Why is it that athletes performing a movement they have done so many times suddenly tear their ACL?  We have been studying ACL injury and prevention for many years now, and despite our best efforts, we have not made marked progress in preventing the number of ACL injuries.  In addition to anatomical variants and perhaps some genetic predisposition, I feel that the earlier push for sports specialization in our society resulting in increased training/competition hours is a major factor.


The term ACL fatigue may or may not be familiar to you.  But in essence, this theory would suggest that after a certain number of impacts/loading, the ACL becomes weakened and less resistant to strain.  You could almost compare this to a pitcher who suffers an injury to his medial collateral ligament with too much throwing.

As someone who is consistently rehabbing athletes with ACL tears and screening athletes to assess injury risk, I am always interested in how we can keep people from suffering such a devastating non-contact injury. A recent article in the American Journal of Sports Medicine sought so assess ACL fatigue failure in relation to limited hip internal rotation with repeated pivot landings.

We already know that hip mobility is often an issue for our athletes.  Researchers at the University of Michigan sought to determine the effect of limited range of femoral internal rotation, sex, femoral-ACL attachment angle, and tibial eminence volume on in vitro ACL fatigue life during repetitive simulated single leg pivot landings.

They used 32 cadaveric knees from 8 males and 8 female specimens without evidence of arthritis, prior knee surgery, or joint deformity.  They designed an apparatus that would simulate a jump landing with a force approximating that of 4x bodyweight with the knee in 15 degrees of flexion.  There were tow primary testing conditions:

1. Knee locked to prevent any femoral rotation at all

2. Knee only restrained by light springs simulating dynamic muscular restraint from the hip rotators but allowing relatively free rotation at the femur


  • 8 graft failures – 7 failures with locked IR and 1 failure with springs
  • 5 female specimens failed at a mean of 15 +/- 13 loading cycles, whereas 3 male specimens failed at a mean of 84 +/- 41 cycles (1 complete tear, 2 partial tears, 2 permanent elongation failures, and 3 tibial avulsions)
  • Knee specimens with limited femoral IR during repetitive in vitro pivot landings had a risk of ACL failure that was 8.3x higher than those specimens with free rotation (using only the rotation as a predictor)
  • The full statistical model including rotation, sex, femoral-ACL attachment angle and tibial eminence volume did not significantly predict ACL failure risk
  • The best and final statistical model used limited femoral IR and sex
  • When counting for sex, failure rate was 17.1x higher
  • When accounting for femoral rotation, females had a 26.9x higher risk for failure, but this did not reach statistical significance
  • Results corroborate current evidence that human ACL is susceptible to a repetitive loading injury and that the femoral enthuses, especially that of of posterolateral bundle is at risk of injury and perhaps this explains why athletes may experience injury with familiar maneuvers

Key takeaways

  1. We need to gain a better understanding of ACL fatigue and look at training volume/intensity for our athletes ranging from youth to elite and consider the efficacy of establishing limits to such high level training that imparts pivot type cutting and landing maneuvers on the knee
  2. We MUST screen our athletes and do a better job of identifying “at-risk” subsets of the population who are competing in higher risk jumping and cutting sports using tools like the FMS and involve physical therapists in the care of those deemed to be at higher risk for injury
  3. We need to educate coaches, parents and youth about the importance of proper mobility, stability and movement preparation techniques that can be implemented prior to practice and play.  In addition, higher risk athletes should have their own corrective exercises or routine that augments this team routine if needed.  In my opinion, the warm-up is the ideal time to implement ongoing injury prevention activity to ensure compliance and consistency.

We have much work to do in terms of finding the answers here.  While we will never put an end to ACL injuries, I feel strongly that we can help reduce injuries through a better understanding of how the body moves and adapts to loading, while utilizing better training techniques for our athletes.

Click here to see an exercise I use to improve hip strength and stability that also facilitates closed chain hip internal rotation.

Article Reference

Beaulieu ML, Wojtys EM, Ashton-Miller JA. Risk of Anterior Cruciate Ligament Fatigue Failure Is Increased by Limited Internal Femoral Rotation During In Vitro Repeated Pivot Landings. Am J Sports Med. 2015 Sep;43(9):2233-41