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:
Itâs the quads until itâs 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 âfunctionalâ as a stand-alone noun or verb to exaggerate the silliness of this word)
Letâs get down to a couple first principles here. From a biomechanical standpoint, the quad has only one job to do: It generates the âinternal knee extension momentâ. From an application perspective, it is the antagonist, essentially the ONLY antagonist, to any âknee flexion momentâ. Again, from an application perspective, quad counters an âEXTERNAL knee flexion momentâ.
I know what youâre thinking. âThat is a big sounding phrase and Iâm not very smart.â Not to worry! Itâs not that complicated and Iâm probably much stoopider than you.
So, letâs break it down so we can understand what this phrase means. Letâs start with the last word âmomentâ. All that word means in this application is ârotational forceâ and is usually described as âtorqueâ. It is the way we refer to the forces that try to turn joints. Simple.
Torque = Force applied times the distance from axis of rotation
Letâs go to the word âkneeâ. 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 shinyâŠ
The next word is âflexionâ which describes the direction of rotation that the moment is trying to achieve â 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 donât result in much movement unless the moments overwhelm the constraints (like ligaments and stuff) and then, well, SNAP! The knee shouldnât bend that wayâŠ
Finally we have âexternalâ. It simply answers the question, âFrom where is the moment originating: Inside or outside the body?â Internal moments are typically generated by muscles (some internal moments can be generated by passive structures but donât 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 âexternal knee flexion momentâ can be restated as a âknee bending force coming from outside the bodyâ. 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 athleteâs mass (probably not going to happen in a split second) or changing their rate of velocity change (âmake me slowerâ 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). Thatâs why a weight feels heavier when you hold it at armâs 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 laterâŠ
Who cares? As long as they can squat, jump, hop, and change directionâŠthatâs all that matters right? They can functional all over the place so problem solved!
Hang on. Iâm sorry. Iâm making a pork roast on a spit and I need to check on it. Itâs Cuban style, marinated in mojo criollo overnight so it has that citrus/garlic/black pepper thing going on and I donât want to mess it up. Iâll be right back. Here, watch this video of tiny hamsters eating tiny burritos while Iâm gone.
Iâm back itâs all done but needs to rest a little bit and the rice and beans arenât 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 canât 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 doesnât 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.
âAh yes! They will go into valgus! We just need to make sure that they donât go into valgus when they functional!â
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 weâre dealing withâŠyet.
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 âstrengthâ). Remember I said earlier that in order to reduce the external knee flexion moment the athlete has only three options:
Option 1. Reduce the athleteâs mass
Option 2. Reduce the athleteâs 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 wonât 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. Iâll 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 problematic.
To explain Option 3 better, we need a real-life sporting activity. Letâs 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). Letâs focus on Step 1: Deceleration.
NOW PAY ATTENTION HERE! THIS PART IS REALLY IMPORTANT TO UNDERSTAND!!!
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 Newtonâs third law). This force that they create is a reaction to the athlete planting their foot into the ground. It is a âground reaction forceâ, 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 athleteâs 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 laterâŠ
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 donât have much of a quad, then what?
Theyâre 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 âhip strategyâ. 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.
NO PROBLEM!!! GLUTES AND HAMS TO THE RESCUE!!!
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 â 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 â I told you that would come up again. Now add a pivot.
Fourth: They can use this strategy to pass your hop tests â once they learn how to do it. It takes a while, so they often wonât pass the tests until late in rehab making it LOOK like theyâve progressedâŠbecause they can now functional.
Ok. You have questionsâŠ
Couldnât 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 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 â 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 Iâm pointing out the obvious here, trying to train them to control that without a quad is, well, like trying to teach my dog calculus â she just doesnât have the capacity! She can barely do fractions.
Couldnât I just functional and look for lack of knee flexion?
Now weâre 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 itâs a start.
The quad index test is not functional
By âfunctionalâ 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 âFUNCTIONALâ. 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 â Iâm sure theyâre strong
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?
No
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 âstrongâ.
But they are 9 months out from surgery; it HAS to be strong!
What if their uninvolved side is weak? Then the quad index doesnât matter.
Well shouldnât 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 DOESNâT MEAN DONâT TEST!
What if I focus on getting really strong glutes and hams to handle âOption 3â?
Look, I want EVERYTHING as strong as possible. Iâll take all the hip and trunk strength that I can get, but not at the expense of the quad. âEverything as strong as possibleâ does not mean âEverything but the quad on one side.â Also, Option 3 is not a typical high-performance strategy.
But doesnât rate of force development matter too?
YES! Just being âstrongâ doesnât mean it can ramp up in the split second it takes to change direction. BUT, if they canât even produce the force with all the time in the world (usually 5 seconds for peak torque measures), then rate isnât their issueâŠyet.
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 contextâŠyet.
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 Iâm told is âstrong but having difficulty with their mechanicsâ, they almost always fail the quad index when tested on an appropriate device. As I said before âItâs the quads until itâs not the quads.â
That is not to say that someone canât 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 canât 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 donât matter?
What? THIS WHOLE POST WAS ROOTED IN BIOMECHANICS! Itâs about more than JUST positions (those matter, too). As I have written before, biomechanics matter even when they donât.
Final Thoughts
I am specifically talking about returning an athlete to sport after their first ACL injury. I donât 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 summaryâŠ
- Measure the quad index
- Thatâs really the whole point of this post
This was originally posted on Erik Meira’s website. You can click here to read more blogs from them.
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Awesome post! Is there any normative values of the Quad index test? It would seem effective to start gathering data on results, and establish age and injury normative values.