Taking a Deeper Look at Hip Strength and Knee Pain (Part I)

Today I’m revisiting the relationship between knee pain and hip strength. That’s because after this post, in which I mentioned a study that showed that strengthening muscles in the hip didn’t improve the biomechanics of runners, I got this comment from “Knee Pain”:

You've cited studies saying that these types of structuralist exercises don't help knees. I'm wondering if there are some other studies that show that these exercises to help knees? Otherwise, gosh, where are our physical therapists getting the guidance to tell us all to do all these hip/glut exercises? (I've been given that advice by three professionals helping me with my knee problem.)

This is a very good point.

It’s not as if a couple of physical therapists were lying in a meadow, gazing at cloud formations on a lazy summer day, and one said to the other, “What about the hips? Try strengthening the hips?” And the other replied, “Sure. Why not?” So they began treating chronic knee pain by focusing on stronger hips, other physical therapists blindly followed their lead, and this treatment gradually became accepted protocol.

On the contrary: Therapists giving you such advice might cite various studies of their own if pressed for evidence. For example, here’s a recent one that looked at 28 female subjects with patellofemoral pain (14 underwent hip strengthening, 14 constituted the control group).

It reaches what looks like a slam-dunk conclusion:

A program of isolated hip abductor and external rotator strengthening was effective in improving pain and health status in females with [patellofemoral pain] compared to a no-exercise control group. The incorporation of hip-strengthening exercises should be considered when designing a rehabilitation program for females with PFP.

Here’s another one -- actually it’s a couple of case studies, so only two subjects were involved. For both individuals, treatment “occurred over a 14-week period and focused on recruitment and endurance training of the hip, pelvis, and trunk musculature.” The result, in part:

Both patients experienced a significant reduction in patellofemoral pain.

Well, that does seem convincing! But let’s take a step back and ask, more deeply: What do such studies really show?

MINDING THOSE P’S and Q’S

First, let’s look at a slightly different kind of study, though it’s very much related. The researchers wrote up its results in the January 2008 edition of the Journal of Orthopaedic & Sports Journal Therapy. It goes by this rather dull name:

Hip Strength and Hip and Knee Kinematics During Stair Descent in Females With and Without Patellefemoral Pain Syndrome.

A prominent star on the online copy of the report signifies that it won an “excellence in research” award by a sports physical therapy group.

The researchers begin the journal article with a sort of embarrassing sidenote, if you’re a believer in the pre-eminent role of structure in causing injuries. For a while, there was a fascination with the relationship between the “Q angle” (the quadriceps angle, which shows the propensity for the patella to track improperly) and patellofemoral pain syndrome. The hypothesis: The size of the Q angle correlates with the incidence of knee pain (the larger the angle, the more problems). Women in particular, with their wider hips, are likely to have a larger Q angle.

However, “many studies have not supported the relationship between an increased Q angle and PFPS [patellofemoral pain syndrome],” we are told.

Oh well.

Moving along.

REACHING FOR A HOLISTIC THEORY 

The “Kinematics During Stair Descent” study delves into why hip strengthening makes sense as a treatment. Now this is actually a very important thing if you want a solid, holistic theory of the relationship between weak hips and bad knees. It’s one thing to show that strengthening the hips helps reduce pain (which we’ll return to later), but why?

Okay, remember the simple “x leads to y” explanation in my earlier post. I’m going to use that, except with some big words thrown in:

Hip abductor and hip rotator weakness --> too much hip adduction and internal rotation --> stress on the patellofemoral joint causing pain.

Again, the short form:

Muscle weakness in hip --> bad form --> knee injury

So, based on this analysis, what would you expect to find in people with PFPS? A couple of things: (1) weak hip muscles (2) bad form.

HOW THE STUDY WAS SET UP 

The study’s methodology appears pretty solid (to my relatively untrained eye).

The experimental group consisted of 18 females with PFPS who reported to the University of Kentucky Biodynamics Laboratory for testing. Each was matched with a healthy female (the control group), in terms of age, weight and height.

Subjects with PFPS were asked to rate their pain. Also, all participants had their leg strength tested, by a handheld dynamometer that was 99 percent accurate. For the researchers to be able to make careful observations about form, everyone in the study was videotaped with a seven-camera system as they descended a short set of stairs while wearing reflective markers at key locations on their bodies.

The results: Weakness in hip muscles was indeed found. The subjects with knee pain “generated 24 percent less hip external rotator torque and 26 percent less hip abductor torque compared to controls.”

So far, so good for the structuralist model. Then the problems start.

FIXING A PROBLEM THAT DOESN’T EXIST

The study’s other major finding, beyond that of weak muscles, undoubtedly made its researchers more than a tad uncomfortable:

Subjects with hip weakness did not demonstrate excessive hip internal rotation, hip adduction and knee valgus compared to controls.

Oops.

Remember our causal chain again:

Muscle weakness in hip --> bad form --> knee injury

In the structural analysis, strengthening the hips should work because it corrects the bad form that caused the injury.

But if there isn’t any evidence of “bad form,” what does that mean? Why are you trying to fix “bad form” if there’s no “bad form” to fix?

That’s a head scratcher, but there’s an even bigger revelation -- a real bomb -- that the authors of the report drop at the end.

THE STUDY’S BOMBSHELL

Before we get to that, imagine that I tell you that 90 percent of everyone with patellofemoral pain syndrome in a study is found to also have something I refer to as “x”. You might think, “Well, let’s find a way to get rid of ‘x’! That should correct their PFPS!”

Then if I told you “x” was “depression,” you might retort, “Of course they’re depressed! They have knee pain. Take care of the PFPS, and you’ll get rid of the depression!”

Cause and effect. It’s absolutely critical to get those in the right order.

Now, for the University of Kentucky study, check out this admission about the weaker hip muscles (my bold):

It remains elusive if such weakness was the cause or the result of PFPS.

CAUSE AND EFFECT: WHAT MAKES THE MOST SENSE?

That’s a hole big enough to drive two trucks through. Think about it. The patellofemoral pain syndrome subjects had weaker hip muscles. But what are the chances they had weaker quad muscles too -- and weaker other leg muscles as well? After all, we’re told that the average duration of their problems was 14.4 months, “indicating a chronic condition.”

What happens when you have a chronic condition that discourages you from using your legs and knees normally, so you use them less? The associated muscles weaken. That’s a powerful argument for PFPS helping to create weak hips, not the other way around.

And, if the structuralist model was correct, you’d at least expect to find evidence of bad form during the stair-descending exercise -- which wasn’t the case.

So the structuralist explanation for weak hips causing knee pain appears to be a long way from proven.

But let’s return to the original studies. They show that strengthening hips did reduce knee pain. So maybe your attitude is this:

Who cares why it works? Maybe it works for a different structuralist reason. Maybe it works for a non-structuralist reason. All that matters to me is it works! Why don’t I do it for that reason?

Next week: Why not, indeed? A look at the Big Picture when it comes to treatments for patellofemoral pain syndrome.

Update: Since writing this, I've found this good essay by Paul Ingraham, "Does Hip Strengthening Work for IT Band Syndrome?", in which he asserts "'weak hips' is a weak theory." Have a look!