The Influence of Hormone Levels on ACL Laxity: Are We Missing a Piece of the Puzzle?

By Steph Allen, DPT, OCS

How many of you out there have torn or know someone who has torn their ACL? How many of you, or the ones you know, are female? Have you/they injured the ACL multiple times? How many have injured BOTH knees one or more times?

If we go with what research statistics have shown, there are likely a large number of you that answered “yes,” “two or more times,” “most are female,” or “both right and left knees.” These are common responses we hear. Not super promising.

The thing is, we seem to know so much about the anatomy of the ACL, injury mechanisms, risk factors, and potential prevention. However, injury rates are not improving, and females remain [continue to be] at a much higher risk of both initial injury and of reinjury.

Frankly, I think we are missing something here, people!

Let’s first take a look at what we DO know, based on years of scientific research:

  1. ACL injuries are often non-contact in nature
  2. ACL injuries are 2-6 times more likely in females than in males
  3. Re-injury and injury to the opposite side are all too common
  4. Rates of return to high level/previous levels of sport are low
  5. Injury prevention programs are starting to take better shape, and it is more widely accepted that a focus on jump/land technique and neuromuscular control is essential
  6. Around puberty, females tend to show a decrease in strength and power and generally poorer LE control and mechanics with jump and land, as well as cutting tasks, as compared to their male peers

This is all extremely helpful, and should be taken into account with all athletes and patients. BUT, I’d like to hop outside of the box for a sec (with pristine control, of course) and explore a possible “X” factor in this conundrum…

I want to look more closely at the impact that hormones have on the ligament complex and how this differs in males vs. females. We have done a great job with researching risk factors, and developing solid injury prevention/risk reduction programs for athletes. However, the sex disparity in ACL injury is still significant.

So let’s dive in a bit with the sparknotes of the research that has been done regarding hormonal influences on ligament laxity…

WHAT THEY DID: They took ACL’s, exposed them to the various hormones present during different phases of the menstrual cycle and measured “laxity” via tensile strength (resistance to pull).

WHAT THEY FOUND: Greatest laxity resulted when the ACL was exposed to Estradiol, which is at its peak in ovulatory phase (10-14 day mark). They also noted increased laxity when it was exposed to relaxin (another hormone floating around during the cycle). AND… they found these exposures to have profound effects on tissue remodeling. [NOTE: due to variability between individuals, it is difficult to assign highest risk to one phase].

Screen Shot 2018-03-20 at 5.39.58 AM

Alright, that’s cool. But, how does this work? How can these hormones actually make the ligament more lax?

Here is the mind blow…There are hormone RECEPTORS ON THE ACL!!

When the hormones bind to the ligament, this can affect gene expression and collagen metabolism in a way that can influence the characteristics of the ACL and other soft tissues around it. (*This is key- it is not just the ACL that is affected- it is ALL the soft tissues).

A bit more about why collagen matters: In tendon tissue; collagen accounts for approximately 60–85% of the dry mass of the extracellular matrix (mostly type I collagen). Tiny collagen fibrils group together within the tendon and form “functional fibers”, whose purpose is the transmission of force between muscle and bone. This is HUGE! If the metabolism/cellular turnover and production of the tissue that is largely involved in force transmission at a joint is being affected, this could be a big contributor to the problem of overall joint stability and injury risk.

pasted image 0


Okay so enough of the cell and biology stuff, let’s take a step back and look at the bigger picture. Let’s take into account movement patterns and neuromuscular control, which we all know is MAJORLY important, both in risk reduction and post-injury rehab/return to sport.

Park et al. found increased loads during cutting tasks in females during their cycle. The most common non contact MOI is deceleration with a quick change in direction, such as a plant and twist that occurs with cutting in sport. In biomechanical terms,  this is combined valgus with tibial IR, putting the ACL on high tension (great explanation in the @cvasps podcast with Tim Hewitt- click here or see references below). These two things, combined with the fact that we now have reason to believe that hormones can increase ligament laxity, may just be what is creating the perfect storm for ACL injury in females…

Lower tissue tolerance/greater tissue laxity + Game type speed and force on ligament with cut/jump/land + physical/mental fatigue and poor mechanics = Tissue failure

pasted image 0-2

Oh man, so what in the world can we do about this? You might argue, hey, we can’t control something such as the menstrual cycle; it’s part of every healthy female’s life. And I can’t disagree with that. However, I cannot, and WILL NOT believe that there are not some actions we can take to combat these biological constants.

First and foremost, let’s make risk reduction/injury prevention programs more of the norm in youth athletics and let’s start these programs EARLY. Thompson-Kolesar et al. found that pre-adolescent female soccer players demonstrated greater improvement in double leg jump tasks after participating in an injury prevention warm up program than did older adolescent females. So if we can reach these kids before skeletal maturity and while they are still learning motor patterns for the first time, we can be more confident that they are able to control their sports specific movements even when they are fatigued or the movements are unplanned.

Second, let’s keep encouraging females to get really strong for their sport, thereby making it “cool” and more socially accepted during teenage years when being cool is as important as breathing oxygen. Thompson-Kolesar et al. also found that the rapid bone growth that occurs in adolescence correlates with an increase in muscle power and strength in males, but corresponds with a decrease in strength and dynamic knee stability in females. So now we have a larger human and larger bones to support, coupled with less of an ability to produce force and decreased neuromuscular control. Yuck. Let’s get ahead of this and address BOTH the strength and motor control deficits that seem to poke their heads out during puberty and adolescence.

Third, let’s stop burning the candle at both ends. This part may be a bit on the opinion end of the spectrum, but I think many would agree. So many young athletes, especially elites, are training SO much and training a single sport. Under-recovery and repeated motor patterns without exposure to movement variability is also a piece of the problem puzzle, if you ask me. I am an advocate of proper recovery and promoting multi-sport participation, regardless of age.

Lastly, let’s please make the conversation surrounding the menstrual cycle as it pertains to training less taboo, more the norm, and a bit more influential. Sex differences in injury cannot be fully explained by strength or motor control alone. There is something else to this! If there is a way we can combat the influence of hormones on ligament laxity and overall injury risk, ultimately leveling the playing field (literally and figuratively), then let’s do it! I think this warrants a deeper dive into research and investigating longer term data so that we can actually solve this puzzle and get these injury and reinjury rates under control.

I hope this piece sparks some interest and discussion, and results in more people asking outside of the box questions. Our athletes, patients, and clients deserve it. Let’s go!

_DSC4770 Blog Post written by Steph Allen, DPT, OCS. Steph has a particular passion for ACL research and rehabilitation and hopes to be instrumental in making positive changes, both in in post op rehab and risk reduction programs.

I asked Steph to write on this topic after hearing her Interview on the CVASPS Podcast, click here to listen. This is an important topic if you have daughters involved in sports, or if you coach, train & rehab female athletes.

Steph is on staff at Boston PT & Wellness in Medford MA. You can find her on Instagram @stephallen.dpt


CVASPS Podcast with Tim Hewitt:

Thompson-Kolesar JA, Gatewood CT, Tran AA, Silder A, Shultz R, Delp SL, Dragoo JL. Age Influences Biomechanical Changes After Participation in an Anterior Cruciate Ligament Injury Prevention Program. Am J Sports Med. 2018 Mar;46(3):598-606. Doi: 10.1177/0363546517744313. Epub 2017 Dec 27. PubMed PMID: 29281799.

Von Rosen P, Kottorp A, Fridén C, Frohm A, Heijne A. Young, talented and injured: Injury perceptions, experiences and consequences in adolescent elite athletes. Eur J Sport Sci. 2018 Mar 3:1-10. Doi: 10.1080/17461391.2018.1440009. [Epub ahead of print] PubMed PMID: 29504456.

Leblanc DR, Schneider M, Angele P, Vollmer G, Docheva D. The effect of estrogen on tendon and ligament metabolism and function. J Steroid Biochem Mol Biol. 2017 Sep;172:106-116. doi: 10.1016/j.jsbmb.2017.06.008. Epub 2017 Jun 16. Review. PubMed PMID: 28629994.

Park SK, Stefanyshyn DJ, Ramage B, Hart DA, Ronsky JL. Relationship between knee joint laxity and knee joint mechanics during the menstrual cycle. Br J Sports Med. 2009 Mar;43(3):174-9. doi: 10.1136/bjsm.2008.049270. Epub 2008 Aug 26. PubMed PMID: 18728055.





Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: