Researchers look at ACL risk from a neuromechanical view

Four times in the past eight years, a group of experts on anterior cruciate ligament injury has published a consensus statement outlining what was known at the time, what was not yet known, and suggested directions for future research. Typically, the consensus statement has been subdivided into five aspects of ACL injury research: biomechanical factors, neuromuscular factors, hormonal factors, structural factors and intervention programs.

The statement from the most recent consensus conference, however, will be a little different. During the meeting, held in April in Greensboro, NC, the subset of attendees who were grouped together to discuss biomechanical factors found themselves addressing many of the same issues as those in the neuromuscular group. Given the body of evidence suggesting that risk of noncontact ACL injury is multifactorial, they reasoned, it makes little sense to consider potentially high-risk movements independently of the muscles involved in facilitating and resisting those movements and the neural processes involved in activating those muscles.

As a result, when this year's consensus statement is published in the Journal of Athletic Training, it will have four subgroups instead of five, with biomechanical and neuromuscular factors combined under a single “neuromechanical” umbrella. The decision reflected a sentiment voiced frequently by conference attendees, during the two days of research presentation and discussion that preceded the consensus process: that when it comes to ACL injury risk, no one contributing factor is an island.

“You need the strength factors, you need the EMG, you need the kinematics and you need the kinetics. All four,” said Christopher M. Powers, PT, PhD, an associate professor of biokinesiology and physical therapy at the University of Southern California. “Until we put the whole package together, we're going to be somewhat limited.”

It may not be surprising, then, that a common thread linking many of the studies presented in Greensboro on neuromuscular factors is that they identified gender-related differences in strength or muscle activation without definitively linking those differences to injury risk. With epidemiological evidence indicating that female athletes are in fact at much greater risk of noncontact ACL injury than their male counterparts, it's certainly possible that neuromuscular characteristics seen more often in female athletes do play a role in those injuries. It's also possible, however, that they are not.

“It may be premature to assume that the neuromuscular variables we measure are a problem,” said Scott G. McLean, PhD, assistant professor of athletic training and movement science and director of the Injury Biomechanics Laboratory at the University of Michigan.

Questions of strength

The focus on quadriceps and hamstrings strength that characterized early ACL injury research has moved up the kinetic chain, such that the majority of presentations on strength in Greensboro centered on the hip musculature. Given that knee valgus and knee abduction moment on landing have been prospectively shown to predict ACL injury in young female athletes,¹ researchers theorize that these apparently high-risk mechanics can be traced to weakness in the hip muscles, which control the lower extremities.

A study from Powers and colleagues, which analyzed kinematics and joint moments associated with “soft” versus “stiff” (more extended) landings in female teenage soccer players, lends support to this theory. Earlier, the investigators had established that the sagittal-plane differences between landing styles were also associated with differences in knee valgus angles and moments (see “Soft landing studies find effects beyond sagittal plane of knee,” November, page 9). In the current study, they calculated a knee-hip extensor moment ratio for each of 58 subjects and found that stiff landings were associated with ratios about one and a half times greater than soft landings.

“The individuals who select a stiff landing pattern seem to be performing the task using more of a knee strategy. We feel they are doing so because they lack the hip strength and/or the neuromuscular control to be able to decelerate effectively,” said Christine Pollard, PT, PhD, an assistant professor of research physical therapy at USC, who presented the group's findings in Greensboro. She noted, however, that actual measurements of hip strength or neuromuscular control would be needed to advance this hypothesis.

Other studies suggest that if hip strength does contribute to ACL injury risk, it probably does not do so on its own (see “Researchers split over strength training for ACL injury prevention,” August, page 41). In a study of 21 male and 21 female collegiate athletes presented at the conference, researchers from West Chester (PA) University found that isometric hip flexor and lateral hip rotator strength was significantly greater in male athletes, but EMG data obtained during landing also suggested that female athletes are less efficient in activation of the gluteus maximus and rectus femoris muscles.

And preliminary data from a four-year prospective study of an armed forces academy population indicate that those who went on to suffer an ACL injury were actually stronger (in terms of hip external rotation and hip abduction as well as knee extension) than those who did not. That study, led by researchers at the University of North Carolina-Chapel Hill, is not yet complete and the number of injuries so far is relatively small (21 injuries in 2563 subjects). It's also possible, the authors theorized, that the injured subjects were more physically active than the uninjured subjects.

But the findings do suggest that poor hip control may be a more important risk factor than hip weakness.

“Maybe the strength is there, but they're just not using it,” said Michelle Boling, ATC, a doctoral student in human movement science at UNC-Chapel Hill, who presented the group's findings in Greensboro.

Activation required

Study of muscle activation and ACL injury risk is only beginning to ascend to the level of the hip, with much of the existing research focused on the quadriceps and hamstrings.

Investigators from the University of Ottawa found that activation of the vastus lateralis, vastus medialis and biceps femoris achieved a higher mean frequency in 15 male soccer players than in 15 female soccer players executing an unanticipated cutting maneuver. They also found that, although biceps femoris intensity decreased in both groups prior to initial contact, a reduction in the median frequency components in that muscle was seen only in the female athletes.

“They're using a different strategy,” said Melanie L. Beaulieu, a graduate student in the department of human kinetics who presented the group's results in Greensboro. “Soccer players use more of a generalized co-contraction during an unanticipated cutting maneuver, so maybe this does put the ACL at risk.”

Research from the University of Michigan also suggested gender differences in co-contraction strategies, in this case identifying a mediolateral imbalance in female recreational athletes that was not seen in their male counterparts. The lateral quadriceps-hamstring co-contraction ratio calculated during a single-leg forward hop was significantly greater than the medial quadriceps-hamstring ratio in 11 female athletes but not in 10 male athletes; the medial co-contraction ratio accounted for a significant amount of the variance in the peak knee abduction moment in the female athletes but not in the male athletes.

“Medial muscle activation is important for resisting abduction loads,” said Riann Palmieri-Smith, PhD, ATC, an assistant professor of athletic training at the University of Michigan and director of the university's Neuromuscular Research Laboratory, who presented the findings in Greensboro. “This strategy that the female athletes are using may be ineffective.”

In a UNC-Greensboro study that analyzed the transition from non-weightbearing to weightbearing using a Vermont Knee Laxity Device,² gender-specific differences included neuromuscular variables in addition to kinematics. Knee varus and external rotation during weight acceptance were significantly greater in 39 female subjects than in 28 male subjects, as were muscle reflex amplitudes for the medial quadriceps and lateral hamstrings.

Moving on up

Researchers from UNC-Chapel Hill took their analysis above thigh level in assessing body-segment coordination in 12 female and 12 male recreational athletes during jump landings; segment pairings included foot-shank, shank-thigh and thigh-trunk. Although they found that coordination strategies were similar between genders, those of the female athletes were significantly more variable. The clinical importance of this finding, however, remains unclear; further research will be needed to determine whether increased variability is indeed a risk factor or even an undesirable characteristic.

“I think there's a case to be made for a good level of variability. Too little variability is probably a bad thing,” said Melanie L. McGrath, ATC, a doctoral student in human movement science at the university, who presented the findings in Greensboro. “On the flip side, if you're all over the place, you're probably at higher risk of ending up in that position of no return. Despite similar coordination patterns, the organization of the female neuromuscular system may be less stable.”

In the only Greensboro presentation to specifically examine hip control, researchers from the Medical College of Georgia measured activation of the gluteus maximus, gluteus medius, and vastus medialis in nine male and nine female recreational athletes as they performed single-leg drop landings. They found no significant gender differences with regard to hip muscle amplitudes but significantly earlier and greater vastus medialis activation in female athletes than in male athletes—a finding consistent with previous researchers' descriptions of “quadriceps dominance,” which is thought to cause anterior tibial shear and destabilize the ACL.³ They also found, however, that male athletes activated the hip muscles significantly earlier than female athletes relative to quadriceps activation, suggesting that poor hip control may also contribute to high-risk landing mechanics.

“The earlier gluteal onset relative to the vastus medialis in the male athletes may serve as preparatory control of the knee for transverse and frontal plane motion,” said Lori A. Bolgla, PT, PhD, ATC, an assistant professor of physical therapy at the college, who presented her group's findings in Greensboro.

Targeted training

A different approach to assessing the role of specific neuromuscular factors in ACL injury is to study the effects of an targeted neuromuscular intervention, to determine if the intervention is associated with improved biomechanics or, better yet, reduced injury rates. Researchers from Cincinnati Children's Hospital are in the early stages of just such an investigation related to the neuromusculature of the trunk and hip.

In a study published in March in the online version of the British Journal of Sports Medicine,⁴ the Cincinnati team found that isokinetic hip abduction strength increased significantly in 14 female high-school volleyball players who participated in a 10-week trunk- and hip-focused neuromuscular training program but did not change in a group of seven control subjects. Further analyses of the study data will reflect the effects of the same training program on trunk strength, trunk and hip biomechanics, and activation patterns in both the hip and trunk muscles, said Greg D. Myer, MS, a sports biomechanist in the hospital's Sports Medicine Biodynamics Center and lead author of the BJSM paper.

“You may have great strength, but if you don't turn it on enough or at the right time then it's not going to be effective,” Myer said. “But you also have to have the available horsepower.”

In an ideal world, the research will identify risk factors that are not only easy to modify through training but also easy to screen for without high-tech equipment or complicated computer algorithms. So even if further research shows that strength is just one of many neuromuscular variables contributing to ACL injury risk, it may still prove to be the measure best suited for screening.

But given the range of variables, neuromuscular and otherwise, currently implicated in noncontact ACL injury, it seems unlikely that a targeted training protocol like the one used in the BJSM study would replace multifaceted interventions for preventing such injuries.

“ACL injury is multivariate,” Myer said. “We know that technique training is important, and that plyometrics are a critical component. If we find that hip strength is a good component, that will be just one small piece to add to our protocol.”

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

References

1.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.

2.Uh BS, Beynnon BD, Churchill DL, et al. A new device to measure knee laxity during weightbearing and non-weightbearing conditions. J Orthop Res 2001;19(6):1185-1191.

3.Myer GD, Ford KR, Hewett TE. Rationale and clinical techniques for anterior cruciate ligament injury prevention in female athletes. J Athl Train 2004;39(4):352-364.

4.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.