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There are a couple of factors that can increase ACL injuries.

1. Hormones. This mostly applies to female athletes but certain points of a female’s menstrual cycle makes her more prone to injuries. The hormones basically tell the body to “loosen up” in prep for potential birth and this instability increases risk of injury.
2. Previous history of injury to that knee/leg, not just ACL injuries but ANY injury. Re-injury is far more common. Any previous meniscus tears, medial/lateral collateral ligament tears, etc, will weaken that area. This area will always be the weakest link and first to go, especially in contact sports. This is why appropriate rehab and adequate time off will greatly reduce chances of re-injury. It takes about a year to fully rehab an ACL injury and reduce risk of re-injury. Unfortunately athletes are not always provided this time off.
3. Mechanism of injury. ACL injuries occur when there is stress on the knee joint, usually because there is a fixed point and another force that creates knee torsion. An example of an external force is when a football player is tackled; the foot is the fixed point to the ground and the tackle creates the knee torsion. This can also occur without an external force like a tackle. An athlete is also prone to ACL injury when she/he quickly changes direction (ie: cutting); the foot planted is the fixed point and the change of direction creates torsion. This can also occur if a basketball player jumps for a shot and lands awkwardly, which can also create the same knee torsion with a fixed foot. My guess is your football player was quickly changing directions when his injury occurred. He could’ve had other previous injuries that might’ve contributed to his risk of re-injury, but a professional level athletes body is asked to do a LOT more than the average person and at higher speeds, which already puts him at risk for ACL injury even if he didn’t have a previous history.

To answer your question on prevention, it would greatly reduce an athletes chances of ACL injury if he/she has a good strength and conditioning coach that focuses on strength, balance and engaging the correct muscles during agility drills. Have you ever lifted something heavy without using your legs/core and then regretted it? You can likely still lift it but your back will just hate you for the next few days. Same concept with athletes, except they are in a much faster paced environment and need to react much more quickly. They don’t have time to think “oh yes need to lift with my legs not my back” - it should be drilled into them like second nature to use correct musculature during their movements. Sounds easy, but it’s not. Our bodies are smart and will adapt/compensate to do the movement but over time, repetitive incorrect movements will result in pain and injury. In addition, fatigue throughout the course of a game will also contribute to more mistakes/injuries. An athlete needs to be conditioned not just to barely make it through the game but be at 100% throughout the entirety. Sport performance coaching is a lot more nuanced than one would think but a good training program/coach will make a big difference to the athlete.

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They suddenly fail.

Simple answer is that ligaments don’t bulk up the way muscles do. And while we’ve created multiple supplements and exercises to increase muscle mass, it’s the same ligament holding bones together. A 180lb high school kid who adds 50lb of muscle in college has practically the same ACL he always did.

ACLs typically fail with a pivot and valgus load to the knee. That’s a twist and the knee bends in toward the other knee, usually from a force like a tackle or other hit.

While ligaments don’t repair themselves as quickly and regularly as bone, they are living tissue so any micro trauma will typically result in repair. I haven’t read any literature on evidence showing cumulative injury to ACLs causing most injury.

Source: I’m an orthopaedic surgeon

While you are correct that hormonal changes have been found to have an effect in ligament strength, you are confusing normal estrogen and progesterone fluctuations with the hormone called Relaxin. Relaxin prepares the body for birth by loosening things up. But that’s only weeks before birth. Not during normal cycles. Do a google on that and you’ll learn more.

To your point though, female soccer players have the highest incidence of ACL tear in noncontact sports. For many reasons neuromuscular control, hormonal changes, and anatomical differences between male and female knees. There are specific physical therapy programs designed to help decrease injury in female athletes.

I wrote a section in my dissertation about female football players and the relation to hormones and acl injuries! It’s fun seeing stuff you actually know about on the internet.

Female anatomy teacher and hypermobility sufferer here: Sadly, that's untrue. Relaxin not only starts as early as 6 weeks into pregnancy, but is also regularly elevated in the luteal (post-ovulatory) phase of a regular menstrual cycle, which not coincidentally is when women are statistically more likely to suffer joint injuries.

I just had a second ACL surgery literally 2 days ago. My first knee was sudden failure (rugby accident) but I was warned it's common for athletes to tear the other one if they over-compensated with it. Considering I eventually did tear the other one and it happened when I was just doing an average paced turn while running I think it could be from gradually wearing down.

I know the typical description is sudden failure, but considering the increased likelihood of tearing the opposite ACL after surgery it would seem to me that both are possible.

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> A 180lb high school kid who adds 50lb of muscle in college has practically the same ACL he always did.

Will they get stronger over time, with increased strength, or do we just live with the ones we're born with?

So Ligaments dont strengthen at all? Rock climbers talk much about finger tendons getting stronger through climbing and training hangboarding. Pro climbers can hang and pull on one finger - I certainly can't.

What is happening there?

The tendons in the fingers are joined to and powered by muscles in the forearm, so grip strength can be increased by muscle training. Ligaments join bone to bone so they can't be developed through training.

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Absolutely bang on with this, but I'd add repeated injuries that cause small amounts of scar tissue each time could theoretically lead to a weakening.

Pretty unusual to have that many significant injuries in exactly the same place though, and I've never seen any studies dealing with it.

you are interested in "tendonosis"

Short answer, both. There are great training programs for tendons. In short, tendons grow in response to:

1 - Heavy Weight (growth signal)

2 - blood flow (non trivial as they do not have natural bloodflow!)

And are worn down by any lack of 1/2. So many athletes engage in chronic, low weight, loading, and do not encourage enough habitual circulation. This wears down the tissue until it is brittle enough to snap. This process is called tendonosis (NOT tendenitis).

The cure is actually 1+2. Let me explain. One needs a tendon specific routine, lets use the wrist:

1. You would lift something REALLY HEAVY for maybe 3-8 seconds in the morning (after warming up). You might do 3-5 repetitions. You are not training muscles, you are just loading weight on the tendons.

2. You then, 10-15 times a day, do EXTREMELY LIGHT exercise (like wrist curls), that put almost ZERO strain on the tendon. Your goal here is to promote circulation,

Have you had to treat any Brazilian jiu-jitsu folk with torn ACL from "heel hooks" a submission that twists the knee?

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Do you enjoy your career as a physician? Or do you have any regrets about the process?

Had a really quick read of all the other comments. I felt like I wasn’t getting a bio mechanical reason as to why ACL tissue fiber can’t slowly be damaged which leads to what seems like “sudden failure” (leg is useable no longer). It seems incredibly counter intuitive to describe all ACL failures as sudden. Stress on the ligament over time should lead to it weakening to the point of what looks like sudden failure to the observing sports fan or coaching staff. With this being the case, for the injured athlete perhaps they have felt strains and tweaks in this ligament throughout their careers until the day it goes pop on them.

Anecdotally: I was a cross country and track runner, and now rock climber, I feel as though my own ACL has told me something weird is going on while it has never failed on me. Can’t imagine that pain sensation isn’t coming with some level of microscopic damage to the tissue.

tendons/ligaments do strengthen over time, they just take much longer to strengthen than muscle, or skeletal tissue. Consequently they tend to be the weak link. Think of a power lifter who can squat 600 pounds. The were not able to do that 5 years ago, but with years of training, when they started at being able to just squat 400 pounds, the muscle and ligament tissue adapted and grew to be able to withstand the stress of their surroundings.

Most grip strength training is really working your forearms. If you look closely at Rock climbers arms they usually have well defined biceps, triceps and forearms.

Nice explanation! Just curious, in non-contact ACL tears, is the process any different? When I was a freshman in college, my team had something like 8 ACL tears over the course of the season, and 1 in particular was an offensive lineman who just came out of his stance and didn’t contact anyone on the play. At the time, some of my teammates blamed all the injuries on the coaches having too many full-contact practices because they thought it would make us tougher.

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I’ve experienced this first hand. I’ve actually plotted charts and have noticed for my height, around 6’3 I feel noticeable strain on my knees at around 230 pounds when playing basketball. Black in college I was able to play football at 235 but being young helps with that lol

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Dynamic knee valgus occurs AFTER the ACL is already ruptured.

Zhang et al (2019) Sports Med, 49(3)

Powerlifters have been shown to have thicker ACLs than the general pop

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3535026/

pivot/valgus load to the knee is the old way of thinking about ACL injuries.

Complete ACL tears have shown to heal in as little as 3 months.

ACLs do adapt! Check out Powerlifter's ACL thickness vs general population.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3535026/

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Tendons attach muscles to bones. Ligaments attach bones to bones. So any answers confusing the two need to be corrected. yersiniaD has the right answer.

Very specific article looking at heavy weight lifting in very specific athletes. "All athletes in our study began training before or during their pubertal growth spurt." And not to be a complete jerk about it, but you've posted a few comments with this article, published in a journal that went out of business in 2014 and looking at 9 (NINE!) total weight lifters. I'm all for looking at new data, but this isn't it.

I've treated a multitude of BJJ athletes. No ACLs though. They are all monsters. Constant injury and constantly looking to get back on the mat to fight. Just monsters.

I think there's some thought to the idea that the fatigue is in relation to the musculature in the knee, and once that fails to protect transition and rotation in the knee, then they tear the ACL. But look, we are constantly learning about the body and changing our thought patterns. Nothing is set in stone.

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Love what I do. It's become more difficult to practice medicine with insurances dictating what you can and can not order. In a perfect world we could order tests as needed and figure things out without regard to cost. But we live in the real world, unfortunately.

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You're correct, and it is so frustrating that an actual orthopedist, if their claim is true, is actively uninformed about half the world's issues with cyclical joint instability. Something something medical misogyny :(

It would also be more useful showing the ligaments getting thicker rather than a comparison to the general population since the people that are able to lift heavier weights might be ones with thicker ligaments.

That's such a great point. Selection Bias. The ones with the thicker ligaments were able to lift heavy weight or stayed in the sport. The ones with regular ligaments either tore their ACLs or dropped out. It would be like saying that playing basketball makes you taller. Great thought. Thanks

Thanks for pointing that out. I didnt realize how early in pregnancy the Relaxin was released. However, I did do some research on the levels and while it does exist during a normal menstrual cycle, it is about 50 times higher during pregnancy. All the research I found recently in response to your post pointed me toward Estrogen being the main culprit in ligament laxity. But again, always open to new info so I love the questions your post raises.

Your claim regarding medical misogyny seems extraordinary however. Do you really feel that I hate women because of my post? I hope not.

I appreciate your taking your own advice and researching to better educate yourself! It's always heartening to see that. No, unfortunately medical misogyny is a far larger issue than your comment, and I meant it more in the widespread sense of "here's someone who really should know this, as a professional regarding a common situation that could affect all their female patients / half the world's population, and yet they don't, very possibly through no fault of their own but because this very basic stuff about female anatomy isn't considered important enough to be taught." Medical misogyny is a pretty common catchphrase for a lot of these issues, whether it's failing to address how teaching male anatomy as the default does a disservice to women, to how much longer it can take women to get diagnosed correctly, etc.

FYI wrt relaxin, yes, sure, lower in the luteal phase than during pregnancy, but yes, still present sufficiently to increase joint issues! (As someone with hypermobility disorder I can validate this personally to boot!) IIRC oestrogen increases expression of relaxin, so they're not unrelated. You might enjoy this writeup: https://www.frontiersin.org/articles/10.3389/fendo.2022.827512/full .

That article review link you sent was interesting. "...the premise of this review is based entirely on inductive reasoning. Additionally, consideration of lower-level of evidence literature and inconclusive literature was necessary."

Here's a group that seems to focus on this issue specifically:

https://www.hypermobility.org/hormones-and-hypermobility

"...studies have not demonstrated a clear relationship between the level of relaxin and the degree of laxity. Other factors must be involved."

Seems like a lot more research is warranted. But it could be very interesting.

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