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.
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:
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:
Despite this angulation, the main purpose of the ACL is still to prevent the tibia from moving too far forward of the femur.
–
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: https://www.youtube.com/watch?v=JWI_Qghqclw
Then this happens:
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:

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:
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:
In this image:
The femur, the thigh, is collapsing in, while the tibia is moving laterally. This is happening:
Causing this to happen:
Notice the femur in effect guillotines the ACL. Back view:
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 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:
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.)
–
How we avoid the above
In the one example of blowing the knee out, the knee is caving inward:
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?
Or laying like this the eight hours we’re asleep?
Nope!
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:
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:
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:
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, b-reddy@hotmail.com.
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:
Sarah
July 1, 2016
Great article! I love how the pictures help to explain it. I did my right ACL and my tib/fib stayed planted and my knee and femur continued laterally (outwards/rightwards) coz I stopped suddenly and my knee couldn’t take the lateral movement. Total opposite of the player in the gif, so maybe my injury is a less common way to do it? Coz of the mechanism of injury I was misdiagnosed as a patella dislocation (that had relocated) for over a week after my injury. Totally agree that it is more than just about the rehab – I think about my knee more than I ever thought I would. But it’s coming more naturally to stand/walk/run in a way that best supports my knee. It is super hard though and I found your suggestions a great addition! Thanks and keep up the great work!
reddyb
July 5, 2016
That’s how I did my knee as well. The guillotine can work both ways!
Thank you for the nice words :).
getontheline
July 1, 2016
In regards to the sitting position, I tend to simply spread my legs (manspread style) to prevent the caving inward motion of my legs and knees. Is that generally a best practice for sitting?
reddyb
July 5, 2016
I hit on this in the manual, and see Sarah’s comment and my reply.
deannavzahablog
January 25, 2017
Hi! I’m now 22 and tore my left knee’s ACL and meniscus (not sure which one as new MRI’s apparently show a new/different tear) in 2013 via my knee caving inward and I’m finally getting ACL reconstruction surgery this March (in the form of a patellar tendon autograft). I’ve read a lot about my ACL and the nature of the injury being female, and now that I have stumbled upon your blog, I’ve been binge reading all your articles. Love how “real” you are about the whole process. As much as I’ve prepared myself these past 3 years by reading countless articles and even watching the surgery I’ll be getting on YouTube several times, this is definitely helping me prepare even more. Thanks for having put this together!!
reddyb
January 27, 2017
You’re welcome! Thank you for stopping by and good luck with your surgery.
Rob
March 26, 2017
Hi Brian. I hope you are well.
My name’s Rob. We spoke a little while back on strength rate of hamstring graft before failure.
I’m now 17 days post op (ham graft) and I’m hoping you may be able to share thoughts on a couple of queries I have?
My main issue I’m having at the moment is swelling. I was put on 2 weeks no weight bearing and the bandage made icing pretty difficult. Since getting the stitches and bandage removed i’m now icing and elevating as before. It’s impeding my ROM especially for flexion. Any suggestions on how to reduce it and, given your experience, any rough timescale on how long it will last before I may consider medical advice (which I am consulting my GP next week on)?
I managed to get full extension by two weeks and now find it no difficulty. I understand you had difficulty with yours and I appreciate everyone is different. As I have slight hypermobility (extension) of my joints is there a risk that in working so hard to get the extension I may have added undue tension to the graft (possibly even hyper extended at times) given I have no pain or difficulty in getting extension? I also ask as a couple of times since walking I seem to have walked on a slightly hyperextended knee. No doubt the atrophy in the hamstrings along with neuromuscular control is to explain but could these occasions have loosened the graft as I had read a source stating that an easy to extend knee early on may be a sign the graft is lax? *source- http://www.perthortho.com.au/resources/keith-holt/ACL-Rehab.pdf page 2 under overarching principles.
Just to add the injury was not sorting related but a fall on slippery ground and I continue to wear the brace with no more than 20 degree extension so I don’t walk onto a fully extended leg.
If you’ve made it this far into my marathon query I would like to finally ask any words of advice on what to be wary of at this stage and the upcoming few weeks in my recovery?
I’d like to say the knowledge you share your site has been invaluable in preparing for this process.
Regards,
Rob.
P.S. Are leg extensions (non machine/ weight resistance) ok to do in building up quad strength or do they add undue tension to the graft?
reddyb
March 28, 2017
Hey Rob,
Regarding swelling and flexion, I’d checkout the ACL guide for that. A lot to consider which is tough to encapsulate in a comment. In general, if the knee is swelling on you, you’re doing too much.
This is also covered in the knee flexion manual, but I recommend people are at least a month post-op before entertaining that.
https://b-reddy.org/2014/04/02/the-most-important-phase-of-acl-rehab-copy/
https://b-reddy.org/2017/03/08/cant-bend-your-knee-heres-what-to-do/
ROM wise you only want to regain what is *your* normal hyperextension. Going further than that will necessitate stretching the graft further than necessary, as the ACL stretches when the knee goes into hyperextension.
The graft should be checked at regular intervals in your post-op with your surgeon. At 17 days, you hopefully have already had one of these appointments. They will be able to immediately assess whether the graft is loose.
I had actually had no problems regaining my extension. Had it back the day of surgery :). Getting it back and getting it to stay there without working on it are different stories though! It certainly took me some time until I didn’t have to work on it anymore. Some do immediately get it back and not have to worry about it from there on out. These are often lax people as you alluded to. That said, during this surgery we’re talking the graft being screwed in a tight position. So whenever somebod immediately has great extension and it’s not a problem to maintain it, it does make sense to gently ring an alarm for the graft being loose. But again, the surgeon should have their eyes (and hands!) on this many times within the first nine months post-op.
Leg extensions like this are fine:
https://www.youtube.com/watch?v=NNRJbfEgQHI
https://www.youtube.com/watch?v=MQgzP3YaG0M
Resisted leg extensions usually aren’t worth it. Beyond graft considerations (common machine version does place stress on the ACL), they are hard on the knees period. Squatting is a much knee friendlier quad exercise.
Thanks for the nice words. Glad you’ve found the site helpful.
Rob
March 28, 2017
Hi Brian,
Thank you for the speedy, and detailed response.
The swelling is not really budging at the moment but certainly I take on board your point about not pushing things too hard too soon. Thankfully last Wednesday I saw my surgeon who tested the graft and was satisfied so hopefully a few days of walking now won’t have been too detrimental.
Yes sorry I was referring to your accounting of the difficulty in maintaining the extension. I’m seeing my physio tomorrow for my second session so hopefully can run through a couple of concerns in person with them.
Forgive me for asking one more question but last night I suffered possibly my biggest set back and chase for concern – I’ve strained my hamstring (the same as the operated knee and where the graft was taken).
It was the end of the day and I was trying to take a sock off of all things, stupid really and felt a pop and sharp pain that then caused tenderness along all the back of the leg and in particular the back of the knee (the pop was just below the belly of the hamstring). I think it’s likely to be a grade 2.
At a guess, how bad will affect my recovery? Am I likely to have more hamstring problems in the future?
To think 24 hours ago swelling was my biggest foe, haha.
Thanks for your advice,
Rob.
reddyb
March 31, 2017
If there is no bruising, then it’s not bad. No bruising and a ~month is when things should be pretty solid.
But it might not have been a strain. Going to write something about this, but after this surgery, and particularly when there is swelling in the knee, all types of sounds and sensations become more common. What you may perceive as a strain may have been fluid all of a sudden moving. (Brief info on this: https://b-reddy.org/2015/06/26/empyting-out-the-mailbag-and-clearing-the-history-7/#Joints )
Or some scar tissue popped. The science behind this isn’t as clear, but it’s a commonly referred to explanation.
But yes, once you get part of your hamstring cut off, you’re more likely to have hamstring issues forever. That issue might not be anything which more than marginally influences your life. It could end up being a weird feeling during certain activities. But one way or another, “issues” are more likely.
Rob
April 2, 2017
Hey Brian, hope you’re having a good weekend.
There is some bruising,surprisingly little, and just six days on I can now do some activation without pain that’s beyond uncomfortable (still feel a sting but just reminds me to ease off). Fingers crossed I can continue to do hamstring work without too many future strains. Swelling is surprisingly better and I think it’s due to the program I’m on and also icing after sessions. My quad control is a bit off still especially as the other day I realised in achieving leg extension I seem to be lifting my leg as much as straightening it which would explain why my affected leg looks so different to the non affected when I straighten them at the same time. I just hope I’ve not left it too late to get the right muscles working and not get into bad habits this early on
Thanks for your advice Brian,
Rob.
Rob
July 6, 2017
Hi Brian. I hope you’re well. My names’ Rob. I messaged a good few months back after I had my ACL surgery (right knee, hamstring autograft from right knee) early March 2017.
I was wondering if you may please share your thoughts on graft stretching out.
I’m 4 months into my recovery and after a cardio workout (using elliptical) I sat on a sofa and out of habit crossed my right leg (I used to do yoga and mma so seem to find myself sitting in odd positions).
Had no pain or problems but when I got up I move my leg out and as I moved it out (it was not weight bearing at this point or tense) it felt like my knee below the joint was out of place and then a second later it got sucked back in.
No pain, no feelings of instability (beyond crunching and cracking I’ve come to get used to with still being relatively early on in the recovery process) but I definitely felt something was under tension though not necessarily compromised.
Would this likely be my ACL graft that was stretching? *I appreciate my description alone makes that hard to answer.
As the graft tissue had no nerves would I feel it tear if it did?
What are the symptoms of graft stretch?
If say it is compromised in some slight way, what may be a prognosis? Treatement options?
Thank you for your time Brian, I’m sure you’re a busy guy so any reply you may be able to give is appreciated. As I’ve said before hands down your site is the go to place for any ACL concerns and I can’t tell you enough how helpful and reassuring it is to have someone like you fielding questions from strangers and putting all these useful articles up.
Kind Regards,
Rob.
b-reddy
July 7, 2017
Hey Rob,
Easiest thing to do here is talk with your orthopedist. They should still manually assessing the graft, and can feel if it’s stretched out. If you’re feeling instability, then that would be one sign. But stretching vs stretching enough to feel instability isn’t the same thing. For instance, you might not feel instability until doing much more demanding activities.
Feeling weird sensations is a normal part of the process, and not being sure what they are is common. That’s where the orthopedist comes in. They should be regularly manually assessing the graft until the nine month mark. They’re there to help distinguish what you’re feeling vs anything wrong with their work.
After a few months the graft is largely believed to be set i.e. the bone has grabbed it. So by this point smaller motions, like a brief sitting period, shouldn’t have an affect. (They hope to not have an affect in the beginning phase earlier, but they’re progressively less likely to matter.)
If the graft is stretched out, the only option to correct that is another reconstructive surgery. Prognosis is the same as a torn ACL. You don’t have to actually tear the ACL to have a functionally torn one. This happened to me. My ACL never tore, but the ligament got so stretched during my injury it became functionally useless. A stretched out graft might not cause the knee to be as lax as a torn ACL, but it could. Regardless, it’s like a tear in that you might be able to live with it, you might not. But you can’t tighten it once it’s deformed.
Thank you for the nice words. Glad you’ve found things helpful.
Rob
July 9, 2017
Hi Brian.
Thanks for your reply.
I am booked to see him in a month but will call tomorrow to see if can get that pushed up.
I guess probably best to prepare myself for idea that may need to have another op.
Is there anything more you’re able to say about stretched out grafts that might help me understand better if I’m looking at that?
Thanks for your input Brian. Hope you’re having a good weekend.
Rob.
b-reddy
July 9, 2017
Hey Rob,
I wouldn’t jump to any conclusions until seeing the orthopedist. Going based off a quick, transient sensation can drive an ACL patient crazy. Important to reiterate weird feelings are part of the process.
Whether a graft being stretched is a problem is the same thought process for whether a torn ACL is a problem.
Rob
July 16, 2017
Thanks Brian. Take care.
b-reddy
July 18, 2017
You’re welcome!
Shannon
February 20, 2018
Would this $8 booklet be applicable to those 1 year post op or more?
b-reddy
February 20, 2018
If you’re interested in thinking about preventing another ACL tear, then yep!
If the primary goal is still working on rehabbing from the previous tear, then it’s not as relevant.