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What is the ACL and Why is it Important?

Soccer players, and most athletes for that matter, often dread hearing the three letters A-C-L in direct succession. “It looks like you’ve torn your ACL.” Ugh. Those words can pretty much guarantee a bad day for anyone 100% of the time.

But what is the A-C-L and why is it so important?

The anterior cruciate ligament, or ACL, is one of four major ligaments that stabilize the knee. A ligament is soft tissue that connects two bones – in this case the tibia (or shin bone) and femur (or thigh bone). The ACL is located in the middle of the knee and forms an ‘X’ with the posterior cruciate ligament. It prevents anterior translation, or forward movement, of the shin bone relative to the thigh bone.


When the ACL is suspect to too much force, it ruptures or tears. This usually happens when an athlete is decelerating (slowing down), cutting/pivoting (changing direction), or landing from a jump. There is usually an audible “pop” and subsequent feeling of instability with walking, running, and/or going up or down stairs. Swelling is often delayed, and other symptoms like clicking, popping, locking, catching, or grinding can occur if there is damage to other adjacent structures like the meniscus.

Okay – so what’s the big deal?

There are between 125,000 and 200,000 arthroscopic ACL reconstructions in the United States each year, with the majority of these patients falling under the age of 25 participating in high-school, collegiate, or recreational-level sports. Although not everyone elects to have their ACL reconstructed – these people are called ‘copers’ – it is largely accepted in the rehabilitation community that young athletes should have the ACL surgically repaired to ensure stability of the knee and prevent future complications later on down the road. Multiple studies have shown that tearing the ACL makes the knee much more susceptible to premature osteoarthritis later in life. Not to mention, a torn ACL requires anywhere from 9 to sometimes even 12 months of intensive rehabilitation, where athletes must first regain range of motion, re-learn how to walk, rebuild strength/endurance, and eventually be reintroduced to sport and other functional activities. Unfortunately even after extensive rehabilitation, approximately 1 in 5 patients suffer re-injury within 2 years.


In addition to the physical toll, there are also financial and mental costs! An ACL reconstruction can range anywhere from $10,000 to $20,000, depending on the extent of damage within the knee joint. Luckily, insurance takes care of most of this, but you’re still left with a pretty hefty bill if you haven’t already met your annual deductible. And don’t forget about your co-pays for physical therapy x number of times a week for x weeks. I won’t even get into the mental side of things. Just know that a torn ACL is a huge buzz kill. I’ll save the sport psych post for a later date.

Shockingly, almost two-thirds of all ACL injuries occur without another player even touching you. These are called ‘non-contact’ injuries and happen when your body moves the ‘wrong’ way. Female athletes are 4-8x more likely to demonstrate these faulty movements due to a number of factors. Let’s call these non-modifiable and modifiable risk factors. Non-modifiable risk factors are things that you cannot change or control. They include things like: joint laxity, hormone levels, femoral notch width, decreased ligament size, increased slope angle, and Q-angle. These are biologic and anatomical things that we can not change.

Modifiable variables are things that you can control.

This is where I come in.

Length-strength muscle imbalances can predispose the ACL to injury. Meaning, if certain muscles are tight and other muscles are weak, your knee may move into positions that your ACL does not like. This typically includes the knee caving inward (called “valgus” in the rehab community) and usually occurs when the knee is at almost-full extension. Add a little bit of twisting or rotation to the equation and you’ve got yourself the perfect storm. To prevent this, you must have adequate hip and ankle mobility, as well as proximal hip, quadriceps, and hamstring strength. Female athletes typically exhibit quadriceps dominance after puberty, meaning that their quads are often much stronger than their hamstrings. When this happens, the quads can expose the knee to higher-than-normal shear forces and translate that tibia forward on the femur. Typical hamstrings to quadriceps ratios are anywhere from 0.5 to 0.8. It has been proven that as this ratio approaches 1.0 (or the hamstrings get stronger), the risk of ACL injury goes dow.


Not only must you possess adequate strength – You must also be able to activate and utilize this strength when you are moving functionally. This is where the concept of motor control and proprioception (or body position sense) comes into play. Motor control refers the communication between your brain and your muscles. Your nervous system must be able to coordinate your muscles and limbs to achieve a desired movement or set of actions – like accelerating, decelerating, cutting, pivoting, jumping, or landing. Even if you have good isolated strength levels, if you cannot use this strength effectively with your functional movement patterns, you may end up on the sidelines.

Now that I’ve got your attention, let’s talk about what you can do. You can work on flexibility of the aforementioned muscles and strengthen your core, posterior chain, and quadriceps muscles. Seek out a medical professional for neuromuscular re-education, or ask them to teach you how to move safely and effectively. While statistics vary from source to source, research has shown that ACL injury prevention programs can reduce the risk of injury as much as 75%, if executed properly. These programs often include strength training, motor learning, balance/proprioceptive training, and plyometrics.

The most important part of these programs is the quality of exercise repetitions. Athletes must not only complete the prescribed exercises, but they must complete the exercises properly. What good is an injury prevention program if you practice jumping the wrong way 3x/week? This is why you should seek out a trained medical professional or movement specialist. While the risk of ACL injury cannot be completely eliminated, it can certainly be reduced. Not only can players learn to move in a safe and efficient manner, but they will also consequently enhance their performance by getting better, faster, quicker, and stronger.


There are a number of resources available to coaches, parents, and sports practitioners who wish to participate in and/or implement ACL prevention programs in their communities. Below, I’ve included links to three of the best and statistically supported programs currently in existence. They include the PEP, the Sportsmetric Program, and the KIPP. Check them out below and let me know what you think!

Santa Monica Sports Medicine Foundation’s Prevent Injury and Enhance Performance Program (PEP):

Cincinnati Sports Medicine Research and Education Foundation’s Sportsmetric ACL Prevention Program:

Lurie Children’s Hospital of Chicago’s Knee Injury Prevention Program (KIPP):

I’ve started this website with the aim of educating athletes and their families on the dangers of ACL injury and to provide them with the appropriate resources to proactively ‘prevent’ these injuries from happening. Every week, different topics relative to injury prevention, rehabilitation, and return-to-sport after ACL reconstruction will be discussed in simple and easy-to-digest blog posts. Please feel free to leave any comments or questions you may have. Thanks for reading and until next time, be well.


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