The Mechanisms of noncontact ACL injury (My insight)

Howdy,

In last decades, ACL (anterior cruciate ligament) injury is a fancy word and numerous debates passed unsolved or at least misunderstood (not clear).

Today, I will try to explain my insight of noncontact ACL injury mechanisms. I will try to convince you that you can’t blame an accident anymore when it comes to this type of injury. I would always blame a S&C coach because somewhere on athlete’s body is definitely a weak link. Please, read the scary fact below…

“Kids ACL injury rates are up 400% over the last ten years.” Kelly Starrett

“We can’t completely prevent ACL injuries but we can greatly reduce the potential with proper strength training, speed and agility technique, and landing technique. Parents, coaches, athletes… you have to do your part and participate in a sound program to support your chances of staying healthy. I have personally worked with many athletes coming back from an ACL saying they had wished they didn’t play as many AAU games and focused more on training.” – Lee Taft

85% of ACL injuries in soccer/football are non-contact (Walden et al, 2015). Unanticipated  and perturbation drills are good warm up strategies to prepare players for training/games (a light plyo unanticipated “Partner Push” exercise is a good example: a simple drill where a small vertical jump is performed and an unexpected push is given mid-air – exercise can be made harder by pushing in multi-directions, and incorporating single leg lands). Invent some modification that suits your sport…

The purpose of ACL is better intracapsular and – of course – general knee stability. In general, knees need to be STABILE (strong surounding structures) but hips and ankles strong and MOBILE enough to provide good knees health because in opposite knees will suffer from various deviations which will make injuries more possible. So, ACL is an extremely important ligament in terms of overall knee integrity and stability. The ACL connects the femur (thigh bone) to the tibia (shin bone).

In my opinion, 3 injury mechanisms exist in practice:

  1. Valgus-position movements;
  2. Landing and deceleration;
  3. Upper leg external rotation with whole foot planted.

Valgus position is position where knees make X position of legs (knee(s) cave in and it looks like upper and lower legs form X letter). Flat feet is a big cause of knee valgus position and hip internal rotation consequently. Loss of proper motor control (a proper walking/running pattern) comes like result very often.

If you are familiar with ACL anatomy, it’s logical that position described above (congenital – you can fix it in childhood; or made during movement – poor motor control and strength) promises high injury risk (ACL could strain, partial rupture or tear by overload force – gravity, external load or both at worst).

The right knee (the ACL runs at an oblique angle from the posterior aspect of the femur to the anterior aspect of the tibia. Due to this arrangement, the ACL is responsible for preventing anterior translation of the tibia or posterior translation of the femur – more on this later)

Wherever you find valgus knee(s) position – some amount of risk EXISTS. But it seems that landing, decelerating and changing directions (everything with valgus knees(s)) are the most vulnerable. The reason is simple – eccentric muscle force that is dominant within mentioned movements is higher than concentric. It means that ACL resilience declines as decelerating force moves up.

Bad landing

Bad landing

Bad changing direction

Bad changing direction

Bad decelerating, or stopping

Bad decelerating, or stopping

If you have a problem with valgus knees you must work on neuromuscular eccentric and concentric control both (motor control, some people are making valgus pos. immediately before jumping – preparing ecc. phase – which is dangerous but maybe less than landing from higher positions and jumping immediately after landing, which is the most dangerous) and strength – with basic movements proper technique (slow unloaded squats, split squats, RNT split squats, Bulgarian split squats, lunges, step ups, various deadlifts, clamshell… etc.). On that way, you will work on ankle-knee-HIP stabilizers proper recruitment and control – which is very important. Also, on that way you will indirectly strengthen glutes (or directly with the last one) which is so important for knees health (a movement/alignment control) along with hams. Plus gain/maintain good hips and ankles mobility and you will probably not have a problem with knee injuries at all – except the cause is a heavy contact.

“If your body could talk it just might tell you: I know my knee hurts but it only does what the foot will allow and the hip can control. Look at those please.” Perry Nickelston

Also, you need to work on m. vastus medialis strength (internal head of m. quadriceps femoris), with compound – not isolated movements of course (it’s impossible in this case). You can “kill two birds with one stone” – everything is included (solved) with phenomenal exercises mentioned above! A proper m. vastus medialis strength is very important for preventing valgus knee/s pos. and ACL injury prevention/reduction. Many of us are aware of the importance of the quadriceps to knee helath, but, often times, the hamstrings get neglected…

Right leg

The right leg

But here is an interesting thing: We see that the best athletes use some of these “bad” positions (valgus) to cut and change direction. Maybe our body (mind) is using it with intention because when you internally rotate the femur you’re prestretching the glute and moves are more explosive consequently? Hmmm who knows, food for thoughts definitely.

Beside valgus position, landing and decelerating by itself could produce an ACL damage (even if limbs are in a proper alignment). Why it’s so?

Namely, when we land or decelerate we do it dominantly with toes (forefoot lands first). That’s mean that we put our quads in a very loaded position (for a moment) compared with posterior chain synergists. In that situation, quads “pull” shins forward-up and if you have weak hamstrings (or improperly turned on – in a proper time) you can lose counterbalance (it produces muscle strength imbalance) and could tear ACL off or make a partial rupture the least (“When contracting, the hams work to bend the knees, but also pull tibia posteriorly – in this way the hams can serve as a dynamic protector of the ACL by limiting excessive anterior displacement of the tibia and strain on the ligament”Dr. Nick Buonforte). You could pass without any injury for years (if “bad force” is small), but keep in mind that you are as close to injury as ACL straining and micro rupture is higher and higher (this is key to noncontact ACL injury understanding – “microtrauma” theory). The noncontact ACL injury can be a sudden (acute) injury almost exclusively in third case which I am going to describe below.

It makes sense that low levels of eccentric hamstring strength could lead to ACL injury because if you are not strong with eccentric your quads will pull the tibia more – there is no counterbalance. I am not saying an injury will be acute, but some ACL micro tears will lead to an “acute” injury in the future.

Ok now, we said that ecc. force is bigger than concentric (active, passive/stimulation of muscle spindles and viscous muscle components are directed toward concentric m. contraction)… So, take a look on a pic below and tell me what do you think? Yeah, you are wondering how that knees/legs don’t “explode”? Plus lower spine of course because of “butt wink” (and curved spine in general) under the huge ecc. force during landing? Muscle/tendon/ligaments tearing risk increases dramatically even if they are prepared because that heights are amazing!

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If you are not familiar with Mr. Aaron “Jaws” Homoki let me tell you that he is the bravest skateboarder in the World! He is well known for its height jumps. Look that valgus knee landing position and look at that height! In short, his like-of-rubber-made genetics made him famous (he didn’t have any serious injury in career – amazing). But I’m afraid to think of what looks like his knees/spine on the scanner (or when he get older – maybe he will need wheelchair). He is an extreme case – don’t put your body on THAT risk.

In order to keep an understanding of injury mechanism closer, there is one more detail noticed in landing movement. Risk of ACL tearing become even higher with landing if you don’t have enough ankle mobility (lack of proper dorsiflexion – below 20 degrees). Here is why!

Your shins would simply stayed almost vertical with landing amortization which is not good because tibial articulation plate has slightly slope toward back – which means easier injury if you got bad ankle mobility and if you don’t have extremely strong hams to support this disadvantage. ACL stabilizes knee during knee extension movement (along with hams) – especially when knee is fully extended. It means that if you want to lower injury risk you have to got STRONG hams (same like quads would be the best) and properly mobile ankles and hips (if not, your body will find it in knees, and this is definitely something you don’t want – you need like an iron STABILE knees)! Take a look at Bret’s awesome ACL biomechanics video below! Plus, by strengthening glutes we can also “save” ACL from injury because hams will not be overloaded/overtired by doing hip extensions like a prime extensors, and we know that hams (along with other hip extensors and external rotators, and m. vastus medialis) play important role in ACL protection from tearing.

Ok, if you didn’t got it – give me one more chance 🙂 . The last doesn’t mean that you will injury ACL if shins stay vertical and torso bended (this could be even more advantageous because on that way you can unload knees – lever arm with bended torso is short, and load stronger – posterior chain muscles when heels are on the floor, that keeps ACL away from injuries). If you got fine amortization with forefoot first and then quickly whole foot contact, this is the best landing technique.

The best landing position

The best landing position (hips BACK)

I just wanted to say that poor ankle mobility (shins relatively straight) in correlation with straight torso during landing amortization gives the highest ACL injury risk (you are realizing now why people with hyperextended knees are under the higher ACL injury risk – joints laxity problem in general). Plus, becomes even worse if you let your knees to pass toes (imagined vertical line) too much forward with heels up – load on quads and ACL injury risk raise… Don’t worry, if you have a good ankle mobility this will not happen. But, nothing is worse than landing on extended knees – I mean nobody normal is doing that but by doing it from just a few inches hight you can immediately tear an ACL at least. If you watched the video above you know why.

Imagine she can’t put her heels down because of dorsiflexion lack or shins stay more vertical with vertical torso – the result is even bigger ACL injury risk!

Imagine she can’t put her heels down because of dorsiflexion lack, or can but shins stay more vertical. Both scenarios (knees too much forward and more vertical shins) with vertical torso are dangerous and the result is even greater ACL injury risk!

“Put simply, there should be hip flexion when landing from a jump. Compared to the glutes, the quads are at a mechanical disadvantage and can cause athletes to be less explosive and more susceptible to injury. As long as we place primary load on the glutes, we’re unloading the ACL and setting ourselves up for a healthy athletic career.”Erica Suter

Upper leg external rotation with whole foot planted is the third way of ACL noncontact injury. As you may know, it’s not a rear situation in sport where you see ACL injury when someone plant the whole foot on the ground and, according to the game situation, rapidly change direction on side (not on toes or with foot off the ground which is the best). On that way, lower leg stays planted in place – upper leg externally rotate and acute injury occurs under tension. On the picture below, you can see improper and proper changing direction.

 A B A – Whole foot planted with undesirable forces; B – Proper movement with heel up

                                             A                                                                                            B
A – Whole foot planted with undesirable forces; B – Proper movement with heel up

On the picture below, you can see described injury mechanism a little closer.

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Also, it’s not a rare ACL injury mechanism in sport where your opponent stand over (trample) your foot and at that moment you try to rapidly change direction on side (it is the moment). It can occur in team contact sports like soccer, basketball, football, etc… Although this is characterized as an opponent’s contact, the injury mechanism is the same like mechanism just mentioned above.

Of course, maybe it’s the case mostly – but the whole foot doesn’t need to be planted when it comes to this last injury mechanism. In some cases it can happen when the heel is off the ground.

Most of these injury mechanisms are more or less connected with other knee injuries (collateral ligaments, PCL, meniscus, etc…), but that is another topic.

You can see here the mechanisms of ski knee injuries (most of them are ACL). I didn’t mention a deep basic squat/flexion position but if you understand the ACL functional anatomy and all the biomechanics around it really makes sense (especially with some load wheter it’s fast eccentric motion or some external load). Ski boots and skis allow you to lean/”sit” back without falling and that’s why the injury risk is high (more vertical shins + body weight is shifted back = ouch 🙂 ). Plus, hams are shortened when it comes to deep squat position (you would say “but it’s stretched in hips”, and I would say “but what about posterior pelvic tilt – butt wink?” 🙂 ) and there is no way to activate enough to support tibia – to keep away potential frontal translation of tibia (it’s even worse if “but wink” shows up because hams are even more shortened with it consequently)

I know this is not a topic, but on the picture below you can see how side direction of force (an accidental punch for example) can influence on multiple structures injury.

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I am sure this text has convinced you that you can’t play with your destiny. This text looks really “OUCH”, right? Contact knee injuries can be more dangerous and more often, but it definitely doesn’t mean that you don’t have to pay attention on NONCONTACT. Therefore, be sure that you gave your best when it comes to prevention/reduction, because whether noncontact or contact – ACL injuries are among the TOUGHEST injuries in the sport when it comes to pain, frustration, rehabilitation process, etc.…

Last but not least, female athletes have to pay attention on this facts because exists increasing incidence of injuries among them, particularly in sports such as soccer and basketball. For example, one study concluded that female basketball players were six times more likely to incur an ACL tear than male players. It’s possible that joint laxity, limb alignment (wider pelvis – Q angle), body movement, skill level, hormonal changes, structures around joint weakness and training deficiencies contribute to the observed difference in the number of knee injuries between male and female athletes.

Thanks for reading and all the best,

Luka

 

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