advantage they confer may not be that great.

(2) The introduction for the MOST cohort study makes a telling confusion.

While drawing comparisons, the researchers cite two other studies. Pay close attention here. In one, “greater quadriceps strength was protective against lateral patellofemoral cartilage loss.” In the other, “strength training did not protect against tibiofemoral joint space narrowing over 30 months in subjects with definite knee osteoarthritis at baseline.” Why the discordant results, they wonder.

Did you notice that one of these three things is not like the others?

Two studies (I’m including the MOST cohort study here) basically say, “If you have stronger quads, you have a lesser risk of bad stuff happening in your knee joint.” But the proposition for the third study begins, “If you attempt to strengthen your quads ...”

Why is this such a critical distinction? Because this gets to the heart of the matter for chronic knee pain sufferers! They’re not spending a lot of time musing, “Hmm, I wonder whether, if my quads are stronger than Jim’s, and he has knee pain too, whose joints will worsen more in a couple of years?” Rather, they’re thinking, “What the heck should I do to protect and hopefully heal my knees?”

That makes the third study the most relevant, hands down. It’s not chasing the obvious and trivial, but setting out to answer the all-important question, “Will strengthening my quads help my knees?”

And its conclusion, remember, was that strength training didn’t help. (Actually, it’s worse than that. Here’s the study abstract and an interpretation of the results. Strength training actually contributed to joint space narrowing among subjects who had no X-ray-indicated osteoarthritis at the outset. One note to keep in mind though: participants were older, averaging 69 years of age.)

While having strong quads is unquestionably a good thing, getting to strong quads -- when you already have knee problems -- may not be so simple or so beneficial. Why? As I’ve already noted, strength training may overwhelm weak knees, hastening cartilage loss and osteoarthritic processes.

Coda: Once again, an objection from my imaginary critic.

All this is okay, but woefully incomplete. What do you ignore studies that do show quad strengthening is good for those with bad knees?

Obviously, only so much can be covered in a single post (even as long as this one turned out to be). But the reply to that objection is really here, in my big picture essay on evaluating treatments for patellofemoral pain syndrome.

I don’t think quad strengthening is necessarily a failure as an approach. Not at all. If your joints are robust enough, if the exercises are smart and not too strenuous, it may work fine. The problems, I think, arise with weaker knees.

So I return to the idea of okay-better-best. Quad strengthening is, in my estimation, somewhere between “okay” and “better” on the spectrum of knee pain treatments. But don’t you want what’s best? That, I’m convinced, is strengthening the joint first. After all, the joint is what’s weak.