Researchers split over strength training for ACL injury prevention

Biomechanics practitioners facing the daunting task of selling high school or youth coaches on the merits of a knee injury prevention program know there are two keys to the sale, neither of which has anything to do with preventing injury. First, the coach needs to believe that the program will not interfere with his or her regular practice schedule. And, if possible, the coach needs to believe that the program will actually improve athletes' performance.

With this in mind, the concept of strength training alone as a potential preventive measure becomes attractive indeed. Coaches understand the value of strength training from a performance standpoint and are likely to already have a program in place that can easily accommodate a few more targeted exercises, so it's an easy sell. And from a biomechanical perspective, it makes sense that poor mechanics may result from muscles too weak to maintain more appropriate postures, and that strengthening those muscles will improve mechanics and, in turn, lower injury risk.

At least, to paraphrase Hemingway, it's pretty to think so.

In reality, as researchers in this area have come to expect, the picture is quite murky. Outcome studies of knee injury prevention programs to date, though sparse, suggest that strength training is at best one of many training components that work in concert to reduce injury risk, with at least one successful program excluding strength training entirely. Despite recent work implicating hip mechanics as risk factors for anterior cruciate ligament tears, researchers have come to mixed conclusions as to whether hip strengthening can help improve susceptible landing mechanics.

"It's of interest to coaches and athletes to only spend the minimal amount of time that they need to have benefit, so the push has been to do less intervention and shorter training," said Ryan L. Mizner, PT, PhD, assistant professor of physical therapy at Eastern Washington University in Spokane.

His group conducted several studies on strength and landing mechanics that were presented in late May at the annual meeting of the American College of Sports Medicine.

"So one thing I wonder is how much time do we need to spend to make the muscles stronger," he said.

A useful adjunct

In a March 2006 meta-analysis,¹ researchers from Cincinnati Children's Hospital assessed six studies of neuromuscular interventions designed to prevent ACL injury in female athletes. Of the three studies that succeeded in reducing ACL injury rates, two included strength training as part of the intervention: the Cincinnati program, which in a 1999 study² was associated with a 72% reduction in noncontact ACL injuries during a single season, and a regimen designed by Santa Monica researchers, which was associated with an 88% decrease in ACL injuries after one year and a 74% decrease in the second year, as reported in a July 2005 study.³

But a balance-focused protocol developed by Norwegian researchers, which did not incorporate strength training, also was associated with a significant reduction in noncontact ACL injury rate after two seasons in a 2003 study of handball players.⁴

"Strength training was involved in two out of the three studies but not in one of the three, which says that strength training is probably a useful adjunct but may not be a prerequisite for injury prevention," said Tim E. Hewett, PhD, director of the sports medicine biodynamics center at Cincinnati Children's Hospital and associate professor of pediatrics, orthopedic surgery, and biomedical engineering at the University of Cincinnati.

"In all of our training we incorporate plyometrics, technique, core, and strength, so we don't know what the main contributors are. My guess is that the effects are going to be combinatorial," he said.

Laboratory studies to date have done little to clarify the extent to which strength training either helps to improve landing mechanics directly or enhances the positive effects of corrective feedback. In a study of 36 healthy female athletes presented at the 2005 meeting of the National Athletic Trainers Association, researchers from Brigham Young University found that a six-week plyometric training program significantly increased knee flexion angle during 46-cm drop landings and single-leg hop tests, whereas a six-week strength training program did not.

At the 2006 NATA meeting, research from the University of Kentucky found that six weeks of strength training in seven female college basketball players had no significant kinematic or kinetic effects at the hip or knee joints during single-leg landing relative to eight players who were not trained.

However, at the same meeting, investigators from the University of North Carolina at Chapel Hill reported that nine weeks of resistance band exercises significantly decreased knee anterior shear force, knee extension moment, and knee flexion angular velocity in 18 healthy recreational athletes when compared with a 12-subject control group.

And in a December 2005 study from the University of Pittsburgh,⁵ eight weeks of either plyometric training or basic resistance exercises significantly increased initial and peak knee and hip flexion in female high school athletes, as well as significantly increasing peak preactive and integrated electromyographic levels for the gluteus medius.

This suggests that both protocols better enable the hip musculature to anticipate the impact forces that result from landing. Interestingly, though, both training protocols significantly improved peak torque in the quadriceps, but not in the hamstrings or hip abductors, raising the counterintuitive possibility that strength training exercises can alter muscle activation patterns without actually improving strength.

Mixed feedback

In keeping with the schizophrenic patterns of evidence, two studies presented at this year's ACSM meeting came to opposite conclusions regarding the effect of strength on athletes' ability to benefit from feedback on landing technique. Researchers from the same group from the University of North Carolina that had previously linked six weeks of strength training to improved mechanics reported that 29 female recreational athletes who underwent nine weeks of strength training were more likely than 29 untrained athletes to respond favorably to video feedback on correct stop-jump technique.

After receiving feedback, athletes in both groups demonstrated significantly reduced peak vertical ground reaction force and increased knee flexion, hip flexion, and hip abduction angles; peak knee anterior shear force was significantly lower and hip abduction angle significantly greater in those who had received strength training than in those who had not.

"Strength training may be able to provide an additional ability to respond to feedback," said Daniel C. Herman, a graduate student researcher in the department of biomedical engineering at UNC-Chapel Hill, who presented his group's findings at the ACSM meeting.

But research from Eastern Washington University, presented at the same meeting, also suggests that strength has no impact on athletes' ability to benefit from verbal instruction on landing technique. Following five minutes of verbal instruction, 37 healthy female collegiate athletes significantly increased absorption time and peak knee flexion angle and significantly decreased ground reaction force and knee abduction angle when landing from a 31-cm drop jump. Although the study did not involve a strength training intervention, investigators found that the improvements were unrelated to the relative strength of the hip abductors, hip extensors, knee flexors, knee extensors, trunk flexors, trunk extensors, or ankle plantar flexors.

"It may be that coordination is more important than strength," Mizner said. "It's probably more of a control issue than a strength issue."

Control issues

Other leading researchers in the field agree. As anecdotal evidence, Hewett cites the observation that within four to six sessions in his neuromuscular training program, an athlete initially unable to maintain even a double-legged squat on a bosu balance trainer can progress to a solid 90 degrees single-leg squat.

"What that tells me is that's not muscle, that's neural control," Hewett said. "Strength is important, but neural control is more important."

The alacrity with which an ACL injury occurs, estimated by Norwegian researchers6 at between 17 and 50 milliseconds from initial ground contact, underscores the relative importance of control, according to Scott G. McLean, PhD, assistant professor of athletic training and movement science and director of the Injury Biomechanics Laboratory at the University of Michigan.

"If injury truly occurs in the first 50 milliseconds and it takes muscles up to 60 milliseconds to produce force, then there's not much you can do once you've landed. Once you land, it's done," said McLean, who is also exploring the effect of neural control on injury risk in the context of fatigue (see "Trunk muscles play role in ACL injury risk," page 9).

"Muscle forces act to oppose external loads. If they don't switch on, no matter how strong they are, then the external loads are not being opposed," he said.

From the hip

Research from Eastern Washington also suggests that strength may be more important in some muscles than in others. In the same group of 37 athletes in the feedback study, Mizner and colleagues found that the strength values for the quadriceps, hamstrings, and calf muscles were significantly correlated with hip and knee positions during drop jump landings; surprisingly, joint position did not correlate with strength of either the hip or trunk muscles. In particular, the lack of correlation between hip strength and peak hip adduction appears to contradict the popular theory that hip motion increases as a result of hip muscles too weak to resist the motion.

"Moving into more hip collapse wasn't related to the hip musculature but was more strongly related to the more distal muscles," Mizner said. "If those muscles are weak, they tend to stiffen in the sagittal plane, and when that happens, we're hypothesizing that the joint needs to go into that secondary (frontal) plane to absorb the impact."

These findings appear to contrast with those of researchers from the University of Kentucky, who found earlier this year that peak hip abductor torque was moderately correlated with hip flexion, hip adduction, and peak knee valgus angle in 15 healthy female subjects but not in 15 male subjects performing single-leg landings.⁷ Because the female subjects also demonstrated significantly greater knee valgus than the male subjects, the authors concluded that hip abductor strength may play a role in neuromuscular control of the knee.

"There has to be a certain level of strength that's required," said Cale A. Jacobs, PhD, ATC, an orthopedic researcher at Lexington Clinic and an adjunct professor of rehabilitation sciences at the University of Kentucky. "Stronger athletes may be able to do it with less activation, while weaker athletes may have to activate the muscle more."

The key question regarding strength training and ACL injury risk, however, may be that of timing. Whereas most of the studies in this area to date have involved high school and college-age athletes, research from Cincinnati⁸ suggests that the optimum window for intervention-of any kind-is between age 11, when girls hit puberty, and age 16, when ACL injuries in female athletes occur most frequently. Both boys and girls undergo a growth spurt following puberty, but boys concurrently undergo a "neuromuscular spurt" that enables them to effectively control their bodies' increased mass and longer arms. Girls do not experience a similar neuromuscular spurt, but researchers hypothesize that training can make up for the deficit, Hewett said.

"The problem is that the majority of people we're retraining now are those in high school and college who've already had ACL problems. Where we have to go now is to more pre-training," Hewett said. "We can't wait until they're 16. We've got to get their neuromuscular control down sooner and hopefully close that window."

Jordana Bieze Foster is a freelance writer based in Massachusetts and the former editor of BioMechanics magazine.

References

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4. Myklebust G, Engebretsen L, Braekken IH, et al. Prevention of anterior cruciate ligament injuries in female team handball players: a prospective intervention study over three seasons. Clin J Sports Med 2003;13(2):71-78.
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8. Hewett TE, Myer GD, Ford KR. Decrease in neuromuscular control about the knee with maturation in female athletes. J Bone Joint Surg Am 2004;86-A(8):1601-1608.