How Necessary is Hamstring Strengthening for Preventing ACL Injury?


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The hamstring muscles work with the ACL (anterior cruciate ligament) to maintain knee stability during sports.  Research over the past few years has shown that females who injured their ACL’s had weaker hamstrings than uninjured females or males.  Because of this, the sports medicine community thought that strengthening the hamstrings would decrease risk for ACL injury.  Furthermore, we thought that training the hamstring muscles on the back of the knee to work with the quadriceps muscles on the front of the knee (aka “co-contraction”) would make the knee even more stable and less vulnerable to injury.  The muscles would basically act as shock absorbers to the knee during sports-  think of all the impact a knee goes through during a run, a soccer match, or a basketball game.

However, a recent study showed that just coaching athletes to “land softly” and bend their knees more during single leg plyometrics lowered the impact forces through the knee.   No strengthening program needed- just modification of athletes’ landing form!   And even more shocking was the fact that when the impact forces decreased, the amount of hamstring/quadriceps co-contraction decreased too.   Yes-  you are reading that right- the hamstrings didn’t really have much to do with the possible decrease in injury risk.

So is hamstring strengthening really the key to reducing ACL injury risk like we thought?  Well…I always recommend that strengthening the posterior chain is VERY important.  And this study doesn’t change my opinion much.  However, in terms of ACL injury risk reduction we may have to increase our focus on re-training athletes’ form with plyometrics and agility.  Proper form with these drills is really important-  while landing from plyometrics and when planting/cutting for agility drills-  bend the knees and hips like you are about to sit in a chair.   Get in the habit of doing this every time  you train to build “muscle memory”



For the Sports Medicine and Strength/Conditioning communities… or anyone who appreciates the “Science” of sports! 

The Anterior Cruciate Ligament, or ACL, works to restrain anterior tibial translation through the knee.  The hamstring muscles are the primary active restraint to anterior tibial translation during weight bearing or athletic activities.  Studies, such as by Myer, et al, showed that female athletes with ACL injury had a bigger strength imbalance in quadriceps vs hamstrings than males or uninjured females.  “Quad dominance” has also been cited as a risk factor for ACL injury, defined as an imbalance between quadriceps and hamstring strength, recruitment, and coordination.  So, for years, we thought that strengthening the hamstrings or trying to increase quadriceps/hamstring co-contraction would decrease risk for ACL injury.

However, in this recent article in the Journal of Orthopaedic and Sports Physical Therapy, athletes after ACL reconstruction were coached to land softly and increase knee flexion during single leg landings.  Doing this decreased ground reaction force in the knee but  it also also decreased the amount of quadriceps/hamstrings co-contraction.   Therefore, training quadriceps/hamstring co-contraction may not be as effective as we thought.  This study further supports the work of Hurd et al, and Kellis et al, who showed that in healthy athletes, the knee absorbs impact through increasing the amount of knee flexion and eccentric quadriceps activity-  without a significant hamstring contribution.

So… Here’s my take:  it’s still important to strengthen the hamstrings, but re-training athletes to increase knee flexion angle with cutting and jumping/landing maneuvers is likely more important to for ACL injury risk reduction.   This is the true way to combat the “quad dominant” risk factor for injury.

PLEASE don’t misconstrue what I’m saying to mean the posterior chain isn’t important-  IT IS!  Furthermore, building glute control is absolutely necessary-  the eccentric control of the glutes helps increase hip and therefore knee flexion, which decreases the shear forces through the ACL.   Which is a whole other topic that I will save for another day…

It’s also important to remember that in terms of ACL rehabilitation, especially with a hamstring graft, hamstring strengthening still needs to be addressed

Thoughts?  Share them below…