Jan 29, 2015
By Leaps and Bounds

The latest research into non-contact ACL injuries has greatly improved our understanding of why they occur–and more importantly, how they can be prevented.

By R.J. Anderson

R.J. Anderson is an Assistant Editor at Training & Conditioning. He can be reached at: [email protected].

For every female athlete playing competitive sports, it lurks. As they lace up their cleats or their high-tops, they know today could be the day that the dreaded ACL tear catches up with them. If not today, then tomorrow–or the day after that–any day.

The past decade has seen great strides in discovering why female athletes succumb to non-contact ACL injuries approximately five times more often than males. While we don’t have all the answers yet, we know various factors, from hormones to muscular imbalances, affect female athletes differently than their male counterparts.

But increased knowledge is little consolation to the thousands of athletes who suffer ACL injuries every year, thus raising the million-dollar question: Can we reduce the risk? The most recent research in this field says, yes, in fact we can. Some groundbreaking work on assessing who is at greatest risk and a team-centered strengthening program are both proving successful for female athletes. As a result, there’s new hope that female athletes can soon replace fear of the dreaded ACL injury with confidence and peace of mind.


It’s not yet fully understood why females tear their ACLs much more frequently than males. Some research shows that hormonal balance may play a role. There are also theories that structural differences are to blame. But one thing seems very clear: There are strength imbalances in many females that make them more susceptible to ACL tears than males.

Some of the most recent research in this area was published in the October-December 2007 issue of the Journal of Athletic Training by Tim Hewett, PhD, Director of the Sports Medicine Biodynamics Center at Cincinnati Children’s Hospital and Professor at the University of Cincinnati College of Medicine. He suggests the difference in injury rates may be directly related to the way boys and girls emerge from puberty.

“We’ve done some meta-analysis of the literature and it’s fairly apparent that before puberty, there’s no disparity in knee injury risk between males and females, but once they hit puberty, there is,” says Hewett, who has authored over 100 studies on non- contact ACL tears in females. “During both boys’ and girls’ growth spurts, the tibia and the femur–the largest bones in the body, which act as long levers and create a lot of torque at the knee–get longer. Meanwhile, the body’s center of mass is pushed upward, making it harder for the elongated levers to exert control. Soon after puberty, boys get a neuromuscular power spurt by developing bigger gluteus and hamstring muscles, which give them the horsepower to better control those levers. Girls, on the other hand, tend to be very front dominant, and develop more in their quadriceps and less in their glutes and hamstrings.”

With the above research as a backdrop, Hewett is now focused on finding some of the exact muscular and neuromuscular problems that lead to ACL weakness. Supported by a $3 million grant from the National Institutes of Health, he has collected data on more than 2,000 female athletes in grades nine through 12 from Boone County, Ky., and says he has seen those same problems show up repeatedly during cross-sectional and longitudinal studies of this population.

Hewett’s lab contains a battery of testing stations that resembles a factory assembly line. Stations record biomechanical and neuromuscular measures such as joint laxity and muscle flexibility; height, weight, body mass index, and body fat; power and vertical jump; hamstring and quadriceps strength; hip abductor and adductor strength; and movement mechanics during landing and cutting.

He has found that the tests can effectively identify athletes who are at risk for non-contact ACL injury by assessing factors such as knee abduction, torque and motion, and trunk forward motion and control. “One way we do this is by observing and measuring vertical jump,” Hewett says. “Landing creates relatively high ground reaction forces, which lead to torque at the knee joint. When the knee is in a compromised position during landing, there is an elevated injury risk.”

Hewett says his research has identified four major deficiencies that, when targeted and improved, can decrease an athlete’s risk of sustaining a non-contact ACL tear:

Ligament dominance. When “ligament-dominant” athletes strike the ground during landing or cutting, the ground reaction forces dictate the directional movement of their knees. “Because they’re not muscle-dominant and the muscles aren’t adequately controlling that joint, the ground forces their knee into an inward collapsed position,” Hewett says.

That inward collapse can be observed by looking at an athlete’s knee abduction during landing. “Knee abduction torque–the torque that tends to push the knee toward the midline of the body–is a pretty good predictor of ACL injury risk,” Hewett says. “So is the angle at initial contact when landing or cutting if the knee pushes into an abducted or valgus angulated position.”

Hewett says the box jump is an ideal tool for identifying ligament dominance. The setup can be as simple as videotaping or observing with the naked eye as the athlete performs a drop off a one-foot-high milk crate or box, then explodes into a maximum vertical jump. “If they have more than five degrees of knee abduction on that initial landing before making the vertical jump, they’re at relatively high risk and intervention is needed,” he says.

To start, Hewett says it’s important to simply tell the athlete they are landing improperly. “Awareness is a huge part of the equation,” he says. “Then, you teach them exercises like double-leg broad jumps and single-leg hops and emphasize the importance of landing without allowing their knee to collapse inward.”

Quadriceps dominance. Some athletes control, stiffen, and stabilize their knee joints primarily by contracting their quadriceps. “The problem is that the quad inserts at the front of the knee joint, so when you fire the quad, it pulls the tibia forward,” Hewett says. “But the ACL is holding the tibia back, so this in turn places a lot of stress on the ACL. In addition, the quad has only a single tendon crossing the front of the knee joint, so it can’t control that valgus position that the ground force often wants to push the knee into.”

Singling out athletes who are quadriceps dominant is fairly easy. Sometimes the imbalance is so obvious that you can spot athletes with oversized quads and relatively small hamstrings with the naked eye. But in most cases, identifying the disparity involves measuring for strength imbalances using a dynamometer or a leg extension/leg curl machine. If an athlete’s quad strength is greater than their hamstring strength by 50 percent or more, they are at risk.

“You can intervene by implementing hamstring strength building exercises,” Hewett says. “We often use Russian hamstring curls and dynamic exercises, as well as squat-position exercises like squat jumps in which the athlete sits with their butt down and shoulders back and has to turn their hamstring muscles on as they jump.”

Symmetric imbalances. Any difference in balancing ability from one side versus the other also puts the athlete at greater risk. To identify athletes with these imbalances, Hewett recommends a test involving a large X taped or chalked on the floor. “They start in one quadrant of the X, and do single-leg hops forward, sideways, diagonally forward, and diagonally backward for 20 seconds each,” he says. “If they’re touching one side of the line more than the other side, they’re probably asymmetric.”

Weak trunk or core. “We know that ground reaction forces are directed at the body’s center of mass, which is located in the trunk,” Hewett says. “We’ve also done some studies that showed trunk control is a good predictor of injury risk, including ACL tears, in collegiate athletes.”

Though there are many ways to measure trunk and core control, Hewett likes putting athletes in a supine position with their heels on a Swiss ball and having them bridge up to a neutral pelvis position. “See if they can hold that position for 15 seconds, and then see if they can do it with each foot on the ball individually,” he says. “Then see if they can hold that plank position and bring their heels up to their buttocks. If they can’t, they probably don’t have good trunk and core control.

“If you can get an athlete to a higher, more balanced level in all four of those risk areas, we’re pretty sure that kid is significantly safer, based on the data we’ve collected,” Hewett continues. “And as a side benefit, there are great performance increases–that kid is going to be significantly stronger, better balanced, quicker, and more athletic.”

Overall, Hewett says it’s important to teach athletes the value of landing on the balls of their feet, with knees flexed, chest over the knees, and no valgus shifting. “ACL injuries don’t happen when athletes have their knees flexed deeply,” Hewett says. “So we teach them to get into a deep, flexed position, while activating all the muscles on the back side of the leg, and at the same time, controlling or stiffening their core.”

He also uses plyometrics to train athletes to land in the correct position, which increases the dynamic stability of the knee joint. To promote proper proprioceptive input and kinesthetic awareness, he prefers single-leg decelerations, dynamic hopping, and dynamic functional movements that focus on proper technique.


While Hewett’s work concentrates on individual athletes, another program is designed for a full team. The Prevent Injury and Enhance Performance (PEP) program, a specialized 15- to 25-minute warmup routine performed three times a week, was recently found to greatly reduce ACL injuries. Conducted by the Centers for Disease Control and Prevention (CDC) and the Santa Monica (Calif.) Orthopaedic and Sports Medicine Research Foundation, a study of the PEP program’s effects was published in the July issue of The American Journal of Sports Medicine.

The PEP study included 1,435 players from 61 NCAA Division I women’s soccer teams. During one regular season, 26 teams used the PEP program an average of three times per week for 12 weeks, while the other 35 teams served as a control group.

The authors reported no ACL injuries on teams using the PEP program, compared to six injuries on the control teams–five of which came in the second half of the season. Even athletes with a history of ACL injury who used the PEP program avoided re-injury, compared to four re-injuries among the non-PEP players with a similar history.

Designed by the Santa Monica Orthopaedic and Sports Medicine Research Foundation, the PEP program focuses on flexibility, strength, balance, and joint proprioception, and requires no specialized equipment. “We wanted to develop a program that addresses the quad strength to hamstring strength ratio,” says Holly Silvers, MPT, the foundation’s ACL Prevention Project Coordinator, “and that also corrects strength and flexibility deficiencies in the lateral hips, lack of core and trunk control, and imbalances between the adductor and the abductor group.”

The 19-part PEP program begins with a warmup consisting of jogging from one side of a soccer field to the other, shuttle runs, and backward running. Next comes a six-minute static stretching session focusing on the calves, quads, hamstrings, hip adductors, and hip flexors. Then, the athletes perform a three-minute strengthening session including three sets of 10 walking lunges, three sets of 10 partner-assisted Russian hamstring curls, and two sets of 30 toe raises for each leg.

A short plyometrics stage follows. Using small cones, the athletes perform 20 lateral hops over cones, 20 backward and forward hops over cones, 20 single-leg hops on each leg, 20 vertical jumps, and 20 scissors jumps. Next comes a two-minute agility stage, consisting of backward and forward shuttle runs, diagonal runs, and bounding runs. The program finishes with a 10-minute cooldown featuring slow jogging, additional light strengthening exercises, and a repeat of the stretching station. (For more information on the PEP program, see “Resources” at the end of this article.)

The Foundation has also developed a PEP program specific to basketball, which was used last year by the women’s team at Pepperdine University. “The programs are very similar, we just used movement patterns that are more applicable to basketball,” Silvers says. “For example, in women’s basketball there is more jump landing and deceleration. When players land there is usually caving of the knee, so that’s what we concentrated on addressing.”

Silvers says the results were outstanding. “With our program, there were zero ACL injuries and a 62-percent reduction in lower extremity injuries from the low back, hip, knee, and ankle compared to nine years of injury surveillance data,” she says.


The PEP program is now being used by several teams and clinics around the country. One is the University of South Florida Sports Medicine and Athletic Related Trauma Institute (SMART), a state-sponsored sports safety outreach program, which implemented it at 10 high schools last year in three sports: girls’ soccer, basketball, and volleyball.

“We chose the PEP program because it is so simple,” says Barbara Morris, MS, ATC, CSCS, Assistant Director of SMART. “It’s easy to understand and doesn’t take up much time. Coaches are quite willing to give up 20 minutes of practice two or three days a week for it.”

SMART athletic trainers help implement the program with teams in the preseason and Morris says after two to three weeks, the athletes are able to perform it on their own–often before the coach even arrives. “The girls who use the program love it,” she says. “It’s more exciting than traditional warmup programs.”

Silvers says compliance is easier if the routine’s performance benefits are pushed. “We saw improvements in sprint and agility times for program participants,” she says. “We also have a paper coming out soon that looks at how the PEP program increases vertical leap.”

Though it’s touted as a warmup, the PEP program is no walk in the park. “It is somewhat taxing, so at the beginning of the season we recommend doing it only before training sessions and practices,” Silvers says. “As the season progresses and the players get better at the routine and better conditioned, coaches or athletic trainers can add it before games.

“I had a coach of an NCAA Division I soccer team that was enrolled in our study call to tell me her players were incredibly sore after the first week,” Silvers adds. “I said, ‘Perhaps they’re a little deconditioned–we have 14-year-olds who can do it. You can scale it back a little, but I recommend sticking with it because clearly there are deficits with your players.’ I followed up with her later in the season, and she said they were doing it three times a week without soreness, including as part of their pregame warmup.”


With the PEP program’s huge success on the soccer pitch and preliminary findings in basketball, the Santa Monica Orthopaedic and Sports Medicine Research Foundation is working to expand its reach. “Right now, we’re trying to raise funding to do a large randomized, controlled trial of the PEP program in NCAA Division I and II women’s basketball,” says Silvers. “We’d also like to develop sport-specific programs for other sports with high incidence rates, including volleyball, lacrosse, and field hockey. Then, we’d like to get some element of the program applied to the President’s Council on Physical Fitness and Sports programs so we can access more athletes, including younger ones.”

Hewett’s next step is to begin a new four-year study that will randomize subjects who have different risk values into groups and apply training programs specifically geared toward each deficit. “This will take us to that next level of identifying kids who are at higher risk by profiling the imbalances they demonstrate,” Hewett says. “We will then–in a randomized, controlled fashion–apply specific training for those deficits and see if we can significantly decrease risk.”

Eventually, this research should translate from the lab to the field, and help keep more athletes safer. “I think it’s only a matter of time before our theories come to fruition,” Hewett says. “Then, we can give athletic trainers all the tools they need to do this assessment in the field.”


For more information on the Prevent Injury and Enhance Performance (PEP) program, visit our Web site: www.training-conditioning.com. Click on “Video Library” to view exercise clips from the PEP Soccer Program instructional DVD, and to access an order form you can use to purchase your own copy.

While on our Web site, you can also click on “Blogs” to access a recent blog about the NATA’s consensus statement on ACL injury research. The association says researchers need to look beyond gender differences to identify the underlying causes of injury.

To learn more about Tim Hewett’s ACL injury risk assessment and prevention research, go to: cincinnatichildrens.com/sportsmed.

To read “A Randomized Controlled Trial to Prevent Noncontact Anterior Cruciate Ligament Injury in Female Collegiate Soccer Players” from the July online edition of The American Journal of Sports Medicine, go to: ajs.sagepub.com/cgi/content/short/36/8/1476.


For a study published in the June 2007 issue of The American Journal of Sports Medicine, scientists from the University of Delaware, Michigan State University, West Chester University, and St. Joseph’s University administered preseason neurocognitive tests to nearly 1,500 male and female athletes from 18 universities who played a variety of sports. The testing, which measured visual memory, verbal memory, processing speed, and reaction time, is traditionally used to gather baseline data for concussion assessment. But the researchers were surprised to learn it could also help predict who would suffer an ACL injury.

Eighty athletes from the testing sample who suffered non-contact ACL injuries during the season were matched up to a control group of 80 non-injured athletes of similar height, weight, age, gender, sport, position, and years of experience at the college level. The athletes with the non-contact ACL injuries were found to have significantly slower reaction time and processing speed than the non-injured athletes, and they performed worse on visual and verbal memory tests.

“What really took me aback were the visual and spatial skill differences between the two groups,” says Charles “Buz” Swanik, PhD, ATC, Associate Professor at the University of Delaware Human Performance Laboratory and the lead researcher for the study. “It seems that where a person chooses to look in the heat of the moment and how they process visual information really matters. If it’s processed slowly, that can lead to an injury.”

Swanik says neurocognitive testing could eventually have a place in biomechanical and neuromuscular injury prevention. “At this point, it’s hard to say how much we can alter these neurological characteristics with training,” he says. “But certainly, the brain has great potential for learning and adaptation.


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