Return to Running After ACLR

That has got to be the most pragmatic title to a blogpost that I have ever written. Usually I write some esoteric thing like “Biomechanics Matter Even When They Don’t” or just make a subtle nod to toilet humor like “Flush Your Stool Down the Funnel”. This one probably falls in line with “Why a Quad Index Matters” and “That Other One I Wrote About Quads After ACLR but I Can’t Remember the Title” (just click the link, I’m not fixing it because I’m writing this after two adult beverages).

There is a good bit of talk and many opinions about when it is a good idea for an athlete to return to simple running after ACLR. As with most recommendations they are made with the best of intentions but are often pretty arbitrary. So for me I really wanted to think this through a bit. Let me shed some light on my reasoning process here.

First, when most patients ask, “Can I run?” my smart-ass response is “I don’t know. Can you?” (Also see, “Can I play?”) There actually is some truth to that reply. The best way to find out if you can run is to simply try it and see what happens. Of course this answers the question of “Can I run?”, but not “Should I run?”

Should I run?

I mean, this is almost an existential question. Why do you run in the first place?

  • Is it for a cardio workout?
  • More for mental health purposes?
  • Simple recreation?

There are all sorts of unusual running mechanics that a person can have and still accomplish these goals. Are these dysfunctional? Are they going to cause overuse injuries or lead to early osteoarthritis in the knee?

You could make arguments that they could, and there are researchers trying to explore these hypotheses, but epidemiology is hard and the further off into the future a possible injury becomes, the harder it is to have any certainty around its cause. Personally if you are able to run and you feel good, I’m typically not going to recommend that you stop.

But would it be different after ACLR?

I’m starting to think so. There are people looking at how altered running mechanics may have an effect on the increased rate of early osteoarthritis that we see in people who have sustained an ACL injury. Again, this gets into more long term forecasting so I will hold my comments on it for now beyond, “Could be.” Also, most of the athletes that I work with are not particularly concerned about long term knee health as much as getting back to sport quickly, effectively, and safely.

In that regard, I have been becoming more concerned with the presence of intralimb compensations associated with returning to functional activities before the quadriceps is ready for them. The amazing Susan Sigward and her team out of USC describe it very well in their JOSPT paper form 2018, and an older episode of PT Inquest. Essentially they are finding that these post-ACLR subjects begin to learn how to shift the load demand off of the quadriceps on their surgical side in a way that has the subject maintain equal weight between legs with almost identical kinematics during squatting.

In other words, they look symmetrically loaded but their quad on that side is still on vacation. They figured out how to do that task functionally but without using their quadriceps.

Who cares as long as it’s functional?

Well, I will admit that there is essentially zero risk of this causing an injury during squatting. But is this having them build confidence in functioning without a quad? Turns out that when you go to perform a max deceleration effort going into a high angle change of direction, you kind of need your quadriceps. Read “Why a Quad Index Matters” for more information on how the biomechanics work here.

This may be why we see so much higher injury rates in those who try to return to sport before their quad function is restored. Something I am wondering is whether or not these “poor mechanics” that we see as we are working on return to sport were actually developed during rehab, as opposed to being present before the injury, as many have proposed.

In other words, the original ACL injury may fall more under the category of “shit happens” and then they learn how to use frontal and transverse plane excursion to make up for a lack of sagittal plane excursion because of a lack of quad function. I mean, if true, that explains a lot…

My concern is that, if an athlete has already learned those altered mechanics early in the rehab, will that make the later restored quadriceps be less engaged during sporting tasks and therefore less protective? Honestly, I don’t know.

So what about running?

This all makes me wonder if this could be going on with running as well. When I think about quad function during running, I think of it taking load during initial contact by allowing knee excursion. We do know that those who have a weak quad tend to run with less knee excursion. A “stiffer” knee, if you will.

As I mentioned before, I don’t like athletes recovering from ACLR learning that avoiding normal knee excursion is an effective strategy. So for me to stay internally consistent in my reasoning (i.e. not be a hypocrite), I should expect there to be a minimum required amount of quad function in order to run with as much knee excursion as they did before the injury.

The problem is, I don’t have a great idea of how much they need.

Let’s think this through

Some people go to quad index here. I love it for return to play, but I don’t like using it for this. Return to play requires max effort intensity which likely requires max physical capacity, whatever that might be for the individual.  Looking at the uninvolved side is a great reference for this – assuming you are getting a true max reading on the uninvolved side. But simply going for a run is not nearly this intense, especially if you are not doing hard decelerations or running down steep hills.

The quad demand during a foot strike while running is simply managing relatively vertical ground reaction force that is a factor of body weight. So to me, it makes sense to consider the torque production of the quad as a ratio to the athlete’s body weight, simply referred to as quad torque to body weight (TTBW).

Quad TTBW tends to fall around 3Nm/kg on most people. Now I hear some very smart people use 70-80% quad index for return to running. But what if your athlete has a normal TTBW that is 6Nm/kg? Is it really necessary that they have a 70% quad index, or 4.2Nm/kg, just to run? That doesn’t seem right.

There is a study that came out in 2021 that we reviewed on PT Inquest that looked at TTBW and running after ACLR. That study came up with 1.45Nm/kg being the minimum, but, as we note in the episode, there are a couple problems applying that study to practice. First, they used a concentric isokinetic assessment which is almost always going to be less than the much more commonly clinically used isometric assessment, because of Newton’s second law. Secondly, they simply looked at running speed to say “Good/Bad”. As I mentioned, I probably care more about knee excursion than I do running speed.

So this makes me think that the isometric reference is a bit higher than 1.45Nm/kg. Let’s go back to those very smart people who say 70-80% quad index. Maybe they are on to something. If TTBW typically falls around 3Nm/kg, maybe 70% of that number would be close? 70% of 3Nm/kg would be 2.1Nm/kg TTBW. Some say 80% which would put us at 2.4Nm/kg TTBW. To me these are both very reasonable.

Personally I like to think more from the perspective of “absolute bare minimum to consider running”. It tells me that “We might be close enough to start trying it”. I don’t like 1.45Nm/kg as an isometric cutoff for the reasons I already mentioned, even though I can cite a study. That said, I think 2.4Nm/kg is a bit high for “bare minimum”, although I would suspect that it is probably where kinematics normalize on average (I’m waiting on that study). So I tend to go with 2Nm/kg TTBW.

Why not 2.1Nm/kg since that is actually 70%? Because “2.1” sounds too certain to me. “2” makes it clearer that the estimate is rough. I like that. I think that it is probably close enough for us to at least start playing around.

Once we get over 2Nm/kg and everything is feeling good, I have them play on the treadmill a little and see what happens. The only cue I give is for them to show me that they can bend those knees when they hit the ground. “Squat into your run by dropping your hips for a few strides” to see if that knee has some give.

If so, let’s begin integrating running into the program. If not, maybe we’ll hold off until quad TTBW gets a bit higher and then try again.

I honestly don’t know how to end this post but, uh, that’s pretty much it. Oh, and this is the kind of crap that I teach in my courses!

In summary…

  • What is your goal for beginning to resume running?
  • The quad matters…
  • …but how much is needed?
  • Quad torque to body weight may be more universal for return to running than symmetry
  • Setting a low bar “bare minimum” may be a good way to say “We might be close enough to start trying it”
  • 2Nm/kg of quadriceps TTBW may be a reasonable “bare minimum”
  • “Squat into your run by dropping your hips for a few strides” to see if that knee has some give.

P.S. Isometric measurement and calculation of the quadriceps TTBW does not need an expensive device. With a little math you can just follow Scot Morrison’s overview and use a Tindeq Progressor.

Featured image is Male Running Free Stock Image by Direct Media on StockSnap (modified)