February 2012

Odd couple: Linking ACL injury and patellofemoral pain

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One is an acute injury, the other a chronic condition. But researchers believe it’s no coincidence that anterior cruciate liga­ment injury and patellofemoral pain syn­drome share many of the same risk factors. The next step is to determine if a single intervention can effectively prevent both.

By Cary Groner

It’s unusual for chronic conditions and acute injuries to share similar causes, but clinicians and researchers suspect just such a relationship when it comes to anterior cruciate ligament (ACL) rupture and patellofemoral pain (PFP) syndrome. If the etiological correlation proves accurate, the result could be preventive strategies targeting both problems, and in fact studies already suggest that such an approach works.

The subject was raised at the annual conference of the National Athletic Trainers’ Association (NATA) in New Orleans last June, in a forum moderated by Darin Padua, PhD, ATC. Padua, an 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, told LER that multiple factors apparently put athletes at risk for these problems. One clear factor is gender: women suffer ACL injuries at rates four to six times those of men and PFP syndrome up to 10 times as often as men.1-5

Although gender itself isn’t a modifiable risk factor, researchers have begun to clarify the biomechanical effects of gender differences and have shown that some may be amenable to change.

“There can be knee valgus collapse due to a multitude of different muscle imbalances,” Padua said. “You need to figure out the underlying cause and design an intervention to address it. In doing so, you’ll be more effective and efficient in being able to prevent these injuries, and in rehabilitating individuals who suffer them.”

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All in the hips

In a 2010 literature review and analysis, Christopher Powers, PT, PhD, outlined the ways in which hip and trunk biomechanics may affect risk of knee injuries.6

Powers, director of the program in biokinesiology and codirector of the Musculoskeletal Biomechanics Research Lab at the University of Southern California, noted that impaired muscular control of the trunk, pelvis, and hip can affect tibiofemoral and patellofemoral joint kinematics and kinetics in multiple planes. In particular, the severe knee valgus often demonstrated by female athletes when landing after a jump results from excessive hip adduction and internal rotation.

Such problems often begin with diminished hip muscle strength, Powers wrote, and this has clear implications for PFP and ACL injuries because of their association with valgus knees and femoral rotation.7

“The risk factors are exactly the same for PFP and ACL injury: hip internal rotation, hip adduction, quadriceps overuse, and lack of hip strength,” Powers told LER. “I wouldn’t be surprised if at some point we figure out that patellofemoral pain is a predictor of who goes on to tear their ACL.”

Padua agrees about the sources of the problem.

“Neuromuscular control at the hip is a big factor, because the knee is pretty much a slave to what is going on at the hip,” he said. “An inability to control frontal and transverse plane hip motion can set the knee up for increased loads that predispose it to patellofemoral pain or ACL injury. Weakness in any of the gluteal muscles, tightness in the hip adductor or flexor muscles, any kind of muscle imbalances there could set an individual up for the lack of neuromuscular control that will predispose them to these injuries.”

Irene Davis, PT, PhD, director of the Spaulding National Running Center in the Department of Physical Medicine and Rehabilitation at Harvard Medical School, also emphasized the role of the hip.

“Analysis of films of someone tearing their ACL often reveals a mechanism of hip adduction and internal rotation,” she said. “These same mechanics are related to patellofemoral pain syndrome. The major contributors to the positioning of the knee in the frontal and transverse plane are not the sagittal plane muscles, but the hip adductors and external rotators. Therefore, interventions are going to be similar between PFP and ACL issues.”

PFP research

Existing research helps elucidate the manifold factors that appear to play into both PFP and ACL injuries. A 2009 paper on PFP—based, interestingly, on data from the Joint Undertaking to Monitor and Prevent ACL Injury (JUMP-ACL) project—noted that risk factors for PFP included decreased knee flexion angle and increased hip internal rotation during jump landing.8

A 2010 prospective study in Clinical Biomechanics reported that in 240 adolescent female athletes followed through the course of a season, those who developed PFP had increased knee abduction moment compared to teammates without PFP. And although 16.3% of subjects began the season with PFP—a surprisingly high number—adding in those who then developed PFP raised the total prevalence to 22%, further underscoring the need for screening and prevention.

A 2009 study in the American Journal of Sports Medicine reported that women with PFP (n=19) had significantly greater average hip internal rotation than controls (n=19), and concluded that patellofemoral pain resulted from diminished hip muscle performance rather than altered femoral structure.9 Another paper by the same authors noted that that hip muscle weakness included the extensors, abductors, and external rotators.10

A literature review from Erasmus University in Rotterdam, presented last September in Ghent, Belgium, at the Second International Research Retreat on Patellofemoral Pain Syndrome and e-published in October by the Journal of Orthopedic & Sports Physical Therapy, shed further light on causes and conditions. It included 37 studies of patients with existing PFP and reported that a larger Q-angle, less hip abduction strength, and lower knee extension strength were associated with increased PFP risk.11

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ACL research

Research into causes of ACL injuries reveals similar factors. For example, a 2005 paper in AJSM found that of 205 prescreened female athletes prospectively measured for neuromuscular control, the nine who later ruptured their ACLs had significantly greater knee abduction angle at landing than those who were not injured, and that abduction moment predicted ACL injury with 73% specificity and 78% sensitivity.12

In a 2008 paper in the Clinical Journal of Sports Medicine, researchers noted that after female athletes enter puberty and begin to grow quickly, gain weight, and increase their tibia and femur length, they may not match those changes with increases in strength and recruitment of the hip and trunk musculature. The result is a loss of core stability and trunk motion control during dynamic tasks—which, in turn, may underlie increased lower extremity valgus and elevate risk for ACL injuries.13 Similarly, investigators reported in 2011 that a 10-week neuromuscular training program affected not only knee abduction on landing, but also hip abduction angle.14

Numerous studies reveal other risk factors including knee joint laxity,15 a combination of knee hyperextension with excessive subtalar joint pronation,16 and acceleration and deceleration with excessive quadriceps contraction and reduced hamstrings co-contraction, as well as valgus loading.17

A 2009 study reported, moreover, that a correlation between quadriceps and hamstring strength and recruitment appears to affect ACL risk.18 Noting that hamstrings and quadriceps co-contraction provides dynamic joint stabilization and protect the knee during sports, the authors found that female athletes who injured their ACLs had a combination of decreased relative hamstring strength and high relative quadriceps strength, and suggested that targeted neuromuscular interventions to increase relative hamstring strength and recruitment could decrease risk.

Researchers have also proposed ways to identify athletes at high risk of ACL injury using clinic-based measurements and freeware computer analysis.19 In this approach, the authors validated a relatively simple way to identify female athletes with high knee abduction moment landing mechanics using an algorithm that incorporates measures of knee valgus motion, knee flexion range of motion, body mass, tibia length, and quadriceps-to-hamstrings ratio. They then used 3-D motion analysis to validate the approach.20

Role players

It remains unclear which of these factors play the biggest role in the risk overlap between PFP and ACL injury, but researchers and clinicians are trying to find out.

“We’re doing what I call prospective biomechanical epidemiological studies,” said Timothy Hewett, PhD, professor and director of research at the Ohio State University Sports, Health, and Performance Institute, and director of the Sports Medicine Biodynamics Center at Cincinnati Children’s Hospital. “We bring the kids to the lab and test their movement patterns, strength, laxity, and other measures; get information about demographic variables, sports and injury history; then let them go out and play their sports.”

Hewett and his colleagues, including Greg Myer, PhD, research instructor of sports medicine at Cincinnati Children’s Hospital, have, in fact, determined that some of the factors that predispose subjects to PFP are similar to those they’ve found for ACL risk.21

“What we’ve previously reported for ACL—valgus torque of the knee, the lack of hip abductor torque—all fed this model of what predicted risk of future patellofemoral injury,” Hewett said.

And even though muscle weakness has often been cited as a source of risk, he drew a distinction between weakness and activation.

“In general, we don’t find hip muscle weakness a problem [for either PFP or ACL injury], but rather under-recruitment of the musculature during high-speed, sport-specific movements,” he explained.

The conclusion reflects on the aforementioned JUMP-ACL study of military recruits that correlated greater hip external rotator strength with PFP.8

“Those findings were directly counter to what would be expected, but it showed that it’s not hip strength that’s most crucial, but rather hip muscle recruitment,” Hewett said.

His colleague, Myer, emphasized the point.

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“We think one reason [PFP and ACL injuries] are more common in women is that they don’t naturally jump and land with the proper hip recruitment needed for control,” he said. “If they don’t have active control from the muscles, they’re going to be more reliant on passive control from their ligaments—but because women often have looser ligaments as well, it’s a double whammy that magnifies the risk.”

Irene Davis has reached similar conclusions in her work in runners, who are typically more prone to PFP than ACL problems.

“Strengthening the muscles doesn’t necessarily change the mechanics,” she said. “That’s why we do gait retraining for people who have anterior knee pain while running.”

In her own research, Davis found that feedback was essential for changing motor patterns.

“You have to give feedback, then remove it systematically so you increase the person’s self-reliance on their own internal cues,” she explained. “It’s brain training, and you’ve got to get them to the point where it becomes their automatic pattern.”

Intervention strategies

Once shared risk factors are recognized, it becomes easier to design interventions to prevent injury.

“In all kinds of sports, you want that leg to be as well aligned as possible, because then the forces will be distributed more normally within the joints,” Davis said. “Interventions should address that abnormal alignment—hip adduction, femoral internal rotation, genu valgus, knee external rotation. This involves engaging the hip abductors and external rotators.”

Myer agreed that the primary focus of prevention should be correcting any underlying biomechanical flaws.

“We’ve been trying to correct these jumping and landing mechanics females tend to have—that tendency to land in a knock-kneed position, which torques the ACL and puts it at risk, and disrupts the patellofemoral joint and puts the knee cap in an abnormal position,” he said.

The training interventions Myers and colleagues have developed aim to improve those knee-flexion dynamics and control the out-of-plane positions, thereby reducing injury risk.

“Recruitment of the gluteus maximus, especially, is a big player in lower extremity alignment, so we try to modify that,” he said.

Several papers by Hewett, Myer, and their colleagues delineate the evolution of their approach. As early as 1996, Hewett published a paper describing a training regimen designed to improve jump-landing mechanics and lower extremity strength in 11 female high school volleyball players.22 All of the subjects demonstrated a marked imbalance between hamstring and quadriceps muscle strength before training. The approach, which the authors termed “plyometric,” included a variety of jumps—wall jumps, tuck jumps, squat jumps (to name just a few), as well as bounding exercises and single-leg hops—and was designed to decrease landing forces by teaching neuromuscular control. The training program corrected the hamstring/quadriceps (H/Q) imbalance, and both adduction and abduction knee moments decreased significantly at landing. The study demonstrated that H/Q balance is critical to knee motion control; 10 of the 11 subjects successfully reduced their peak landing forces.

In 2005, Myer, Hewett, and colleagues reported in the Journal of Strength and Conditioning Research that 41 female teenage basketball, soccer, and volleyball players who underwent six weeks of neuromuscular training increased their knee flexion-extension range of motion during landing and significantly decreased both valgus and varus torques.23 The training included four main components: plyometric and movement, core strengthening and balance, resistance training, and speed training.

Other studies by these researchers have shown that plyometric training and a combination of dynamic stabilization and balance training each reduce lower extremity valgus measures during drop landings;24 that each of these approaches alone also increased lower extremity neuromuscular power and control;25 and that female high school athletes deemed at high risk for ACL injury based on measures of knee abduction moment significantly reduced abduction following neuromuscular training (though not to the level of those in the low-risk group, who had little reduction in knee abduction torque following training).26 They also showed that 10 weeks of trunk-focused neuromuscular training in 21 female high school volleyball players significantly increased standing hip abduction strength, which could improve the athletes’ control of lower limb alignment and decrease motion and loads resulting from increased trunk displacement during sports.27

In the past few years, research has begun to show that such interventions affect injury rates. In a study of 1263 high school athletes, untrained female athletes (n=463) had an incidence of knee injury 3.6 times higher than female athletes given neuro­muscular training (n=366). The latter group, moreover, had rates roughly equivalent to untrained male athletes (n=434), suggesting that the training erased the usual difference in injury risk between genders.28

In a study of a Santa Monica intervention program called PEP (Prevent injury, Enhance Performance), researchers reported that neuromuscular and proprioceptive training lowered the ACL injury rate in adolescent female soccer players by 88% in the first year.29 A later study by some of the same investigators found that in 1435 female NCAA soccer players, those who received the training had an ACL injury risk 1.7 times lower than controls.30

Chicago researchers developed a neuromuscular warm-up based on the techniques described by Hewett, Myer, and their associates, as well as the PEP program, and implemented it in the city’s urban public schools. Ninety coaches and 1492 athletes completed the randomized trial, in which student athletes received either the 20-minute neuromuscular warm-up or a standard warm-up before practices and games.31

The results were striking: Athletes who received the intervention had roughly a third to half the injury rate of controls for gradual-onset lower extremity injuries (including PFP), knee sprains, and ACL sprains. Incidence of ACL sprains dropped from 0.26 per 1000 athletic exposures in the control group to just 0.07 in the intervention group.

“We targeted this population because most of the research had been done in more homogeneous suburban populations with better resources, and we wanted to know if coaches functioning in this environment could learn the program and implement it con­sistently,” said Cynthia LaBella, MD, the paper’s lead author.

LaBella, an associate professor of pediatrics at Northwestern University’s Feinberg School of Medicine and medical director of the Institute for Sports Medicine at Children’s Memorial Hospital in Chicago, believes there is significant overlap between ACL and PFP risk factors, and that the training program addresses both. In a previous paper, she reported that similar training reduced knee pain in adolescent female athletes.32

Lindsay DiStefano, PhD, ATC, an assistant professor of kinesiology at the University of Connecticut and one of the panelists at the NATA forum, believes that, fundamentally, all such approaches go back to teaching young people to move well.

“We have to teach kids how to control their bodies—how to land well, cut well, with good technique,” she said.

She noted that in the JUMP-ACL study, for which she was a research assistant, individuals who developed PFP didn’t have significant strength or postural differences from those who did not develop the condition.

“The risk factors for both injuries are movement-based,” she said. “People who don’t absorb forces as well, who have a lot of frontal and transverse plane motion, are most likely to get injured.”


One consideration when assessing such programs is how well athletes and their coaches will persist down the stretch.

“There’s good evidence that prevention programs work; the problem is that often you go back a year later and people have stopped doing them,” DiStefano said. “We have to figure out ways to make exercises more efficient and effective so coaches and athletes adopt these programs as part of their normal practices.”

In Chicago, according to LaBella, the prevention program was so popular that it’s spreading rapidly as students move and coaches change jobs, taking it with them.

“These folks are hungry for professional development and education to better their coaching skills,” she said. “They’re very appreciative, and they feel like it’s an important program that they want to keep using.”

Cary Groner is a freelance writer based in the San Francisco Bay Area.


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9. Souza RB, Powers CM. Predictors of hip internal rotation during running: an evaluation of hip strength and femoral structure in women with and without patellofemoral pain. Am J Sports Med 2009;37(3):579-587.

10. Souza RB, Powers CM. Differences in hip kinematics, muscle strength, and muscle activation between subjects with and without patellofemoral pain. J Orthop Sports Phys Ther 2009;39(1):12-19.

11. Lankhorst NE, Bierma-Zeinstra SM, van Middelkoop M. Factors associated with patellofemoral pain syndrome: a systematic review. J Orthop Sports Phys Ther 2011 Oct 25. [Epub ahead of print]

12. Hewett TE, Myer GD, Ford KR, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med 2005;33(4):1-10.

13. Myer GD, Chu DA, Brent JL, Hewett TE. Trunk and hip control neuromuscular training for the prevention of knee joint injury. Clin Sports Med 2008;27(3):425-429.

14. Greska EK, Cortes N, Van Lunen BL, Onate JA. A feedback inclusive neuromuscular training program alters frontal plane kinematics. J Strength Cond Res 2011 Sept 14. [Epub ahead of print]

15. Myer GD, Ford KR, Paterno MV, et al. The effects of generalized joint laxity on risk of anterior cruciate ligament injury in young female athletes. Am J Sports Med 2008;36(6):1076-1080.

16. Loudon JK, Jenkins W, Loudon KL. The relationship between static posture and ACL injury in female athletes. J Orthop Sports Phys Ther 1996;24(2):91-97.

17. Shimokochi Y, Shultz SJ. Mechanisms of noncontact anterior cruciate ligament injury. J Athl Train 2008;43(45):396-408.

18. Myer GD, Ford KR, Barber Foss KD, et al. The relationship of hamstrings and quadriceps strength to anterior cruciate ligament injury in female athletes. Clin J Sport Med 2009;19(1):3-8.

19. Myer GD, Ford KR, Hewett TE. New method to identify athletes at high risk of ACL injury using clinic-based measurements and freeware computer analysis. Br J Sports Med 2011;45(4):238-244.

20. Myer GD, Ford KR, Khoury J, Hewett TE. Three-dimensional motion analysis validation of a clinic-based nomogram designed to identify high ACL injury risk in female athletes. Phys Sportsmed 2011;39(1):19-28.

21. Hewett TE, Myer GD, Ford KR, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med 2005;33(4):492-501.

22. Hewett TE, Stroupe AL, Nance TA, Noyes FR. Plyometric training in female athletes. Decreased impact forces and increased hamstring torques Am J Sports Med 1996;24(6):765-773.

23. Myer GD, Ford KR, Palumbo JP, Hewett TE. Neuromuscular training improves performance and lower extremity biomechanics in female athletes. J Strength Cond Res 2005;19(1):51-60.

24. Myer GD, Ford KR, McLean SG, Hewett TE. The effects of plyometric versus dynamic stabilization and balance training on lower extremity biomechanics. Am J Sports Med 2006;334(3):445-455.

25. Myer GD, Ford KR, Brent JL, Hewett TE. The effects of plyometric versus dynamic stabilization and balance training on power, balance, and landing force in female athletes. J Strength Cond Res 2006;20(2):345-353.

26. Myer GD, Ford KR, Brent JL, Hewett TE. Differential neuromuscular training effects on ACL injury risk factors in “high-risk” versus “low-risk” athletes. BMC Musculoskeletal Disorders 2007;8:39.

27. Myer GD, Brent JL, Ford KR, Hewett TE. A pilot study to determine the effect of trunk and hip focused neuromuscular training on hip and knee isokinetic strength. Br J Sports Med 2008;42(7):614-619.

28. Hewett TE, Lindenfeld TN, Riccobene JV, Noyes FR. The effect of neuromuscular training on the incidence of knee injury in female athletes: a prospective study. Am J Sports Med 1999;27(6):699-706.

29. Mandelbaum BR, Silvers HJ, Watanabe DS, et al. Effectiveness of the neuromuscular and proprioceptive training program in preventing anterior cruciate ligament injuries in female athletes: two year follow-up. Am J Sports Med 2005;33(7):1003-1010.

30. Gilchrist J, Mandelbaum BR, Melancon H, et al. A randomized controlled trial to prevent noncontact ACL injury in female collegiate soccer players. Am J Sports Med 2008;36(8):1476-1483.

31. LaBella CR, Huxford MR, Grissom J, et al. Effect of neuromuscular warm-up on injuries in female soccer and basketball athletes in urban public high schools. Arch Pediatr Adolesc Med 2011;165(11):1033-1040.

32. 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.

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