By: Matt McManus
Every year, thousands of athletes around the country will tear their ACL. While advances in surgery and rehab have cut down the recovery time for these injuries and helped athletes come back as strong as ever, it’s still a devastating season-ending injury with long-lasting implications. Some of these injuries are impossible to prevent. There are ways, however, to manage and mitigate the risks of suffering non-contact injuries, which account for over 70% of all ACL tears (Cyphert). The four most important characteristics to identify are mobility, knee valgus, strength, and injury history.
Oftentimes when there is a lack of mobility around a joint it isn’t that joint that is at risk for injury, but rather the joints above or below. Think about how often low back pain can be attributed to tightness in the hamstrings and hip flexors, for example.
In regards to protecting the knees the two joints we’re most concerned with are the hip and the ankle. If the hips or ankles are tight, force cannot be dissipated at those joints during cutting or landing and instead stresses the knee. A bodyweight squat is a great test to determine if an athlete possesses adequate mobility at both of these joints. If they can’t reach proper depth in a squat (crease of the hip below the knee) with good upper body posture further tests may be necessary to determine exactly where the problem is. My two favorites are the hip rotator stretch and ankle lunge, shown below.
Hip Rotator Stretch- Ideally, the lower leg will stay flat on the box and the athlete should be able to get their chest close to touching the box. An inability to get the shin to lay flat on the box means the athlete’s hips, specifically the piriformis, are tight.
Ankle Lunge- Standing 4-6 inches away from a wall, the athlete pushes their knee as far over their toe as possible while keeping the heel flat. Failure to maintain heel contact with the ground or get the knee to touch the wall indicates tightness in the ankle and calf.
In addition to serving as evaluations, these two stretches are great for improving mobility when done regularly.
Knee valgus is the “knock kneed” position individuals display when jumping or landing. Sometimes this is a structural issue, called Q angle (described below). Other times, however, it is caused by weakness in the glutes and an inability to control the head of the femur in the hip socket. Excessive knee valgus can lead to ACL injuries by increasing the torque on the knee when jumping, landing, and cutting.
Q angle is the angle of articulation between the femur and tibia. It is particularly problematic with females because they have wider hips, meaning the femur inserts farther away from the midline of the body. Unfortunately, you can’t “fix” your Q angle. You can’t change how narrow your hips are or where your femur attaches on your pelvis. What we can do, however, is work to strengthen the glutes and ingrain proper motor patterns so the problem isn’t exacerbated with activity.
In addition to extending your leg backwards, the major function of the glute is external rotation of the femur. The stronger your glutes are, the more easily they can resist and absorb the forces encountered in sports and prevent your knees from caving in.
One of my favorite exercises to activate the glutes is the band squat hold, below. Set your feet shoulder width or slightly wider, depending on the tension of the band. Hold onto a rack for support and squat down below parallel while maintaining good posture. Push the knees out hard and hold for time.
Mini band squat hold- 3x20 seconds prior to or in between sets of squats
In addition to specific strengthening work like the band squat hold, verbal cues during lower body exercises can be important in teaching the athlete to activate the glutes without thinking. Reminding athletes to push their knees out when squatting or during plyometrics can help this become second nature. While this doesn’t guarantee they will do the same thing on the field, learning to activate the glutes under a heavy load in a controlled environment can help ingrain the motor pattern to ensure knee valgus isn’t amplified during play.
There are two ways to measure strength:
Maximal strength- The most an athlete can lift regardless of bodyweight.
Relative strength- The most an athlete can lift, divided by bodyweight.
While maximal strength is obviously useful and beneficial, relative strength is the more important quality when evaluating ACL injury risk. The stronger an athlete is relative to their bodyweight, the more easily and effectively they can absorb the high impact forces associated with cutting, landing, and decelerating.
The data in the chart to the right was compiled during my time at Utah State. It shows every athlete that came through the women’s soccer program at USU over six recruiting classes. After testing on squat, each athlete was ranked based on relative strength. Athletes with prior ACL injuries, whether it was during their college career or before their arrival on campus, are highlighted. Obviously, lower levels of relative strength don’t guarantee that an athlete will suffer an ACL injury, but there’s an undeniable trend of weaker athletes getting hurt more often.
No amount of strength protects you from a contact injury and there are still incredibly strong athletes who tear their ACL (look no further than Adrian Peterson). The data above, however, while a small sample size, shows that females in the 1.4-1.5 range, even 1.3, are pretty safe. For males this number is higher, a minimum of 1.5, with 2 times bodyweight being ideal.
In addition to relative strength, hamstring strength plays a crucial role in preventing ACL tears. The ACL attaches in the back of the femur and the front of the tibia. One of its major functions is to prevent the tibia from sliding forward relative to the femur. This is why athletic trainers use an anterior drawer test when diagnosing a tear. The hamstrings, attaching on the posterior side of the tibia, also exert a posterior force on the tibia when flexed. Strong hamstrings, therefore, can actually take some of the stress of the ACL during activity, allowing it to bear less of the burden. Strong hamstrings also help stabilize the knee by equalizing the forces of the quadriceps acting across the front of the knee. In fact, one of the main criteria physical therapists and doctors look at before clearing an athlete after an ACL tear is the quad:hamstring strength ratio. For complete development of the hamstrings, make sure to include a variety of hip and knee dominant hamstring exercises, like RDLs, good mornings, glute ham raises, and Russian leans, and load them HEAVY.
The best predictor of future injury is past injury. Two startling statistics show how true this is:
-Athletes who have had an ACL injury are six times more likely than others to suffer another tear.
-29.5% of athletes suffer a second ACL injury within 24 months of returning to activity.
This is because athletes who have suffered an ACL injury in the past usually had red flags in strength or mobility tests to begin with. If these issues aren’t corrected, the risk still exists. In these cases we need to be especially thorough in identifying risk factors. Bringing strength up to the aforementioned 1.4 level can take a lot of time, but any increase in strength will be beneficial and things like glute activation and mobility work can be implemented immediately and improved relatively quickly.
Along with the factors discussed above, there are other programming principles that can be helpful in preventing these injuries, including sensible plyometric progressions, teaching proper landing and cutting mechanics, and balance/proprioceptive training. This article is simply a synopsis of the qualities that are most important and an explanation of what we do within the framework of our programs to identify at-risk athletes and minimize the chances of them suffering a non-contact ACL injury. Having a system like this is important. Quite simply, the more accurately you can predict those at risk for injury the more successful you'll be at preventing these injuries from happening.
Brotzman, Brent S. Clinical Orthopaedic Rehabilitation. Mosby Elsevier, 2003. pp. 266-92.
Cyphert, L., Denegar, C.R., Woodford-Rogers, B. 1994. Risk Factors for Anterior Cruciate Ligament Injury in High School and College Athletes. Journal of Athletic Training. 29(4): 343–346.