In recent years, much of the focus in athletics has been on concussions, which is an emerging area of both concern and research. Although this is an important area of research, it is vital to not forget about the proven success of ACL prevention programs.
For the many athletes each year that suffer ACL tears, the question is always "when can I return to sports?" ACL tears usually require surgical reconstruction of the ACL which has varying results and outcomes. Some athletes are able to return to sports without issue, others suffer reinjury, and some never return to competitive sports.
Mechanism of Injury
ACL injuries typically involve a non-contact event with a valgus (inward) movement of the knee. The athlete is usually running, cutting, or landing from a jump.
About 70% of ACL tears are non-contact while 30% are contact events. Although contact events can’t be avoided, non-contact injuries have been shown to be preventable by implementing a 20-minute warm up that is performed 2-3 times a week for a 12-week period in conjunction with a focus on correcting poor mechanics.
Gender has been shown to be one of the largest predisposing factors to ACL tears. In general, females are 4-8 times more likely than males to have an ACL injury. This is due to differences in movement patterns and muscle strength. Those kinematic differences in females are as follows:
- Reduced knee flexion
- Reduced hip flexion
- Increased hip internal rotation
These factors can be seen in the image below on the right side. Compared to the image on the left the person on the right has decreased bending in the hip and knees as well as inward caving of the knees.
Muscle Activation Patterns
In studies involving muscle activation patterns they have found increased quadriceps activity and decreased gluteus maximus activity in those who land from a jump with poor mechanics. For this reason prevention programs focus on teaching athletes to use what is called a “hip strategy” (use of the hip and gluteal muscles) versus a “quad strategy” (primarily using the quadriceps muscles at the front of the thigh).
With a good hip strategy, an athlete is able to land with both hips and knees flexed while keeping control of their knees and not allowing them to come together or bend forward past the toes.
The video below depicts an athlete performing a broad jump with good form. Notice how his knees do not extend past his toes and his hips are bent in equal proportion to his knees.
These principles apply not only to jumping but running and cutting as well. The images below illustrate an athlete how is at high risk for tearing their ACL (right), compared to an athlete who is at low risk (left).
Biomechanical Predictors of ACL Injury
The following is a list of movement patterns that may predict an ACL injury:
- Knee valgus angle and moments (inward collapsing of the knee)
- Increased knee extensor moments (landing with a straight knee)
- Decreased hip and knee flexion (decreased bending of the hip and knee during landing)
- Lateral trunk motion (sideways movement of the trunk and upper body)
Conclusion and Solution
Research has shown in athletes who injure their ACL there is an imbalance between quadriceps activation and hip activation during sports activities. At risk athletes are more likely to overuse their quadriceps versus gluteal muscles, land straight legged, and have decreased hip and knee flexion. All of these components result in a valgus moment at the knee (knees caving together during motion), which leads to increased risk of tearing the ACL.
Incorporation of an ACL prevention program has been shown to be effective in decreasing incidence of torn ACL’s in athletes who are at risk. The Santa Monica ACL Prevention Project developed a program called PEP, which stands for Prevent injury, Enhance Performance. This program was a prospective study on the incorporation of a 20-minute warm-up which involved the following five components:
In the study, this warm up was performed 2-3 times per week for a total of 12 weeks and led to an 88% reduction in ACL tears in the exercise group versus the control group. To read more about this study click here.
Much of the content from this blog originated from a presentation by Chris Powers, PhD, who is a leader in research at USC. If you are interested in learning more about ACL prevention and biomechanics click here to watch the presentation by Chris Powers.
If you are concerned you may be at risk for an ACL injury a physical therapist can help! A physical therapist will be able to identify any predisposing factors and help develop an exercise plan to keep you at low risk for injury. At AOPT we are offering free screens where you will have the opportunity to discuss any questions or concerns you may have with one of our physical therapists.
Mandelbaum BR, Silvers HJ, Watanabe DS, Knarr JF, Thomas SD, Griffin LY, Kirkendall DT, Garrett W Jr
Am J Sports Med. 2005 Jul; 33(7):1003-10.