An underappreciated aspect of ACL injury prevention and rehab

Posted on July 1, 2016

(Last Updated On: July 1, 2016)

Working on jump landing, strength of the hamstrings, strength in general, squatting and not letting the knees come in, bosu balls, there is a lot out there on preventing ACL injuries and rehabbing from them.

One thing rarely discussed is the other 23 hours of the day you’re not doing the above. Like how you stand, sit, lay down.

Briefly recapping anatomy of the ACL

The anterior cruciate ligament goes from the back of the top of the knee- the femur, to the front of the bottom of the knee- the tibia.
ACL anatomy

Next, notice the ACL goes from the outside of the top of the knee to the middle of the bottom of the knee. This would be the front of your right knee:

ACL anatomy with fibula

If you’re looking at your right knee, the fibula is the bone on the outside. So, the ACL starts towards the fibula (the outside), and comes back in some:

ACL anatomy with fibula and arrow

Despite this angulation, the main purpose of the ACL is still to prevent the tibia from moving too far forward of the femur.

ACL Lachman with drawing and anterior translation

The point of prevention and rehab then…

Is to help the ACL do this job, and or make it so the ACL doesn’t have to do this job. In other words, make it so we don’t test its limitations. For instance, we want to avoid positions of anterior translation of the tibia. If the tibia moves too far anteriorly- if this happens too much:

Animations made from this video:

Animations made from this video:

Then this happens:

ACL injury animation

One way this commonly happens is excessive extension of the knee (hyperextension). When the knee hyperextends, notice the tibia can move forward of the femur:

ACL through knee extension GIF

Notice the femur moving back those last few degrees (meaning the tibia is moving forward *relative* to the femur).

But this isn’t the only way the ACL can rupture. Another very common way is what we call excessive knee valgus. The knee collapses in:

ACL injury knee valgus turning in

Let’s think about why this can blow our knee out, because anterior translation of the tibia isn’t the only problem here. Remember this:

ACL anatomy with fibula

In this image:

ACL injury knee valgus turning in

The femur, the thigh, is collapsing in, while the tibia is moving laterally. This is happening:

ACL injury knee valgus turning in with rotational lines

Causing this to happen:

ACL injury mechanism valgus GIF

Notice the femur in effect guillotines the ACL. Back view:

acl injury animation

If interested, here are two research papers finding relationships between the amount of (hyper)extension people have and ACL injuries, as well as generalized joint laxity and ACL ruptures:

The effects of generalized joint laxity on risk of anterior cruciate ligament injury in young female athletes.

The risk of anterior cruciate ligament rupture with generalised joint laxity.

The relationship between static posture and ACL injury in female athletes.

Long story short- a knee which is more lax, this can be in a forward and back manner (like hyperextension) or in a side to side manner (like our guillotine), is more susceptible to an ACL tear.

You may have noticed females are often the focus of this. Many of us likely remember how girls always seemed to be able to do this better than boys when growing up:

Seated Toe Touch Hamstring Stretch

Women are more lax and tend to have a greater ability to let the tibia anteriorly translate. That’s not good for ACL prevention! This is one reason hamstring strengthening gets so harped on. The hamstrings can assist the ACL in preventing this translation. (As can other muscles. Getting stronger anywhere and everywhere tends to help.)

ACL lachman with hamstring line

Notice the similarities in the line of pull between the ACL and hamstrings.

How we avoid the above

In the one example of blowing the knee out, the knee is caving inward:

ACL injury knee valgus turning in

ACL injury mechanism valgus GIF

Naturally then, from an ACL point of view, this isn’t a motion we want to get good at. It’s not a position we want to be promoting; it’s not a habit we want to have. Think about flexibiilty in general. You tend to get stiff if you hold something tight, right? Like being in cast. You also tend to get lax if you hold something in a stretched position. In other words, would we want to be sitting like this the eight hours we’re at a desk?

Sitting bad knees in with lines

Or laying like this the eight hours we’re asleep?

Laying on side knee in tibia out 2

Rotated to clarify how tibial tuberosity (pink spot) is lateral to kneecap.

Rotated for clarification.


Then let’s think about forward / backward translation. Again, this is what happens as we get into full extension of our knee, then into hyperextension:

ACL through knee extension GIF

During the last bit there the femur starts sliding back on the tibia, meaning the tibia, relative to the femur, is moving forward. That is, our ACL is stretching.

So, would we want to stand with our knees really extended then, like this:

Standing knee hyperextension GIF

Jennifer knee hyperextension

Nope! We don’t want to get hypermobile into this position. If anything, we want to be good at avoiding this position! So it’s not something we want to be practicing every time we’re standing up.

If you want examples of more scenarios to avoid, as well as help getting out of those scenarios, like knowing how to sit so you can place the least amount of tension on your ACL (particularly helpful if you have a new ACL and want to do everything you can to not stretch it out), check out ACL Prevention and Rehab: The Other 23 Hours. It’s $8 and can be purchased here:
Buy NowAdd to Cart

It comes in the form of a password protected link. You’ll be redirected to the page with the password, as well as receive a copy of it in your email for safe keeping. Any credit or debit card can be used for payment.

If you’re unhappy for any reason with it, all purchases come with a full refund option, and you can email me directly with any issues,

Note: if you already own The most important phase of ACL rehab, this would provide some nice information for things to be on the lookout for after that i.e. those past the first month or so. Due to the constraints of the first month -like needing to be in a brace, use crutches, etc.- much of this won’t be applicable until those first 2-4 weeks are over.

Eight bucks:

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