The Lurking Sideliner: The Risk and Reasoning of Female ACL Injuries

The frequency of female knee injuries compared to their male counterparts has become increasingly recognized.  There are numerous hypothesized female ACL tear factors.  Supposed “research” has suggested everything from anterior cruciate ligament size to hormonal fluctuation and everything in between.  This piece focuses on the concrete factors that have been proven to increase the susceptibility of ACL tears in females; factors which if improved upon can reduce the risk of injury.  With an understanding of why girls are at higher risk for this type of injury in the first place, knee injury reduction can be systematically approached from the core outwards. 

According to the National Federation of State High School Associations, 15,190 more girls played a high school sport last year marking the 24th year in a row participation has increased.  Compared to boys, girls are 8 times more likely to undergo an ACL injury and “as many as 70% of ACL injuries involve little or no contact with the other player” (National Institutes of Health Medicine Plus, An Athlete’s Nightmare: Tearing the ACL).  Basketball, volleyball, and soccer pose the most serious threat because they involve the 3 most significant movement patterns employed when a knee injury occurs: deceleration, lateral pivoting/cutting, and landing.  These three sports are the number first, third, and fifth most popular sports amongst girls, measured by participation.  In case this has not captured your attention, research done by Josh Sandell published in a document entitled “Noncontact ACL Injuries: Risk Factors and Prevention Strategies,” proved that “females are 7.8X more likely” to tear their ACL than males and 5X more likely while playing soccer.  Not good news for young female athletes (or their parents). 

As an athlete, sister to female teenagers, high school girls’ basketball coach, and athlete preparation coach, the truth about female ACL injuries is sobering.  In order to truly prepare female athletes for competition we cannot merely focus on strength.  We must target their true weaknesses and use preemptive measures to help them not only compete well, but stay safe and healthy.  Women were created different than men and ACL statistics have repeatedly proved this point.  Following are the primary internal factors leading to ACL injuries in the female population:

Poor proprioception
React poorly to unexpected movements
Girls are more quadriceps dominant

Hamstring/quadriceps ratio is disproportionate

Poor hip muscle activation which leads to poor hamstring activation
Electromechanical delay

Delayed hamstring activation (co-contraction is ideal!)
Leads to poor quad/hamstring coordination

Poor joint stiffness (knee stabilizers)
Significant internal hip rotation especially when landing

Girls have a greater femoral angle – essentially wider hips

Weak/inhibited glutes
Poor core strength

The quadriceps will often respond prior to the hamstrings while women are performing deceleration movements.  The “electromechanical delay” – the time between initial electrical activity and actual muscle tension -- has been shown to be longer in females versus males.   If these antagonist muscles are not responding simultaneously the joint, stabilizers, ligaments, and tendons of the knee suffer unneeded and unnatural amounts of pressure.  In order to prevent the ever common valgus knee collapse, the hamstring must balance the force generated by the quad muscles.  The hamstring muscle should be greatly assisting in the stabilization of the knee whenever the lower extremities are called upon in eccentric loading.  Without proper recruitment of the hamstring, ligaments around the knee handle far more force production than they were meant to. Not only is the hamstring slower to fire, it is commonly weaker making a bad situation worse.  ACL injury research has shown that female athletes often land with limited hip and knee flexion, which also requires the quad to handle stabilization during these movements, a force the hamstrings cannot counterbalance due to weakness.  This leads to extra stress on the ACL. 

Another key issue is “joint stiffness,” a fancy term describing weak knee joint stabilizers.  The quads tend to fire first in girls, putting undue stress on the knee; research has shown joint stiffness as a contributing factor to knee injuries.  The knee is already dealing with undue stress because of this electromechanical delay differential between quadriceps and hamstrings.  If the stabilizing muscles around the knee do not create significant tension around the joint, the knee will lack overall stability.  When the knee joint lacks this stability, the force created during athletic movements (specifically decelerating or cutting movements) will be primarily directed towards the ligaments of the knee…such as Anterior Cruciate Ligament and Medial Collateral Ligament.  Can you say ACL injury?  And last but not least, a weak core (from shoulders to knees) is evident and correlated with ACL injuries in females because the core muscles can help protect the ACL if they are properly strengthened.  In a video analysis published in The British Journal of Sports Medicine, Hewett, Torg and Boden noticed that lateral trunk motion and the torque from aforementioned knee valgus are interrelated. 

With all the information out there, one thing is clear: athletes are suffering an increasing amount of knee injuries each year.  Solid evidence has shown similar traits amongst those who have undergone such injuries.  Many of these traits involve controllable factors which have simply been unseen or neglected.  With steady, calculated training, injury rates can be reduced and athletes can continue their athletic endeavors with an extra confidence knowing they have tended to weaknesses and imbalances that put so many at high risk for injury.