ACL Injuries: Why They Happen & How To Prevent Them
Imagine this: you’re tearing down the field with the soccer ball, ready to score, when a defender shows up in front of you. You try to quickly make a cut to the left. Something pops, you go down, and suddenly your knee is on fire.
“What the heck just happened? No one even touched me!” you’re thinking. The athletic trainer makes their way onto the field and helps you hobble off, but you can’t put any weight on your leg. What the heck is going on?
Another way to picture this: you’re sitting in the stands, watching your kid run down the soccer field, and suddenly: boom, they’re on the ground. You feel your heart rate rise and your mind starts to race. What’s going on? Are they hurt? What just happened? They hobble off the field, and you’re anxiously waiting to see them run out again, good as new.
This scenario is probably familiar to many athletes and many parents of those athletes. Injuries are common in sports, and, while they’re often minor, sometimes they’re a lot more significant. Some of these injuries can be season- or career-ending.
Nearly 200,000 people tear or rupture their ACL every year in the United States. That’s a lot of sports seasons that end in an instant. So, the questions are, (1) how do they happen, (2) who’s at risk, and (3) how can they be prevented?
What Is The ACL Anyway?
The anterior cruciate ligament (ACL) is a ligament inside the knee joint. It attaches the back of your femur (your thigh bone) to the front of your tibia (your shin bone). The goal of the ACL is to stop your femur from sliding back and stop your tibia from sliding forward - basically, it works in conjunction with another ligament (the posterior cruciate ligament, or PCL) to keep your femur and your tibia connected and in line. Without the ACL, these two bones can slide around significantly, and your knee becomes unstable and somewhat unusable.
How ACL Injuries Happen
When an ACL is injured or ruptured, there is one common mechanism of injury. This common mechanism happens like this: an athlete puts weight on their foot (most often when landing or cutting), and the knee falls inward (into “knee valgus”) while the toes are pointed out. The athlete’s weight and the force coming up from the ground put strain on the knee and the ACL, the ACL is too weak to handle those forces, and the ACL goes “snap.” (Click here and look at the first image (1) for a visual of this position.) Nearly 70% of ACL injuries happen like this, when the knee falls into a “valgus” and there’s no contact with anyone else.
About 30% of ACL injuries happen through contact. A contact injury occurs when someone gets hit, either directly in the knee or indirectly somewhere else on the body. Most often, this occurs through a hit that pushed the knee into that valgus position. But sometimes, if the athlete’s foot is caught or planted, a hit elsewhere on the body can cause this same valgus and create the same injury, though this is much less common. These injuries are hard to prevent, as sports are unpredictable and you never know when someone’s going to take you out at the knee. Most commonly, these contact ACL injuries occur in football.
Again, 70% of ACL injuries occur through the non-contact mechanism described above: the athlete puts weight on their foot, the knee falls in, and boom, the ACL ruptures. These injuries most commonly occur in sports like soccer, basketball, volleyball, and skiing. As an added bonus, these injuries are typically preventable to some degree, since there are often biomechanical issues, weaknesses, and imbalances that lead to that ACL-tearing noncontact knee valgus.
But before we can discuss prevention, we should discuss WHO should focus on prevention. Most people could surely benefit from an ACL injury prevention program, but not everyone is at a high risk for ACL injury, and many people might have more important things to focus on (like ankle stability, low back stability, and so on).
Who Is At Risk For An ACL Injury
The short answer to this question is most female athletes, particularly those in the sports listed above: soccer, basketball, volleyball, and skiing (in that order). This doesn’t mean that males can’t fall into a high-risk category, and this doesn’t mean that only athletes can tear an ACL. But statistically, the highest risk group tends to be these female athletes, particularly around the high school age.
So, let’s break down some of these ACL injury risk factors:
Being female - Females are at anywhere from a 2-8 times higher risk of ACL injury than male athletes playing the same sport. There are a variety of possible reasons for this, with two of the most prominent being differences in hormones and differences in anatomy. Females have more estrogen and progesterone than males, which may have an effect on the ACL itself as well as motor control throughout the menstrual cycle. Females also tend to have a larger “Q-angle” (the angle at which your thigh connects to your pelvis), which creates a larger natural knee valgus in females than in males.
Your activities - Your sport can increase your risk of ACL injuries. If you’re a female, soccer, basketball, volleyball, and skiing are higher-risk activities. If you’re a male, football, wrestling, and lacrosse tend to be higher-risk activities due to the risk of a contact ACL injury.
Anatomic Variations - Certain variations in the structure of your knee, like having a tibia that’s angled forward, can put you at a higher risk for ACL injury. Typically, you won’t know about this until after an injury, unless you’ve had previous x-rays or other imaging of your knee. In most cases, your individual anatomic variations aren’t super helpful in preventing injury, so it’s not something you necessarily NEED to know.
General Hypermobility - If you’re generally hypermobile (often called double-jointed), this might be an additional risk for ACL injury. Sometimes this hypermobility is localized (meaning just in one area), usually as the result of previous injury or lifestyle. Other times, these hypermobility issues are genetic and exist throughout the body, like in the case of Ehlers-Danlos Syndrome. Click this link for a screening tool for generalized hypermobility.
Knee Laxity/Hypermobility - This is related to general hypermobility, and is particularly relevant if you’ve had any knee injuries in the past. These can affect your ligaments and other connective tissues, making them less able to support the knee and potentially adding stress on the ACL.
Biomechanical & Motor Control Issues - There are four major biomechanical and motor control issues related to ACL injury: ligament dominance, quadriceps dominance, leg dominance, and trunk dominance. Don’t worry, we’re going to break these down in the next section.
Biomechanical Risk Factors For An ACL Injury
These are some of what could be considered the “important” risk factors, as these are the ones that are modifiable. Sure, you could quit your sport to prevent injury - but that’s no fun, and athletes aren’t typically interested in that anyway. So, we look for things we CAN change. What can we do, looking at your individual biomechanics, to prevent an ACL injury?
(Side note - you might hear the word biomechanics thrown around a lot, both in our content and elsewhere - basically, biomechanics looks at your body during activity, evaluating how it’s built and how it’s functioning. Clinically, biomechanics allows us to look at you as an individual to determine what’s happening as you move.)
As we discussed above, there are four major biomechanical factors to look at when evaluating your risk for an ACL injury. Let’s break them down here:
Ligament Dominance - Normally, your muscles, joints, and ligaments all work together to manage incoming forces during physical activity. If your muscles are too weak, too tired, or too uncoordinated to help disperse and manage forces, then your joints and ligaments have to take up the extra work. This is called ligament dominance. As far as the ACL is concerned, this is usually related to weakness in the “posterior chain” - the calves, the hamstrings, and the glutes. These muscles are essential in dispersing forces coming up through the leg, and when they’re weak, your leg is more likely to fall into that ACL-injuring valgus position. (One key physics idea here - when you hit your foot on the ground, the ground hits your foot back with the same amount of force, and that force travels up your leg and body. It’s this “ground reaction force” that’s often responsible for rupturing the ACL, in conjunction with your own body weight.)
Quadriceps Dominance - Typically, your knee should be stabilized by your quadriceps, adductors, and hamstrings, acting as a cohesive unit to adjust for changes in force and position. However, many people (females in particular) tend to be quadriceps dominant - meaning that their quadriceps do most of the muscular stabilizing at the knee, and the hamstrings and adductors don’t do very much. This is an issue for two reasons. First, your ACL works to hold your thigh back, while your quads work to pull your thigh forward at the knee; these are two opposing forces, so quadriceps dominance adds more stress on the ACL, potentially pushing it over the edge in an at-risk position. Second, your quads only connect to your tibia in one spot, right below your kneecap; this makes it hard for them to react to forces in multiple directions. On the other hand, your hamstrings and most other muscles around your knee attach in multiple places, allowing them to manage forces in more directions and ultimately creating more stability.
Leg Dominance - Leg dominance occurs when one leg is significantly stronger than the other. Some degree of leg preference is natural, but both legs should generally be similar in strength, mobility, and overall function. If one leg is too dominant, you’re going to tend to use that leg too often or in situations where you should use the other, putting you at a higher risk for ACL injury. Additionally, the other leg will be weaker and less coordinated, and thus potentially less able to do the things you need it to do without injury. This is particularly important for those sports that involve a lot of one-legged actions, like cutting, landing, or jumping.
Trunk Dominance - Trunk dominance occurs when your ability to sense the position of your core and create stability is impaired. Normally, your brain should know exactly where your body is in space at all times, and it should be able to control the movement at any point. When you’re trunk dominant, you can’t do that. This happens for a variety of reasons, like a growth spurt or puberty (which can change your center of gravity, and thus the motion of your body) and a sedentary lifestyle. This inability to control your trunk creates two problems. First, you need a stable core to create powerful movements of your legs and create stability in those legs (think of anchoring something in sand (an unstable core) vs cement (a stable core). Second, an unstable, trunk-dominant core will have excess motion during physical activity, creating a variety of stresses on the knee and the ACL; if you’re in an at-risk, knee valgus position, lack of control of your trunk might well push the ACL over the edge.
So, now that we know what these modifiable, biomechanical risk factors are, how do you know if you have any of them? The answer here is more challenging. In our clinic, we use a screening test: the 10-second tuck jump test. This test will typically show us if one or more of the four risk factors above are present.
To do this test, set a timer for 10 seconds (or use this 10-second timer on YouTube, which will give you some time to get into position and clearly tell you when to stop). When the timer starts, start doing tuck jumps, doing as many as you can in 10 seconds without letting your form go. When the timer stops, you stop.
The idea here is not how many tuck jumps you can do in 10 seconds. It’s all about how your form is while doing them. If your form falls apart within 10 seconds, we might suspect you have an issue that’s putting you at risk for an ACL injury.
Additionally, there are some things to look for. If your knees drop in, that’s a possible issue. If your thighs don’t get parallel with the ground, that’s a possible issue. If you’re drifting or landing/jumping with an uneven stance, that’s a potential issue. There are a lot of things to notice and consider when using this test to assess your biomechanical function, and you probably won’t be able to notice these things on your own. If you have a phone, you can take a video of yourself from the front and from the side (do two separate tests, don’t turn during the test), and watch it in slow motion to look for any abnormalities. More preferably, you should schedule an appointment with a healthcare provider, as they might be able to pick out some things that you won’t.
(Notes and disclaimers - This test is not diagnostic unless performed under the supervision of a healthcare provider. Additionally, this test is really mostly designed for the conditioned athlete, as tuck jumps are a super strenuous exercise. If you’re not a conditioned athlete, it’s very possible that you being deconditioned or out of shape will put you at a higher risk for injury than a biomechanical issue that you have, so it might be a better idea to start working on getting into shape first. If you have had an ACL injury in the past and want to prevent another as you get back into shape, it might be a good idea to work with a professional like a PT or certified personal trainer as you start your journey.)
How To Prevent ACL Injuries
The answer here is variable, depending on your specific biomechanical variations and issues. Ideally, you match the preventative exercises/ideas to your specific issues (for example, if you’re leg dominant and quadriceps dominant, you address those two things specifically). That said, there are some things to offer up here:
Practice Good Landing Form - This one is particularly for athletes that do sports with a lot of jumping, like basketball and volleyball. When you land wrong, you put more force up through the leg & knee and put yourself at risk for ACL injury. Here’s how to do it right: as you come down from a jump, your knees and hips should be bent slightly. You should land on the balls of your feet (or your midfoot at the very least), and roll back onto your heels - don’t land on your heels. You should keep your knees pointing relatively straight forward - they should not collapse/fall inward. This is important to address ligament dominance, as well as the other factors, as this will help you use your muscles in the correct order and the correct way.
Core Stability - There are three big reasons you should care about your spinal stability: (1) Stability is necessary for your spine to successfully manage load and movement, (2) Building stability can eliminate micromovements creating pain and dysfunction, and (3) Having the ability to stabilize your spine and core improves your athleticism and performance. Core stability training should involve some static core exercises, as well as some dynamic/perturbation training. Click this link for an article about lumbar stability and the “Big 3” exercises to increase it.
Posterior Chain Exercise - Your “posterior chain” includes your glutes, hamstrings, and calves. Activating this posterior chain can help to address ligament dominance and quadriceps dominance, by ensuring that these posterior chain muscles have enough strength, stamina, and coordination to do their job. Exercises that activate the posterior chain include squat jumps, kettlebell swings, and medicine ball throws/slams. Bridges can also activate the posterior chain to some degree, as can Romanian deadlifts (RDLs). Exactly what you should do varies on your level of conditioning, but bridges and kettlebell swings with no or light weight are potentially good starting points for a beginner. A more conditioned athlete could add in squat jumps, medicine ball throws, and weighted RDLs.
Hamstring Curls - Hamstring curls will help to address quadriceps dominance, while also training the posterior chain. There are a number of ways to do it, but one great option is to use an exercise ball, as shown in this image. Other options include nordic curls, glute-ham raises, and machine-based hamstring exercises. In my opinion, the more accessible an exercise is, the easier it is to do and the more likely you are to do it, so the exercise ball hamstring curl and the
Single Leg Activities - Your body is an interesting system, in that if you train one leg or arm, the other leg or arm also gets some benefit from that exercise, even if you don’t do anything with it. This tells us that your two sides are intricately connected. That said, we don’t see equal results between both sides, so we do need to train both sides. Doing single-leg activities, like single-leg balancing, pistol squats, or lunges can train your legs and help to equal them out if you have any leg dominance going on. As an added bonus, these single-leg exercises can also activate and train your posterior chain, another common issue with ACL injury.
Ultimately, preventing an ACL tear involves a few things: increasing coordination and motor control, increasing core stability and control, strengthening the posterior chain, and training both legs equally (or until they’re the same strength). A good prevention protocol should incorporate exercises like plyometrics, isolated strength training, single-leg activities, and core training.
If you’re a higher-risk athlete or playing/coaching a higher-risk sport, you should also consider a more comprehensive, evidence-based ACL prevention program, like the Santa Monica PEP (Prevent injury and Enhance Performance), or think about finding a personal trainer or physical therapist who specializes in ACL injury prevention. It can be a challenge to know what to do and keep doing it on your own, so help from a team, a program, or a professional can really help keep you on track.
Particularly a note for coaches and ATs here - certain ACL injury prevention programs, like the PEP, have been shown to reduce the risk of ACL injury. Even if you haven’t had a ton of ACL injuries, it makes more sense to start working on prevention BEFORE you have an injury, rather than waiting until you’ve had a slew of them in a short time. Dr. Tim Hewett, Ph.D., is an expert in ACL injuries and prevention, and he noted in a recent podcast that most of his clients call him in for a consult AFTER they’ve had a bunch of their star athletes tear their ACLs. If you only take prevention steps after an athlete’s been injured, you’re not serving all of your athletes.
Parting Thoughts
Ultimately, it’s important to think about preventing an injury BEFORE it happens, not after. There are a number of things that put you at risk, but the summary is that if you’re a female athlete in high school/college, participating in soccer, basketball, volleyball, or skiing, you should consider taking some steps to prevent ACL injury. If you’ve injured your ACL before, you definitely should take some steps to prevent reinjury (but you probably know that already).
In addition to all the above, chiropractic care may be a good option to consider for injury prevention. If your hips, knees, ankles, feet, or low back aren’t moving well, that can significantly affect your overall mobility and ability to handle forces, putting your ACL at risk of tearing. In addition to chiropractic care, we do a biomechanical assessment to guide you in your ACL injury prevention, nutritional support as you work on your sports performance, and soft tissue manipulation to help with recovery and overall mobility.
At Northbound Chiropractic, we are here to help you get on the right path. Whether it’s improving your sports performance, managing or preventing an injury, or just getting rid of pain, we’re here for you. Click the button below or call or text us at (763) 373-9710 to learn more about scheduling an appointment with us.
Giving you my best,
Dr. Nicholas Carlson
Northbound Chiropractic
Serving Delano, MN, Maple Plain, and the rest of Wright and West Hennepin Counties
**Note: These recommendations do not replace advice given by your healthcare provider. Always consult your healthcare provider for personalized medical advice. These are general recommendations, not specific to any one person or condition. Never perform any stretch or exercise that causes pain or discomfort. If you have physical discomfort or pain, stop immediately and contact your healthcare provider. Recommendations and Information provided are designed for informational purposes only, they are not intended to diagnose, treat, or cure any disease.
Citation for the biomechanical risk factors & prevention strategies:
Hewett TE, Ford KR, Hoogenboom BJ, Myer GD. Understanding and preventing ACL injuries: current biomechanical and epidemiologic considerations - update 2010. N Am J Sports Phys Ther. 2010;5(4):234-251. Link to article