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Stack Up: Three Basic Exercise Progressions to Help Prevent ACL Injuries


The Anterior Cruciate Ligament (ACL)The Anterior Cruciate Ligament (ACL)The Anterior Cruciate Ligament (ACL) is located in the middle of the knee and helps stabilize the femur over the tibia (see image). The ACL is one of the most commonly injured ligaments in the knee with 150,000 ACL injuries occurring in the United States every year (Boden et al., 2005). In aggressive contact sports, the ACL can be stretched or torn following hits to the leg (many contact ACL injuries are unavoidable and are hazards of playing the game). However, ACL injuries can also occur in the absence of contact; in fact approximately 70% of all ACL injuries occur in the absence of contact. There are numerous reasons for non-contact ACL injuries including: valgus collapse at the knee, large Q-angles, hyper-pronation in the foot, poor hamstring to quadricep strength ratio and poor coordination. Although a comprehensive prevention program is beyond the scope of this article, three exercise progressions will be presented which will address strength, balance and deceleration, helping to mitigate the possibilities of sustaining an ACL injury.

Learning Objectives:

  1. Readers will be able to articulate the function of the Anterior Cruciate Ligament.
  2. Readers will be able to identify one of the most important muscles surrounding the knee in regard to ACL prevention strength training.
  3. Readers will be exposed to ACL prevention drills focusing on: strength, balance & deceleration.

Strength

The musculature at the hip and ankle are equally important as the surrounding knee musculature in preventing ACL injuries. The hip abductors are of huge importance as they help stabilize the femur during single-leg support, and the muscles of the foot and ankle need to properly function to avoid collapsing of the foot arch - which can lead to a valgus collapse at the knee. One of the most important muscle groups in the lower extremity is the hamstring group. The hamstrings act as secondary restraints for forward movement of the upper leg on the lower leg (the primary restraint being the ACL). “When the quadriceps contract they pull the tibia anterior relative to the femur. The resultant biomechanical problem is that the ACL serves to hold the tibia posteriorly (or check anterior translation), and when a female athlete uses her quadriceps to stabilize the joint she induces an anterior shear stress to the tibia and therefore also to the ACL.” (Hewitt, et al., 2010.)

The hamstrings should be one of the primary focal points in any ACL injury prevention program. Although there are a multitude of hamstring strengthening exercises, the single-leg bridge is one of the best. This exercise not only involves hamstring strength, but core strength. The single-leg bridge involves the client laying supine with knees bent at 90 degrees. One leg is straightened and held around a 45 degree angle off the ground (the lower the angle the harder the exercise). The client is asked to stabilize their core and bridge their body as a plank towards the ceiling. Make sure at the top of the motion to cue gluteal activation as well. Most clients will have an asymmetry in strength from left to right; make sure to perform an additional set on the weaker side (2-3 sets of approximately 15 repetitions is solid work for this exercise).

bridge on floor bridge on bench bridge on stability ball
Single-leg Bridge on Floor Single-leg Bridge on Bench Single-leg Bridge on Stability Ball

As the client improves in strength, the same exercise can be performed off a bench (see image), and eventually on a stability ball (see image). The bench increases force so make sure to look for rotation from spine and pelvis; also, cue full hip extension at the top range of motion. The stability ball adds a layer of instability activating more musculature and requiring more cognitive focus.

Balance

Another critical component in ACL injury prevention is balance. Both the vestibular system and proprioception system are important in helping maintain one’s center of gravity over his or her base of support.  Superior balance is a hallmark of high-level athletes and is precipitated by proper strength and mobility. Whether playing sports or engaging in activities of daily living, we often find ourselves having to maintain our body positions and transfer energy on one leg. For this reason, a leg swing progression is an excellent tool used to enhance balance and reduce the risk of knee injuries.

The easiest leg swing motion is performed in the sagittal plane; simply have the client lift one leg and swing it front to back. Make sure the spine stays in a neutral alignment (especially going through the end ranges of motion). The more exaggerated the R.O.M., the more difficult the exercise.

sagittal leg swing lateral leg swing crescent leg swing
Sagittal Leg Swing Lateral Leg Swing Crescent Leg Swing

The next progression is a lateral leg swing. In order to properly perform the lateral leg swing, the client will need to slightly lean toward the side of the base foot. Again, focus on a full range of motion without spine deviation for best results.

Finally, the crescent pattern is performed. The crescent pattern is tremendously important as it involves all three cardinal planes of motion (the rotational component being the most difficult to control). It is critical to cue that the knee remains pointing forward during rotation (i.e., the rotation needs to come from the hip and the ankle, not the knee). The key during these exercises is full range movement and control.

To add another level of difficulty, and to incorporate the vestibular system, leg swings can be performed with eyes closed. If clients have difficulty with the first progression, simply have them balance on one leg until they can maintain 30 seconds prior to progressing into the dynamic swing. Additionally, it is critical to screen for left/right asymmetries in balance and aggressively correct. In a 2005 article, Hewett found that asymmetries in single-leg balance were indicative of a greater future risk of injury (Hewett et al., 2005).

Deceleration

Plyometric training is an integral part of any ACL injury prevention program. Non-contact ACL injuries ineviteably occur during deceleration (the eccentric loading phase). “The benefit of plyometric training is that it aids in the training of adaptations in the sensory motor system that enhances the athlete’s ability to brake, sometimes referred to as the “restrain mechanism” (Swanik, et. al., 2002).

Although sagittal plane jumping seems like an easy skill to teach, it is the hardest mentioned thus far. To start, have your client place his/her feet one shoulder-width apart and pre-load by flexing at the hips, knees and ankles. With arms back and spine in neutral position, drive arms towards the ceiling and extend through the hips and ankles jumping straight up. Land softly with the knees directly over the feet (don’t allow them to collapse together). Valgus collapse of the knees (knees falling towards the midline) has been a predictor for ACL injury (Hewett, Myer, Ford, et. al, 2005).

sagittal plane jump start sagittal plane jump finish
Sagittal Plane Jump Start Sagittal Plane Jump Finish

Deceleration needs to be practiced in multiple planes of motion (not just the sagittal plane as seen in the previous exercise). Once your client can successfully accomplish the sagittal plane jump with good mechanics, introduce lateral jumps to simulate more of a “game day” environment.  The set up is the same as the sagittal plane jump, but the jump occurs laterally. Focus should be on landing with vertical alignment of hips, ankles and knees. Upon landing with a brief pause, the movement is performed to the opposite side. Using tape on the ground or a small barrier increases the intensity and focus needed for this drill.

lateral jump start lateral plane jump finish
Lateral Jump Start Lateral Jump Finish

The final progression is a single-leg lateral bound (skater). Again, stacking of the hip, knee and ankle of the outside leg is critical in power transfer, agility and injury prevention. This exercise can start with a lateral step and tap with the inside leg for less conditioned clients and progress to lateral bounds for distance.

skater start skater finish
Single-leg Lateral Bound Skater Start Single-leg Lateral Bound Skater Finish

Plyometric exercises should only be performed after proper strength and balance levels are achieved and should be introduced with low volumes. Additionally, these exercises should follow a thorough cardiovascular warm-up and dynamic flexibility routine.

Check out this video for an even more in-depth explanation and demonstration of these exercises:

Conclusion

Suffering contact ACL injuries is often unavoidable in sports. However, many athletes and non-athletes suffer non-contact injuries, which could be avoided by better preparing the body. Non-contact ACL injuries often occur during deceleration and result when the neuromuscular system fails to “stack” the joints ensuring stability at the tibiofemoral joint. Proper alignment or “stacking” allows for energy transfer trough the kinetic chain; maximizing power and minimizing injury risk. An ACL prevention program including hamstring strength exercises, single-leg balance drills and multi-planar deceleration will help prevent ACL injuries and maximize performance.

*If you are rehabilitating from an ACL tear you must consult your orthopedic surgeon or physical therapist prior to engaging in any form of these exercises.

References

Boden, B.P., Dean, G.S., Feagin, J.A., et al. (2005). Mechanisms of anterior cruciate ligament injury. Orthopedics, 200(23), 573-578.

Hewett, T.E., Myer, G.D., Ford, K.R., Heidt, R.S. Jr., Colosimo, A.J., McLean, S.G., van den Bogert, A.J., Paterno, M.V., Succop, P. (2005). Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med., 33(4), 492-501.

Hewett, T.E., Ford, K.R., Hoogenboom, B.J. & Meyer, G.D. (2010). Understanding and preventing ACL injuries: current biomechanical and epidemiological considerations update. N Am J Sports Phys Ther., 5(4), 234–251.

Hewett, T.E., Myer, G.D., Ford, K.R., et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: A prospective study. Am J Sports Med, 33(4), 492–50.

Swanik, K., Lephart, S., Swanik C., Lephart S.P., Stone, D., and Fu, F. (2002). The effects of shoulder plyometric training on proprioception and selected muscle performance characteristics. Journal of Shoulder and Elbow Surgery, 11, 579-586.