Research suggests that training protocols can reduce the risk of anterior cruciate ligament tear, but epidemiological evidence suggests that in the real world, training doesn’t work as well as it should. The challenge now is to determine why.
By Barbara Boughton
Although training programs aimed at preventing noncontact anterior cruciate ligament (ACL) injuries in sports have been available for more than 15 years, the rate of these injuries remains high—particularly among women. Studies have documented the effectiveness of some ACL injury prevention programs, but researchers are now questioning why these programs are not making more of a difference.
They’re finding that the reasons may be complex—involving poor compliance, limited adoption, and whether elements of the programs are specific enough for each sport and different types of athletes, as well as whether they are age-appropriate. More emphasis also needs to be placed on identifying those most at risk for injury—an effort that may have the most promise for improving the effectiveness of ACL injury preventions programs, researchers say.
Meanwhile, at least one devil’s advocate is suggesting that the studies demonstrating the effectiveness of ACL injury prevention programs are marked by flaws in scientific methodology. By that logic, expectations of training interventions may have been inflated to begin with.
New research indicates that injury rates may respond to improvements such as tailoring ACL injury prevention to specific sports, younger athletes who are just starting to develop the neuromuscular patterns that may lead to injury, and those who exhibit the signs of poor biomechanics. But researchers say other important issues include the limited adoption of ACL injury prevention programs and poor compliance with the programs—when athletes fail to do injury prevention exercises or do them improperly. Even though some protocols are free and downloadable from the Internet, many coaches and athletes don’t use them because they put a higher priority on performance-based training.
“It’s not surprising that there’s a disconnect between the efficacy of neuromuscular training and the number of ACL injuries,” said Tim Hewett, PhD, professor in pediatrics and orthopedic surgery and director of the sports medicine biodynamics center at Cincinnati Children’s Hospital and professor and director of research at the Ohio State University sports medicine center.”We have barely addressed the tip of the iceberg of women and girls who are at risk for tearing their ACLs.”
Hewett, whose group developed one of the first neuromuscular training programs for ACL injury prevention, has worked with sports teams at the elementary, high school, college, and professional levels. Although there are no statistics available on how many coaches and athletes are using ACL injury prevention programs, Hewett believes that use of these interventions is not widespread, considering the numbers of children and athletes who play on athletic teams.
How can these programs be made more popular among coaches and athletes? A review study published in the August 2009 issue of Knee Surgery, Sport Traumatology and Arthroscopy by researchers from the Cincinnati Children’s Hospital, the Santa Monica Orthopedic Sports Medicine Foundation and Blanquerna University in Barcelona Spain, delved into this issue.1
The scientists noted poor compliance rates—with athletes failing to take enough time for the exercises or not doing them at all after they are prescribed—as low as 28%. However, compliance rates increased—as high as 80% to 90% in some cases—when programs incorporated training that could also improve athletic performance in addition to lowering injury risk. The higher compliance rates make such programs more likely to be effective in preventing ACL injuries, and emphasizing the performance benefits might also make them more appealing to coaches of both pro and amateur teams, the authors noted.
Darin Padua, PhD, ATC, a researcher who has conducted studies on ACL injury prevention programs in athletes of varying ages, notes when these programs are first implemented, coaches and team sports players are often enthusiastic about them. But as time goes on, compliance tends to drop off.
“Many teams have a fixed amount of time in which the coach can work with the players, and they want to spend their time on skill development that will help them win games. No one wants to spend even 15 minutes doing injury prevention because they see that as taking away time that should be spent on skills important to the sport,” said Padua, associate professor in the department of exercise and sports science and director of the sports medicine research laboratory at the University of North Carolina at Chapel Hill. “We really need more research to understand the behavioral aspects of implementing these programs and how we might convince athletes, coaches and parents to make them a priority.”
Pressure to perform
Yet the question of how much currently used ACL injury prevention programs actually improve performance is an open one. There are sparse scientific data on performance metrics after implementation of ACL injury prevention programs, and current programs tend to vary widely in terms of what kind of exercises they include, so it’s difficult to make generalizations.
In the February 2005 issue of the Journal of Strength and Conditioning Research, researchers from Cincinnati Children’s Hospital reported that a comprehensive neuromuscular training program including exercises aimed at injury prevention did improve athletic performance. In the study, 41 female basketball, soccer and volleyball players underwent a six-week program that included plyometric and movement exercises, strengthening and balance, and resistance and speed training. Athletes who underwent the training improved their performance on squats and bench press, single-leg hop and vertical jump distance, and speed in a 9.1-meter sprint. Twelve control subjects—matched for age, height and weight—did not experience significant changes in athletic performance.
Yet in a more recent study published in the June issue of the Scandinavian Journal of Medicine and Science in Sports, Canadian researchers from York University investigated the effects of the Prevent Injury Enhance Performance (PEP) program on physical performance in adolescent female soccer players.2 Four soccer teams were randomly assigned to either PEP or a control group and assessed at baseline, six weeks and 12 weeks. Sprint times improved by more than 0.10 second for longer distances after six weeks, but the researchers characterized those improvements as “small and transient;” there was no change seen for agility or countermovement jump abilities at any time point. “ACL injury prevention programs designed as a structured warm-up routine seem to lack the necessary stimulus to enhance athletic performance,” the authors wrote.
Another problem may be that ACL injury prevention programs are typically implemented when female athletes are most likely to tear their ACL—in the late teenage years, from age 15 to 18.
“That’s when you get a rash of injuries and it’s also when we try to implement the intervention,” Hewett said. “But insurance data indicate that the disparity in risk (between females and males) actually occurs right at early puberty, when boys and girls diverge in neuromotor control and patterning. So we’ve got a big window—between the ages of 10 and 16—when we’re not addressing kids who are potentially at highest risk.”
Padua agrees that working with athletes in childhood and adolescence—as they’re developing faulty movement patterns that lead to increased ACL injury risk—is vitally important. In a study published in the March 2009 issue of the American Journal of Sports Medicine, he and other researchers implemented either a “one size fits all” ACL injury prevention program or one tailored to individual athletes’ biomechanics among 173 soccer players from 27 teams.3 Both programs had benefits in correcting movement errors in high school subjects, but younger players showed little improvement after either program.
“That led us to believe that the way we train kids may need to be modified to be age appropriate,” he said.
As a result, the researchers launched a study to assess the efficacy of an ACL injury prevention program designed for athletes younger than age 12. The kid-specific program, developed based on recommendations from the literature for teaching skills to children, includes more progressions, more variety, continuous feedback, an internal focus of attention, and a reduced training frequency in the first three weeks to avoid overuse injury.
“We used specific types of verbal cues for children versus those we might use for an adult population,” Padua said. “With adults you can be more vague and say ‘land as light as a feather,’ but with children you have to be very specific and say things like ‘keep your knees over your toes when you land from a jump.’”
The study, which was presented in June at the annual meeting of the National Athletic Trainers Association, randomized 59 children to a traditional ACL training program, a pediatric ACL training program, or a control group. After nine weeks, results showed that the pediatric program significantly increased peak knee flexion and reduced peak ground reaction forces during a jump landing task, whereas no changes were seen in the other two groups.
Another important step in improving the success and effectiveness of ACL injury prevention programs is to gear them toward those who are at highest risk. By using videotapes or just observation with the naked eye, coaches and trainers could identify athletes who display high risk movement patterns that put them at risk for injury, Padua said. Other clues for identifying high risk athletes would be previous ACL injury or a family history of such injuries, he added.
A simple low-cost method of identifying athletes at high risk for ACL injury was unveiled at this year’s meeting of the American Orthopaedic Society for Sports Medicine by Gregory Myer, MS, and fellow researchers at Cincinnati Children’s Hospital. The method would require clinicians to calculate an athlete’s tibial length, quadriceps-to-hamstrings ratio, and body mass index, and use a standard camcorder to assess knee flexion range of motion and knee valgus motion during landing. When tested on 744 female basketball and soccer players from a public school district, these five variables predicted high knee abduction moment during landing with a sensitivity of 73% and a specificity of 70%.4
Unfortunately, the definition of just what contributes most to risk for ACL injury is still developing—although elements that are thought to be significant include knee valgus during landing; flexion at the knees, hips and trunk; and core strength and stability. Current ACL injury prevention programs do target changing problematic movement patterns—but unfortunately, risk factor research has not led to a definitive understanding of why these patterns develop, said Sandra Shultz, PhD, associate professor and director of graduate studies in the department of kinesiology at the University of North Carolina at Greensboro.
“We need to understand more about what causes someone to fall into these poor movement patterns, so we can more effectively target interventions,” she said.
Shultz organized this year’s ACL Injury Research Retreat, held in March at UNC-Greensboro, where researchers met for the fifth time in the last decade to discuss the complex and formidable challenge of advancing knowledge on how to best prevent ACL injuries. The proceedings of the retreat—including its consensus statement, keynote summaries, and abstracts—will be published in the Journal of Athletic Training in the fall.
“The consensus was that the ideal ACL injury prevention program cannot be realized until we have a better understanding of the causative risk factors,” she said. “We also don’t know which components of ACL injury prevention programs are most effective or the ideal timing of the intervention.”
Shultz’s recent research indicates that anatomical differences between men and women, chiefly joint laxity, may contribute to elevated risk of ACL injury in females. But she notes that it’s also important to delve deeper into other possible causes for poor biomechanics and elevated ACL injury risk, such as muscle weakness or imbalances, she said. If more were known about the reasons why some athletes are at higher risk for ACL injury, prevention programs could be tailored to possible causative factors such as muscle weakness with strength training. Yet programs designed to address injury risk caused by anatomical factors, might seek to strengthen the weakest anatomical areas, she said.
“Since anatomical differences don’t go away, we may also need to use injury prevention programs for extended periods to address these factors,” she said.
Evaluating the evidence
To analyze what makes ACL injury prevention most effective, Shultz reviewed the scientific literature on these strategies for a February lecture at the Combined Sections Meeting of the American Physical Therapy Association. She found that the best randomized controlled trials and meta-analyses to-date indicate that the most effective programs include training in technique, strength conditioning and plyometrics, as well as balance. Those that just incorporate balance training without an emphasis on technique have not been as successful, she said. A June meta-analysis published in KSSTA by researchers from Soonchunhyang University in South Korea supports this conclusion, reporting that plyometrics and strengthening components were the most important for effective ACL injury prevention programs, particularly in those under age 18.5
Shultz also found that ACL injury prevention programs are most effective when they are performed before activity at a minimum of twice a week for six weeks, incorporate verbal and visual technique and training, and are of sufficient exercise intensity and specificity to mimic the demands of an athlete’s sport. In fact, high intensity progressive programs tend to produce more biomechanical improvements in athletes than those composed of low intensity exercises that don’t progress. Initiating preventive exercises early in the pre-season and continuing them throughout the season was also important, according to both Shultz and the South Korean meta-analysis.
It’s also worth noting, Shultz added, that most injury prevention programs that have been studied were designed for soccer and team handball. In the limited studies that have been performed on athletes in other sports, such as basketball, ACL injury prevention programs have proven to be less effective, she said.
“We don’t have good evidence that these prevention programs reduce injuries in basketball, although the injury rates in this sport are as high as they are in soccer,” she said.
Even for soccer players, the scientific evidence for ACL prevention programs is not as strong as it should be, according to Kevin Shea, MD, an orthopedic surgeon who practices in Boise, ID, and is an associate adjunct faculty member at the University of Utah and Boise State University. Shea presented a meta-analysis in July at this year’s annual meeting of the American Orthopaedic Society for Sports Medicine which showed that many of the studies documenting the effectiveness of ACL injury prevention programs are marked by problems in statistical methods.6
“These are design flaws such as patients dropping out of the study before completion that introduce statistical bias,” he said in an interview.
None of the studies published up to early 2009, which he and fellow researchers analyzed after a search of three medical databases, met the strict criteria established by The Journal of Bone and Joint Surgery for “Level 1”—or the highest level of scientific evidence—among prospective randomized controlled trials. Seven of 12 studies that were prospective but not randomized showed that prevention programs significantly reduced knee injuries, but many were flawed, according to Shea.
Since the research for that analysis was conducted, however, several large randomized clinical trials7-12 have demonstrated the efficacy of ACL injury prevention programs. Shea is in the process of analyzing the quality of these new additions to the literature.
“Over the last 10 years, the quality of research has improved significantly, and our goal is to have several research studies that show the least amount of design flaws,” he said.
However, Shea acknowledged that such “gold standard” studies take years to conduct, are expensive, and require a significant amount of time and commitment from researchers. And orthopedics research simply does not receive as much funding as in other fields of medicine, he said.
Yet the issue of ACL injury is one that requires serious and quality scientific investigation, he said.
“Many athletes who develop ACL injury don’t return to the same level of sports play afterward, and many will develop arthritis in the injured knee within 20 years,” he said. “So this research is very important to all of us, and we need to develop better methods to prevent these injuries.”
Barbara Boughton is a freelance health and medical writer who is based in the San Francisco Bay Area.
- Alentorn-Geli E, Myer GD, Silvers HJ, et al. Prevention of non-contact anterior cruciate ligament injuries in soccer players,. Part 2: A review of prevention programs aimed to modify risk factors and to reduce injury rates. Knee Surg Sports Traumatol Arthrosc 2009; 17(8):859-879.
- Vescovi JD, VanHeest JL. Effects of an anterior cruciate ligament injury prevention program on performance in adolescent female soccer players. Scand J Med Sci Sports 2010;20(3):394-402.
- DiStefano LJ, Padua DA, DiStefano MJ, et al. Influence of age, sex, technique, and exercise program on movement patterns after an anterior cruciate ligament injury prevention program in youth soccer players. Am J Sports Med 2009;37(3):495-505.
- Myer GD, Ford KR, Khoury J, et al. Validation of a FIELD based prediction tool to indentify high ACL injury risk female athletes. Presented at The American Orthopaedic Society for Sports Medicine 2010 Annual Meeting, Providence, July 2010. Abstract 9150.
- Yoo JH, Lim BO, Ha M, et al. A meta-analysis of the effect of neuromuscular training on the prevention of the anterior cruciate ligament injury in female athletes. Knee Surg Sports Traumatol Arthrosc 2010;18(6):824-830.
- Shea KG, Grimm NL, Jacobs JC, et al. ACL and knee prevention programs for young athletes: Do they work? Presented at The American Orthopaedic Society for Sports Medicine 2010 Annual Meeting, Providence, July 2010. Abstract 9121.
- Gilchrist J, Mandelbaum BR, Melancon H, et al. A randomized controlled trial to prevent noncontact anterior cruciate ligament injury in female collegiate soccer players. Am J Sports Med 2008;36(8):1476-1483.
- Soligard T, Myklebust G, Steffen K, et al. Comprehensive warm-up programme to prevent injuries in young female footballers: cluster randomized controlled trial. BMJ 2008;9(337):a2469.
- Engebretsen AH, Myklebust G, Holme I, et al. Prevention of injuries among male soccer players: a prospective, randomized intervention study targeting players with previous injuries or reduced function. Am J Sports Med 2008;36(5):956-960.
- Kiani A, Hellquist E, Ahlqvist K, et al. Prevention of soccer-related knee injuries in teenaged girls. Arch Intern Med 2010;170(1):43-49.
- Emery CA, Meeuwisse WH. The effectiveness of a neuromuscular prevention strategy to reduce injuries in youth soccer: a cluster-randomised controlled trial. Br J Sports Med 2010;44(8):555-562.
- LaBella CR, Huxford MR, Smith TL, Cartland J. Preseason neuromuscular exercise program reduces sports-related knee pain in female adolescent athletes. Clin Pediatr 2009;48(3):327-330.