The Odd Tale of the HQ Ratio

My latest post lashing out at physical therapy for obsessing over “quad strengthening” (to treat knee pain) prompted this comment:

There is a lot of evidence that weak quadriceps lead to increased knee pain. Ditto for *tight* quadriceps, which are often caused by over-exertion of weak quadriceps. Below are some sources from scientific journals.

There are a lot of causes for knee pain. For chrondomalacia patella, the root cause is often a misaligned patella, improperly tracking over its surrounding cartilage. I argue, based off of my own experience and research, that a first step towards addressing this misalignment should be achieving an optimal balance of strength between the hamstrings and quadriceps.

A couple of links to studies followed. So I thought, “What the heck. Why not see where this leads?” What I discovered was so remarkably unremarkable that I felt compelled to share it with everyone.

Today’s installment: The Odd Tale of the HQ Ratio.

Way back in 1988, when a first-class stamp cost 24 cents and the drug Prozac was a brand-new treatment for depression, a Finnish researcher by the name of Pekka Kannus published a study: “Ratio of Hamstring to Quadriceps Femoris Muscles’ Strength in the Anterior Cruciate Ligament Insufficient Knee: Relationship to Long-Term Recovery.”

Okay, the title is a bit dense.

The “Hamstring” and “Quadriceps” part refers to a strength ratio between the muscles, known as the (what else?) HQ ratio. (Quick aside: the deeper I get into structuralist hypotheses for chronic knee pain, the more their quest begins to look like a decades-long Easter egg hunt for the structural deficiency that explains away much of the problem.)

Kannus decided to investigate whether an optimal balance of strength exists between the hamstring muscles and quadriceps femoris. His study included 41 subjects, from top-level athletes to nonathletes, who had an anterior cruciate ligament injury to one of their knees.

An average of 8.2 years after the injury, he performed various measures of strength of their hamstring and quad muscles. He also noted the condition of their bad knee, both functionally (Do they walk with a limp? Do they have problems climbing stairs?) and subjectively (Do they often feel knee pain?)

His objective was simple:

The main hypothesis was that the patients with an optimal absolute HQ ratio (50% - 80%) have better outcomes than other patients.

So, to step back for a moment, here was the structuralist paradigm for the HQ ratio at the time:

There is an ideal balance between the strength of a patient’s hamstring and quad muscles, which falls within a range, where the hamstring should be 50% to 80% as strong as the quadriceps femoris. When the ratio lies outside this range, the patient will have more knee problems.

And (drumroll, please), the study found ...

Nothing of the kind.

First, Kannus discovered that the HQ ratio varies a lot naturally, even for healthy knees. Second, and to the point, he learned “absolute HQ ratios do not correlate with long-term outcomes.” So, whether your HQ ratios are 65% (in the middle of the “ideal” range) or 30% or 150%, this has no bearing on how your knee fares. He ends on a rather sheepish note: “No general recommendation can be made about an optimal absolute HQ ratio as a goal for rehabilitation.”

But wait, there is something else.

Kannus did find that the HQ ratio of a subject’s bad knee was consistently higher than that of his good knee (meaning the hamstring muscles were stronger relative to the quads). So even though he couldn’t support the contention that an HQ of 65% in one individual is better than a 30% or 150% ratio in another, he could assert that the appropriate HQ for the injured-knee leg may be that of the same person’s uninjured-knee leg.

Which is ...

A big nothingburger in the Great Deli of Knee Pain Studies.

Why is this conclusion all air, no meat? Kannus himself tells us in his write-up of the research (my bold):

“The higher HQ ratios on the injured side ... resulted from the weak quadriceps femoris muscle ... computerized tomographic studies of cross-sectional areas of atrophied thigh muscles have demonstrated that atrophy of muscle tissue is greater in the quadriceps femoris muscle than in the hamstring muscles after knee ligament injury.”

Got that? After such an injury, this quad muscle weakens faster than the hamstring does. So what would you expect to find in subjects? Hamstring muscles that, relative to the quadriceps, are stronger in the injured knee than in the uninjured one.

Now, did the HQ ratio die an ignominious death after Kannus failed to prove his hypothesis?

Of course not.

Check out this study from 2010, more than 20 years later! It had many subjects (almost 2,000). So sample size wasn’t an issue, unlike in the much smaller Kannus study. Among other things, researchers looked at the relationship between the HQ ratio and knee joint-space narrowing in a population that either had either knee osteoarthritis or known risk factors for it.

The study found (another ironic drumroll, please) ...

No evidence that the HQ ratio is linked to the risk of narrowing space in the knee joint.

Hmm. How much do you bet someone will still be studying the HQ ratio 20 years from now? :)

Coda: Now for an anticipated objection:

This study isn’t a “nothingburger” -- far from it! Kannus made a valuable finding! He showed us that the goal of physical therapy is to restore the HQ ratio of the injured leg to that of the non-injured one. This gives us an identifiable, concrete objective.

So what is wrong with using the HQ ratio as a relative, not absolute, guide? For starters, here are two things:

(1) There are a number of muscle ratios you could focus on, and a number of reasons you could come up with for doing so -- so why this one?

Once your knee is bad and you can’t use your leg properly, many muscle ratios probably get out of whack. What if the hip muscles become relatively stronger than the quadriceps femoris? Why not focus on correcting that ratio instead? I’m sure you can concoct a theory why that would make sense. Here’s one off the top of my head: The muscle imbalance draws the kneecap off its proper tracking pattern, causing a misalignment.

But then again, a creative structuralist might conclude it’s not the hip-to-quad strength ratio that matters most. Maybe it’s the calf-to-quad. Or hamstring-to-gluteus. Or gluteus-to-quad. Or hamstring-to-hip.

The possibilities are endless once you go down the structuralist rabbit hole.

(2) Cause and effect! It’s more likely that bad knees cause a distorted HQ ratio, rather than the other way around. So an out-of-whack HQ ratio means little; it’s just a symptom of having an injured knee.

Here’s an illustration to drive home the point: Suppose I discover something I call the Leg Irregular Muscle Propulsion ratio. This proportion is derived by measuring a subject’s fluidity of motion during ordinary walking, compared with a perfectly smooth gait. The lower the LIMP ratio, the closer a leg is to walking normally.

Further, let’s say I observe that, for a healthy individual, the LIMP ratios of their right and left legs are very similar. But among subjects with knee pain, the LIMP ratio is higher in the injured leg.

Ostensibly, as a physical therapist, I now would have a goal: Restore the bad LIMP ratio to that of the good knee. Teach the patient to walk more smoothly. But that ignores the fundamental reason he doesn’t walk more smoothly in the first place. He has knee pain!

Kind of silly, huh? If you want to fix the LIMP, why not focus on improving what’s causing the LIMP, the bad knee?