Does Previous Knee Injury Impact Performance & What Can We Do About It? - Part II

Last week, we looked at whether or not an ACL injury impacts an athlete’s future performance.  Based on what we learned from the Read et al and Mai et al studies, we know that an ACL injury and ACLR does have an impact on future athletic performance.  So, the obvious take home is that we need to somehow try to prevent these injuries.  How do we do that?

If we take football in isolation, the obvious question becomes, how do NFL Players’ ACLs get injured?  Most would assume that in a violent collision sport like football that the injury occurs from a contact with another player.  Some in the media have suggested that as a result of the new concussion rules and players being required to hit lower, that this is resulting in an increase in ACL injuries.  This assumption is based on the assumed fact that the majority of ACL injuries in football are contact in nature. 

Based on an analysis of NFL data from 2013 to 2016, there were a total of 202 ACL injuries.  Of the 202 ACL injuries during this time, 73% (147) were non-contact in orientation.  This means that the mechanism of injury was not another player striking the knee or the player during the injury.   Further analysis reveals that 66% of ACL injuries in the NFL are isolated to 5 key positions:

• Wide Receivers – 19.4%
• Linebacker – 15.5%
• Cornerback – 11.7%
• Offensive Line – 10.7%
• Defensive End – 8.7%

This data is based on public record and what information is available through the NFL.  But does this hold true in the research and if so, what is there a common mechanism that results in the player rupturing their ACL?

Johnson et al published a paper in the American Journal of Sports Medicine in 2018 that looked to determine just that. 

Purpose:  The purpose of this study was to describe the mechanism, playing situation, and lower extremity limb position associated with ACL injuries in NFL players through video analysis to test the hypothesis that a majority of injuries occur via a contact mechanism.

Methods:  In this study the authors performed a retrospective cohort of National Football League (NFL) players with ACL injuries from 3 consecutive seasons (2013-2016).  Of the 156 ACL injuries identified, 77 (49%) occurred during the regular season and playoffs with 79 (51%) occurring in pre-season and training camps.  Video analysis was available for 69 injuries which was reviewed to determine the mechanism of injury. 

Results: The majority of ACL injuries analyzed occured vai a noncontact mechanism (50 of the 69 or 72.5%).  The only exception to this was offensive lineman who had only 20% of their ACL injuries by noncontact mechanism.  The most common activity leading to the noncontact injury was pivoting or cutting.  The most common position of the limb during the injury was hip adduction/flexion, early knee flexion/abduction and foot abduction/external rotation.  There was no association between injury mechanism and time of play or playing surface.

Discussion:  The results of this study suggest that the majority of ACL injuries in the NFL occur from noncontact mechanism with the lower extremity exhibiting a dynamic valgus moment at the knee.  This lead the authors to suggest that to reduce these types of injuries prevention programs should assess for these movement patterns and develop intervention programs which address.

Aside from these points, there are also some additional key points that came out of this study. 

1. Lineman are more susceptible to contact ACL injuries.  This fact has led a lot of teams at the professional and collegiate level to proactively brace players.  Conceptually this might seem like a great idea, but there is very little support that this prevents ACL injuries in contact sports.  Additionally, we need to think about what this brace does to gait (how you walk and run) and muscle EMG activity.  What we do know is that players wearing two ACL braces walks and runs with an altered gait and EMG activity of the quads and hamstrings is decreased.  So what is the answer?  We may not have the answer to that yet but we do need to think about how we address this.
2. Over 1/2 of ACL injuries are in the preseason.  In preseason, a lot of players are coming into preseason and camp in a deconditioned state.  Knowing the majority of these injuries are non-contact in orientation, that certain biomechanics add to increased risk of these injuries and that targeted training programs have been shown to decrease risk for these injuries, it seems implementing a post season and preseason movement training program would be beneficial.  
3. No association between injury rates and time of play.  This is basically stating that fatigue did not play a role in the non-contact ACL injuries.  There are several authors that suggest that fatigue does play a role and a whole other contingency that states fatigue does not play a role.  For those in the fatigue does not play a role camp, they site that the literature simply does not indicate this is the case. 

We are going to save this for our next discussion to look at, does fatigue play a role.  Stay tuned next week as we look at this in more depth.  Also, please make sure to check out our new website at www.iceperform.com where our goal is to help you help others.  #ViPerformAMI

Dr. Nessler is a practicing physical therapist with over 20 years sports medicine clinical experience and a nationally recognized expert in the area of athletic movement assessment.  He is the founder | developer of the ViPerform AMI, the ACL Play It Safe Program, Run Safe Program and author of a college textbook on this subject.  Trent has performed >5000 athletic movement assessments in the US and abroad.  He serves as the National Director of Sports Medicine Innovation for Select Medical, is Vice Chairman of Medical Services for USA Obstacle Racing and movement consultant for numerous colleges and professional teams.  Trent is also a competitive athlete in Brazilian Jiu Jitsu.