Why a Quad Index Matters

I test the quads on every knee patient who is returning to sport. Actually, I test a lot of things but it all starts with the quads. As I have said many, many times before:

“It’s the quads until it’s not the quads.”

This is especially true after ACL injury.

Always always start there. The isolated quad index test, almost always measured via some variation of leg extension, compares max effort quad output of the involved knee to the uninvolved knee.

But why test it? It is not a functional!

(I like using “functional” as a stand-alone noun or verb to exaggerate the silliness of this word)

Let’s get down to a couple first principles here. From a biomechanical standpoint, the quad has only one job to do: It generates the “internal knee extension moment”. From an application perspective, it is the antagonist, essentially the ONLY antagonist, to any “knee flexion moment”. Again, from an application perspective, quad counters an “EXTERNAL knee flexion moment”.

I know what you’re thinking. “That is a big sounding phrase and I’m not very smart.” Not to worry! It’s not that complicated and I’m probably much stoopider than you.

So, let’s break it down so we can understand what this phrase means. Let’s start with the last word “moment”. All that word means in this application is “rotational force” and is usually described as “torque”. It is the way we refer to the forces that try to turn joints. Simple.

Let’s go to the word “knee”. That just tells us what joint the moment is acting upon. If that is confusing to you, maybe stop here and go play with something shiny…

The next word is “flexion” which describes the direction of rotation that the moment is trying to achieve – bending the knee in this case. You might think that flexion and extension are our only options for moments at the knee but you can also have internal/external rotation, abduction, and adduction moments as well. They just don’t result in much movement unless the moments overwhelm the constraints (like ligaments and stuff) and then, well, SNAP! The knee shouldn’t bend that way…

Finally we have “external”. It simply answers the question, “From where is the moment originating: Inside or outside the body?” Internal moments are typically generated by muscles (some internal moments can be generated by passive structures but don’t worry about that now). External moments are usually coming from the Ground Reaction Force (GRF) but not always. For example, colliding with another athlete generates external moments as well.

So “external knee flexion moment” can be restated as a “knee bending force coming from outside the body”. Simple right?

Now you can make this moment smaller by changing one of two things: Reducing magnitude of the GRF (or whatever is the generator) or reducing the size of moment arm. Reducing magnitude of the force requires changing the athlete’s mass (probably not going to happen in a split second) or changing their rate of velocity change (“make me slower” is not a common request from most athletes).

This leaves us with reducing the size of the moment arm. The moment arm is the distance of the force vector (like GRF) from the axis of rotation (the joint). That’s why a weight feels heavier when you hold it at arm’s length as opposed to holding the same weight at your side. By reducing the moment arm, you reduce the moment. Because math. More on this later…

Who cares? As long as they can squat, jump, hop, and change direction…that’s all that matters right? They can functional all over the place so problem solved!

Hang on. I’m sorry. I’m making a pork roast on a spit and I need to check on it. It’s Cuban style, marinated in mojo criollo overnight so it has that citrus/garlic/black pepper thing going on and I don’t want to mess it up. I’ll be right back. Here, watch this video of tiny hamsters eating tiny burritos while I’m gone.

I’m back it’s all done but needs to rest a little bit and the rice and beans aren’t done yet either so I have time to finish this post. Where was I?

Oh yeah! Quads. Remember that I said that the quad is essentially the only thing that can counter (antagonist) an external knee flexion moment? Now, if you can’t COUNTER a large external knee flexion moment then you can do one of two things: FALL DOWN or REDUCE THE SIZE OF THE MOMENT. Since falling is the opposite of a functional, the only option would be to reduce that external knee flexion moment so the athlete doesn’t have to generate an internal knee extension moment.

ENGLISH TRANSLATION: Since they got no quad they need to find a way not to need the quad

This means that they must compensate.

“Ah yes! They will go into valgus! We just need to make sure that they don’t go into valgus when they functional!”

Dammit to hell. No. No! NO!!!!! Valgus does not REDUCE AN EXTERNAL KNEE FLEXION MOMENT! That is a result of a knee abduction moment which is a thing but not THE thing we’re dealing with…yet.

They need to bring the SIZE of the external knee flexion moment to equal or less than the SIZE of the internal knee extension moment they can generate (quad “strength”). Remember I said earlier that in order to reduce the external knee flexion moment the athlete has only three options:

Option 1. Reduce the athlete’s mass
Option 2. Reduce the athlete’s rate of velocity change (make them move slower)
Option 3. Reduce the external knee flexion moment arm

Options 1 is limited here. Obviously they can lose weight in general over time, but it is not a strategy to deal with forces coming at them in the middle of a change of direction task. Unless your athlete is some kind of shape-shifting lizard thing this is not a real option (please contact me if you do have an athlete who is a shape-shifting lizard thing).

Option 2 is common during rehab or sport-specific drills. They will go out there and functional at lower speeds making the movements look better. But this won’t help them during full speed/max effort which is usually a goal of most athletes (admittedly not all).

The athlete is left with Option 3. Option 3 does not require a change in mass and it allows a higher amount of velocity change allowing them to have more functional.

Hang on. Pork is getting cold. I’ll be right back. MORE HAMSTERS!!!

Ok. I ate the pork with some rice and beans, but I also had two margaritas, so I might get a little sloppy here. We were talking about why Option 3 is shit problematic.

To explain Option 3 better, we need a real-life sporting activity. Let’s use a very common scenario.

The athlete is sprinting max effort and in a split second decides that they need to change directions. In order to do that, they need to QUICKLY change their velocity in one direction (decelerate), pivot, then change it into another direction (accelerate). Let’s focus on Step 1: Deceleration.


The athlete has momentum (a head of steam) going forward. In order to change that momentum, they need to create a force that is EQUAL(-ish) AND OPPOSITE to their momentum (because Newton’s third law). This force that they create is a reaction to the athlete planting their foot into the ground. It is a “ground reaction force”, if you will. (You see what I did there? That deserves another margarita!)

Below is a simple picture of this that I use in my courses. The fat blue vector represents the athlete’s momentum. The fat red vector represents the resulting GRF. The smaller red line is the length of the external knee flexion moment arm. This results in a really large external knee flexion moment. This is VERY similar to coming to a stop on hop tests (full head of steam and then STOP). More on this later…


So, what does the athlete do in this situation? Well, if they have a well-functioning quad on that leg, they counter the large external knee flexion moment with an even larger internal knee extension moment (via quad contraction), absorbing all of that energy and redirecting it into the new direction. This is a high-performance move that requires a certain amount of quad strength. But if they don’t have much of a quad, then what?

They’re left with Option 3: Reduce the external knee flexion moment arm. How will they achieve that? What is most commonly seen is what is called a “hip strategy”. They keep the knee straight and flex hard at the hips. This throws center of mass forward and downward creating a smaller external knee flexion moment reducing the demand on the quads.


But that force has to go somewhere. It ends up creating a very large external hip flexion moment.


First: Look at the knee. It is close to straight. The most common position for the knee to be in when an ACL ruptures. It typically occurs between 20 and 30deg of knee flexion (the same position you put the knee in to do the Lachman – FOR A REASON).

Second: This is Step 1 of a multi-step task. Step 2 of a rapid change of direction is to initiate the pivot. Pivoting on a straight/stiff knee at a high rate of velocity change is not a good situation.

Third: They are asking A LOT out of their hip musculature. If that gets overwhelmed it needs to compensate AGAIN via some other shift like lateral trunk lean which can now increase the knee abduction moment arm – I told you that would come up again. Now add a pivot.

Fourth: They can use this strategy to pass your hop tests – once they learn how to do it. It takes a while, so they often won’t pass the tests until late in rehab making it LOOK like they’ve progressed…because they can now functional.

Ok. You have questions…

Couldn’t I just functional and look for valgus, then train them not to do that?

No. Read the post again. Also read this post where I talk about nothing but valgus. Valgus in a deep knee flexion position, although ugly as fuck not pretty to look at, is probably less of an ACL risk than valgus with a straight knee (you may or may not have other problems there – read the other post).

And if you are actually seeing straight-knee valgus with your own eye-holes then the athlete needs to back way off on the magnitude/intensity of that task! They are not ready. And, fearing that I’m pointing out the obvious here, trying to train them to control that without a quad is, well, like trying to teach my dog calculus – she just doesn’t have the capacity! She can barely do fractions.

Couldn’t I just functional and look for lack of knee flexion?

Now we’re getting somewhere! So, this is definitely a start but there are major limitations here.

First, are you actually getting max effort? It is much easier to get max effort out of a simple isolated quad test than during a more complicated and variable functional.

Second, everything happens fast. At least video record it.

Finally, you are relying on an UNVALIDATED and SUBJECTIVE judgement call. BUT, it is at least plausible and getting to the point of quad dysfunction, so it’s a start.

The quad index test is not functional

By “functional” do you mean transfer to a sporting task? Unless the test is max effort, full speed, and with an externally focused athlete NO TEST IS “FUNCTIONAL”. The quad index answers a simple CAPACITY TO DO WORK question. Before capacity can transfer to a task it has to, you know, exist.

I already work on quads with my patients – I’m sure they’re strong

They’re not.

But when I push on them they feel strong!

The quad is normally crazy strong, and the goal is MAX EFFORT. Do you use a FIXATED and VALIDATED device/setup to accurately measure a quad index?


I stand by my statement. When we first got the tools to test this accurately I was blown away with how large these deficits can be when the athlete seemed “strong”.

But they are 9 months out from surgery; it HAS to be strong!

It’s not.

What if their uninvolved side is weak? Then the quad index doesn’t matter.

Well shouldn’t they then AT LEAST MATCH THEIR WEAK-ASS OTHER LEG?! Then make the uninvolved side as strong as possible AND TEST THEM AGAIN! There are other things to consider like peak torque to body weight on the uninvolved side but THAT DOESN’T MEAN DON’T TEST!

What if I focus on getting really strong glutes and hams to handle “Option 3”?

Look, I want EVERYTHING as strong as possible. I’ll take all the hip and trunk strength that I can get, but not at the expense of the quad. “Everything as strong as possible” does not mean “Everything but the quad on one side.” Also, Option 3 is not a typical high-performance strategy.

But doesn’t rate of force development matter too?

YES! Just being “strong” doesn’t mean it can ramp up in the split second it takes to change direction. BUT, if they can’t even produce the force with all the time in the world (usually 5 seconds for peak torque measures), then rate isn’t their issue…yet.

Through the current body of literature we know that peak torque quad index is consistently the most predictive test for performance and 2nd injury risk (you now know why). There is just not much published on rate of force development in this context…yet.

Does anything else matter other than the quad index?

Of course, but lacking a quad predicts just about everything else in this population. You need to clear that first and you need to do it objectively and accurately. I will say, anecdotally, when I consult on a case who I’m told is “strong but having difficulty with their mechanics”, they almost always fail the quad index when tested on an appropriate device. As I said before “It’s the quads until it’s not the quads.”

That is not to say that someone can’t pass a quad index and still have issues. Anecdotally, in the clinic we find that this is where rate of force development comes in.

I can’t afford a $50K isokinetic dynamometer so why bother?

Well, first you can get them used, but that is still a good chunk of change. Although that is the gold standard, you can get a pretty good measure out of an inline setup. And those are under $900 for the complete setup. Still too much? How much did you pay for your ultrasound machine?

So biomechanics don’t matter?

What? THIS WHOLE POST WAS ROOTED IN BIOMECHANICS! It’s about more than JUST positions (those matter, too). As I have written before, biomechanics matter even when they don’t.

Final Thoughts

I am specifically talking about returning an athlete to sport after their first ACL injury. I don’t know how or if any of this applies regarding preventing the first injury. That is an inherently more complicated question that I would rather not comment upon in this post.

Athletes who have torn more than one ACL create another layer to the problem. As the research suggests, they probably never restored their quad the first time and probably learned those compensations that I described above. They probably have some more complex issues as well. That said, they still need to get their quad back first and foremost. And you need to test it.

In summary…

  • Measure the quad index
  • That’s really the whole point of this post

The featured image on this post is “EKFMA2” by Erik Meira and is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License
Creative Commons License

Also included on this post is “EKFMA” by Erik Meira and is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License
Creative Commons License

Torque_animation gif is from Wikicommons

Ryan Reynolds Facepalm gif is from GIPHY