Why Are Athletes Always Tearing Their ACLs?

Answers to your questions about the news.
May 4 2012 3:16 PM

Exit Sandman

A torn ACL may have ended Mariano Rivera’s career. Why are ACL injuries so common?

Mariano Rivera
Mariano Rivera

Photograph by Al Bello/Getty Images.

New York Yankees relief pitcher Mariano Rivera tore his anterior cruciate ligament while shagging balls during batting practice on Thursday. On the first day of the NBA playoffs last weekend, both Derrick Rose of the Chicago Bulls and Iman Shumpert of the New York Knicks tore their ACLs, too. Why do so many athletes suffer this injury?

Because we weren’t built to leap and cut. The cruciate ligaments are located inside the knee joint, connecting the underside of the femur (thigh bone) to the top of the tibia (one of the shin bones). The anterior cruciate ligament’s main duty is to prevent the tibia from sliding in front of the femur and out of joint. (See this video.) The dynamic forces created by leaping and cutting side-to-side tend to stress the ACL by pushing the tibia out of position. When the ACL tears, it’s usually because the athlete rotated his hips at the wrong moment, multiplying the forces on the ACL. This can happen when someone makes a slightly awkward movement while planting his foot. The ACL tears more often than any other ligament—there may be as many as 200,000 ACL injuries per year in the United States—because of the lack of muscle support for twisting or rotational movements around the joint. The quadriceps and hamstrings are much better at protecting the ligaments on the outside of the knee joint (the collateral ligaments), which can be injured by side-to-side movements. Injuries of the posterior cruciate ligament, or PCL, are rare in sports because under most circumstances, the tibia would not get forced backward by a movement. PCLs do get torn in car accidents, when the tibia strikes the dashboard.

Weak knees may be an evolutionary consequence of our rapid shift to bipedalism around 6 million years ago. As quadrupeds stride, they are able to spread the force of the impact across four sets of muscles. Our two spindly legs have to absorb all of our body weight on their own. To compensate, we developed a straight-knee gait, allowing our bones and joints to absorb the shock that the muscles can’t handle. (Contrast the walking style of a chimpanzee, which keeps its knees slightly flexed throughout its stride, with that of a human, who locks out his knees when the heel touches down.) The system works well enough during a leisurely stroll, but our knee joints aren’t well-adapted for leaping, twisting, and changing direction.


There’s no solid evidence to suggest that ACL injuries are more common now than they were 50, 100, or 1,000 years ago. Still, as athletes have gotten bigger and faster, their ligaments have had to absorb greater forces on the field. At the same time, no one has figured out a way to strengthen the ACL. That’s not to say ligaments can’t be toughened up with training—the ones in a pitcher’s throwing arm tend to be a lot hardier than the ones in his other arm, for example. But it’s very difficult to isolate the ACL in an exercise. Biomechanics experts have proposed repetitive movements that force the tibia forward, like walking backward downhill, but there’s little research on whether this actually prevents ACL injuries. Despite the many ACL-injury prevention programs that are out there already, which focus on strengthening the muscles around the knee, among other things, the rate of injury hasn’t budged.

Torn ACLs seem to discriminate by gender: Women athletes are four to eight times more likely to suffer the injury than men who play the same sport. Researchers have concocted many theories for this imbalance. Some think it has to do with skeletal structure: female tibias tend to slope backward more at rest, leaving more room to accelerate as they land from a jump. Hormones may also play a role, as the tightness of ligaments appears to fluctuate throughout the menstrual cycle. Women also tend to have a different pattern of musculature, with more dominant quadriceps muscles and weaker gluteals.

Got a question about today’s news? Ask the Explainer.

Explainer thanks Scott McLean of the University of Michigan.

Brian Palmer writes about science, medicine, and the environment for Slate and the Natural Resources Defense Council. Email him at explainerbrian@gmail.com. Follow him on Twitter.


Justice Ginsburg’s Crucial Dissent in the Texas Voter ID Case

The Jarring Experience of Watching White Americans Speak Frankly About Race

How Facebook’s New Feature Could Come in Handy During a Disaster

The Most Ingenious Teaching Device Ever Invented

Sprawl, Decadence, and Environmental Ruin in Nevada

View From Chicago

You Should Be Able to Sell Your Kidney

Or at least trade it for something.

Space: The Next Generation

An All-Female Mission to Mars

As a NASA guinea pig, I verified that women would be cheaper to launch than men.

Terrorism, Immigration, and Ebola Are Combining Into a Supercluster of Anxiety

The Legal Loophole That Allows Microsoft to Seize Assets and Shut Down Companies

  News & Politics
Oct. 19 2014 1:05 PM Dawn Patrol Justice Ruth Bader Ginsburg’s critically important 5 a.m. wake-up call on voting rights.
Business Insider
Oct. 19 2014 11:40 AM Pot-Infused Halloween Candy Is a Worry in Colorado
Oct. 17 2014 5:26 PM Judge Begrudgingly Strikes Down Wyoming’s Gay Marriage Ban
  Double X
The XX Factor
Oct. 17 2014 4:23 PM A Former FBI Agent On Why It’s So Hard to Prosecute Gamergate Trolls
  Slate Plus
Slate Picks
Oct. 17 2014 1:33 PM What Happened at Slate This Week?  Senior editor David Haglund shares what intrigued him at the magazine. 
Oct. 19 2014 4:33 PM Building Family Relationships in and out of Juvenile Detention Centers
Future Tense
Oct. 17 2014 6:05 PM There Is No Better Use For Drones Than Star Wars Reenactments
  Health & Science
Space: The Next Generation
Oct. 19 2014 11:45 PM An All-Female Mission to Mars As a NASA guinea pig, I verified that women would be cheaper to launch than men.
Sports Nut
Oct. 16 2014 2:03 PM Oh What a Relief It Is How the rise of the bullpen has changed baseball.