Gain lower body strength and prevent ACL injury by training for structural balance. Research shows 50 to 80 percent of ACL injuries in field and court sport are non-contact and could be prevented.
A new review into why non-contact ACL injuries occur points to a few things you can do to decrease risk. Despite a number of conflicting and inconclusive studies, a few key points came out of the analysis:
• Movement analysis of ACL injuries show that a low knee flexion angle, knee valgus, and minimal internal knee rotation were linked to increased injury risk.
• Higher ground reaction forces, particularly during cutting motions, increase ACL injury risk. Imbalances between the lower body muscles increase risk of injury when the body experiences high GRF. Aside from a finding that greater soleus activation aids in stabilizing the tibia from the posterior and thereby reduces ACL injury risk, no conclusions were offered about how structural imbalances influence injury risk.
• There was a higher ACL injury risk when an athlete was sidestepping (when the athlete pushes off the right foot to change direction to the left, for example) than when cross-cutting (pushes of the left foot to change direction to the left) or doing a jump and step cut (athlete jumps to pivot point, lands, and sidesteps off right to change direction to the left).
• Knee joint laxity, which can be defined as passive or active, was found to contribute to ACL injury risk. Neuromuscular control and strength as well as hormone levels contribute to knee joint laxity. Greater hamstring flexibility and anterior tibialis volume were linked to increased ACL injury risk in one study, however, this connection doesn’t consider how structural balance of the lower body muscles influence injury rates.
• No study tested the effect of increasing strength on ACL injury risk. Nor were there any studies assessing knee joint strength ratio and injury risk.
• There was a lack of studies on how fatigue and proprioception influence injury risk. However, we do know from previous studies that a fatiguing lower body strength training protocol of deadlifts, back squats, and bench presses resulted in significant changes in athletes’ movement patterns in a body weight squat. During the descent, athletes recruited the hip adductors to function as hip extensors to eccentrically control the body. The hip adductors were also overly engaged at the expense of the hip and knee extensors when producing concentric force to return to neutral.
Although much remains unknown about ACL injury prevention, the take away is that training for structural balance in the lower body so that athletes can manage high GRFs most effectively will keep them healthier. The effect of muscle imbalances can be seen with a recent study that showed that having subjects do body weight squats with the ankle dorsiflexed 12 degrees on a wedge board resulted in greater knee valgus and higher soleus activity, which decreased activity of the vastus medialis obliquus.
This is one example of how imbalanced muscles and range of motion restrictions can influence movement patterns. Naturally, the body weight squat is a diverse movement from a cutting motion at high speed, but it points to the training factors that need to be addressed to prevent injury and improve performance. A structural balance screen that looks at the body unilaterally and bilaterally will allow you to identify problems. Split squats, step ups, and eventually deep squats can help develop greater structural balance.
Serpell, B., et al. Mechanisms and Risk Factors for Non-Contact ACL Injury in Age Mature Athletes Who Engage in Field or Court Sports. Journal of Strength and Conditioning Research. 2012. 26(11), 3160-3176.