Secondary Logo

Journal Logo

An Integrated Approach to Change the Outcome Part II: Targeted Neuromuscular Training Techniques to Reduce Identified ACL Injury Risk Factors

Myer, Gregory D.1,2,3,4,5,6,7; Ford, Kevin R.1,2,3; Brent, Jensen L.1; Hewett, Timothy E.1,2,3,4,8,9,10,11,12

Journal of Strength and Conditioning Research: August 2012 - Volume 26 - Issue 8 - p 2272–2292
doi: 10.1519/JSC.0b013e31825c2c7d
Methodological Report
Free

Myer, GD, Ford, KR, Brent, JL, and Hewett, TE. An integrated approach to change the outcome part II: targeted neuromuscular training techniques to reduce identified ACL injury risk factors. J Strength Cond Res 26(8): 2272–2292, 2012—Prior reports indicate that female athletes who demonstrate high knee abduction moments (KAMs) during landing are more responsive to neuromuscular training designed to reduce KAM. Identification of female athletes who demonstrate high KAM, which accurately identifies those at risk for noncontact anterior cruciate ligament (ACL) injury, may be ideal for targeted neuromuscular training. Specific neuromuscular training targeted to the underlying biomechanical components that increase KAM may provide the most efficient and effective training strategy to reduce noncontact ACL injury risk. The purpose of the current commentary is to provide an integrative approach to identify and target mechanistic underpinnings to increased ACL injury in female athletes. Specific neuromuscular training techniques will be presented that address individual algorithm components related to high knee load landing patterns. If these integrated techniques are employed on a widespread basis, prevention strategies for noncontact ACL injury among young female athletes may prove both more effective and efficient.

1Human Performance Laboratory, Sports Medicine Biodynamics Center, Cincinnati Children's Hospital Medical Center, Cincinnati, Ohio

Departments of 2Pediatrics; and

3Orthopedic Surgery, College of Medicine, University of Cincinnati, Cincinnati, Ohio

4Division of Athletic Training, School of Allied Medical Professions, The Ohio State University, Columbus, Ohio

Departments of 5Athletic Training

6Sports Orthopedics; and

7Pediatric Science, Rocky Mountain University of Health Professions, Provo, Utah

8The Ohio State University Sports Health and Performance Institute, The Ohio State University, Columbus, Ohio

Departments of 9Physiology and Cell Biology

10Orthopedic Surgery

11Family Medicine

12Biomedical Engineering, The Ohio State University, Columbus, Ohio

Address correspondence to Dr. Gregory D. Myer, greg.myer@chmcc.org.

Back to Top | Article Outline

Introduction

High knee abduction moments (KAMs) during landing predict noncontact anterior cruciate ligament (ACL) injury risk in young female athletes with high accuracy (4,17,25,43). Therefore, a field-based assessment algorithm was systematically developed and validated, which aimed to improve the potential to identify and target injury prevention training to female athletes with increased KAM (38,39). The validated field-based assessment algorithm delineated 5 biomechanical factors that combined to identify high KAM during landing with high accuracy (38,39).

The established links between lower limb mechanics and noncontact ACL injury risk led to the development of neuromuscular training interventions designed to prevent noncontact ACL injury by targeting deficits identified in specific populations (14,17,33–35,40,41). Injury prevention protocols have resulted in positive preventative and biomechanical changes in female athletic populations at high risk for knee injury (14,15,32,41). More specifically, pilot work indicates that female athletes categorized as high risk for noncontact ACL injury, based on previous coupled biomechanical and epidemiologic studies (17), may be more responsive to specially designed neuromuscular training (34). Neuromuscular training targeted to the specific biomechanical components that drive high KAM may provide the most efficient and effective training strategy to reduce noncontact ACL injury risk. Female athletes who demonstrate high KAM risk factor for noncontact ACL injury may be ideal for targeted neuromuscular training. The purpose of this commentary was to provide an integrative approach to identify and target mechanistic underpinnings to increased ACL injury in female athletes. Specific techniques will be presented that can be used for athletes for whom their high KAM is related to one or more components assessed in the ACL injury risk prediction algorithm.

Back to Top | Article Outline

Integration of Screening Techniques to Identify Specific Mechanisms That Underlie High Knee Abduction Moments and Methods to Target Risk Factors With Neuromuscular Training

The previously described methodology (36) demonstrates techniques to accurately capture and analyze measures of tibia length, knee valgus motion, knee flexion range of motion, body mass, and quadriceps to hamstrings ratio (QuadHam ratio), which are used in a high knee load prediction algorithm. Certain athletes are at a higher risk of noncontact ACL injury and may demonstrate one or more specific mechanisms that underlie predicted increased KAM. The following discussion will present representative athletes' test measurements of specific landing mechanics or anthropometrics that result in the prediction of high KAM. Accordingly, specific neuromuscular training techniques will be presented that can be used to target each deficit that contributes to high KAM that is captured in the prediction algorithm.

Back to Top | Article Outline

Rapid Musculoskeletal Growth Contributes to High Knee Abduction Moments

Rapid growth during maturation initiates increased stature and, in turn, an increased height of the center of mass. The increased musculoskeletal height, added to increased total body mass, may initiate greater joint forces that are more difficult to balance and dampen during high velocity maneuvers (13,16,18). Unlike adolescent male athletes who naturally increase hip abduction strength relative to body mass as they increase in age from 11 to 17 years, female athletes demonstrate no similar adaptation in hip abduction strength measures (5). The absence of adaptation-relative hip abduction strength to match the demands of growth and development in adolescent women as they mature may create decreased “core control.” The maturational invoked inertial imbalance (increased trunk load without adaptive hip and trunk control) may lead to abnormal joint alignments, increased KAM, and may be related to their increased risk of noncontact ACL injury compared with men following this developmental stage. Figure 1 presents an example of a representative subject whose combination of increased tibia length and mass, associated with her rapid growth, contributed to increased inertial demands on her lower extremity and increased risk to demonstrate high KAM landing mechanics when using the field-based ACL injury risk prediction algorithm. The completed algorithm for the representative subject (tibia length: 47 cm; knee valgus motion: 3.0 cm; knee flexion range of motion (ROM): 55.9°; mass: 71 kg; QuadHam ratio: 1.19) predicted that she would have a 96% (126.5 points) probability of high KAM during the drop vertical jump. Her actual KAM measure for the presented drop vertical jump quantified simultaneously with three-dimensional (3D) motion analysis was 48.5 N m of knee abduction load. Conversely, Figure 2 presents a representative subject whose combinations of decreased tibia length and mass before her rapid growth spurt diminishes her risk to demonstrate high KAM landing mechanics. As demonstrated in her completed algorithm, the representative subject (tibia length: 33.0 cm; knee valgus motion: 4.0 cm; knee flexion ROM: 66.4°; mass: 30.7 kg; QuadHam ratio: 1.64) would have a 14% (63.5 points) chance to demonstrate high KAM during the drop vertical jump. Her actual KAM measure for the presented drop vertical jump that was quantified simultaneously with 3D motion analysis was 13.4 N m of knee abduction load.

Figure 1

Figure 1

Figure 2

Figure 2

Thus, after the onset of puberty and the initiation of peak height velocity, increased tibia and femur lever length, with increased body mass and height of the center of mass, in the absence of increases in strength and recruitment of the musculature at the hip and trunk may lead to decreased core control and ability to control inertial forces of the trunk during dynamic tasks (11). As female athletes reach maturity, decreased “core stability” may underlie their tendency to demonstrate increased KAM during dynamic tasks and increased ACL injury risk during competition (Figure 1) (13,16,18,21,22,47).

Back to Top | Article Outline

Targeted Training for Rapid Growth Risk Factor

Reduced ability to activate the hip stabilizers may allow increased lateral trunk positions that can incite increased KAM (48). Decreased core control and muscular synergism, along with decreased control of inertial loads from the hip stabilizers, may affect performance in power activities and may also increase the incidence of injury secondary to lack of control of the center of mass, especially in female athletes (23,49,50). Targeted trunk and hip control neuromuscular training increases standing hip abduction strength in female athletes (29). Figure 3 presents a neuromuscular training progression that can be instituted with athletes to target deficits in trunk and hip control (29,30). Progressive exercise phases are used to facilitate incremental advancements that are designed to improve the athletes' ability to control the increased inertial loads during dynamic activities. The progressive increases in intensity to given exercise techniques facilitate the adaptations that prepare the athlete for end-stage progressions that will incorporate lateral trunk perturbations. These perturbations require the athlete to decelerate and control the trunk in the frontal plane to successfully execute the prescribed technique. Similar exercise progressions have demonstrated that increased hip abduction strength improves the ability of female athletes to control the body center of mass and lower extremity alignment, which results in a decreased KAM during sports activities (7,11).

Figure 3

Figure 3

Back to Top | Article Outline

Excessive Knee Valgus Motion Contributes to High Knee Abduction Moments

Neuromuscular control deficits are defined as muscle strength, power, or activation patterns that lead to abnormal joint alignments and increased KAM (19). Female athletes demonstrate neuromuscular control deficits that increase lower extremity joint loads during sports activities (10,12,39). One neuromuscular deficit, which can be termed “ligament dominance,” is defined as an imbalance between the neuromuscular and ligamentous control of dynamic knee joint stability (19). This imbalance in control of dynamic knee joint stability is demonstrated by an inability to control lower extremity frontal plane motion during landing and cutting (10,12,39). Figure 4 provides an example of a representative subject with excessive knee valgus motion during the drop vertical jump that contributes to her increased risk of high KAM landing mechanics. Figure 4E presents the completed algorithm for the representative subject (tibia length: 41.0 cm; knee valgus motion: 9.0 cm; knee flexion ROM: 59.8°; mass: 67.5 kg; QuadHam ratio: 1.90). Based on her demonstrated measurements, this subject would have a 98% (137.5 points) chance to demonstrate high KAM during the drop vertical jump. Her actual KAM measure for the presented drop vertical jump, which was quantified simultaneously with 3D motion analysis, was 44.1 N m of KAM. Figure 5 provides an example of a representative subject with optimal neuromuscular control that limits her potential to demonstrate high KAM risk factor and ultimately reduces her risk of noncontact ACL injury. Based on her demonstrated measurements (tibia length: 38 cm; knee valgus motion: 0.0 cm; knee flexion ROM: 63.8°; mass: 44.9 kg; QuadHam ratio: 1.89), this subject would have a 30% (76.5 points) chance to demonstrate high KAM during the drop vertical jump using the ACL injury prediction algorithm (Figure 5E). Her actual KAM measure for the presented drop vertical jump, quantified simultaneously with 3D motion analysis, was 7.6 N m of knee abduction load, indicating a likely reduced risk on noncontact ACL injury.

Figure 4

Figure 4

Figure 5

Figure 5

Back to Top | Article Outline

Targeted Training for Excessive Knee Valgus Motion Risk Factor

Although prior research demonstrates the benefit of injury prevention training among a wide variety of athletes, it appears that those athletes who demonstrate neuromuscular imbalances, as evidenced through poor frontal plane knee control and increased knee valgus motion, may benefit the most from training (34,39). Wall jumps are an example of an initial exercise that could be used to target excessive knee valgus motion (Figure 6). This low-to-moderate intensity jump also allows clinicians to begin analysis of the athlete's level of frontal plane knee control by instructing the athletes to keep their knees apart when landing and which produces decreased ACL load in knee flexion angles used in this jump (27). Another useful exercise to target the high KAM driven by knee valgus motion is the tuck jump (Figure 7). The tuck jump is an exercise that is on the opposite end of the intensity spectrum from the wall jump and requires a high effort level from the athlete. Initially, the athlete will likely place a majority of their cognitive efforts on the performance of this difficult maneuver. Clinicians can readily identify an athlete who may demonstrate abnormal levels of frontal plane knee displacement during jumping and landing because the athlete usually devotes minimal attention to technique on the first few repetitions. In addition, tuck jumps can be used to assess progressive improvements in lower extremity biomechanics. The broad jump and hold (Figure 8) allows clinicians to assess the athlete's propensity to demonstrate jumping mechanics termed as “active valgus,” a position of hip adduction and knee abduction that is the result of muscular contraction rather than the inability to control ground reaction forces. Active valgus occurs during taking off from a jump, rather than at landing, and should be corrected during neuromuscular training.

Figure 6

Figure 6

Figure 7

Figure 7

Figure 8

Figure 8

The final progressions of the plyometric and movement training targeted toward reduced knee valgus motion should use unanticipated cutting movements during training. Knee abduction moment loads on the knee can double during unanticipated cutting maneuvers similar to those observed during actual sport (3). Figure 9 shows an athlete who demonstrated excessive dynamic knee valgus positions during agility and unanticipated cutting drills. Extensive verbal and visual feedback (via video tape) can be used to help the athlete with high KAM arising from inordinate knee valgus motion to correct unsafe biomechanics during these movements (44). Neuromuscular training that incorporates techniques to focus on unanticipated cutting maneuvers effectively reduces KAM in female athletes (41). By teaching the athlete to use movement techniques that produce lower KAM during unanticipated cutting maneuvers that inherently invoke high KAM loads on the joint, they may ultimately transfer the mechanics that reduce the risk of injury onto the field during competitive play (3,17).

Figure 9

Figure 9

Back to Top | Article Outline

Reduced Knee Flexion Contributes to High Knee Abduction Moments

In addition to limiting lower extremity frontal plane motion and KAM, a reduction in sports-related ACL injury rates in women may be achieved via improved sagittal plane biomechanics, especially increasing knee flexion during dynamic activities (4,43). A sagittal position of the knee close to full extension when landing or cutting is commonly observed in video analysis of ACL injuries in female athletes (4,43). In addition, a prospective study has shown that female athletes who subsequently sustained ACL injuries demonstrated significantly less (10.5°) knee flexion during a drop vertical jump than those who did not sustain injury (17). Current reports also indicate that increased sagittal plane moments and decreased knee flexion range of motion influence the propensity to demonstrate high KAM landing mechanics (37,39). Figure 10 provides an example of a representative subject whose knee valgus motion is exacerbated with small knee flexion ROM during the drop vertical jump that contributed to her high likelihood for a high KAM landing. In evaluation of her completed algorithm, the representative subject (tibia length: 42 cm; knee valgus motion: 4.6 cm; knee flexion ROM: 38.3°; mass: 65.3 kg; QuadHam ratio: 1.51) would have a 94% (123.0 points) chance to demonstrate high KAM during the drop vertical jump. Her actual KAM measure for the presented drop vertical jump that was quantified simultaneously with 3D motion analysis was 77.6 N m of KAM which is over 3 times the cut-score of 21.7 used to classify athletes as high risk for noncontact ACL injury (37,38). Thus, the current subject's data indicate that control of knee valgus motion is critical when landing with knee angles close to full extension. Beyond her high KAM, the present landing mechanics may amplify direct ACL loads from forceful quadriceps contraction with her dynamic valgus positioning at small knee flexion angle (27).

Figure 10

Figure 10

Back to Top | Article Outline

Targeted Training for Reduced Knee Flexion Range of Motion Risk Factor

To decrease the tendency toward utilization of small knee flexion ROM during dynamic sports-related tasks, exercises are employed that emphasize early (pre and initial contact) co-contraction of the knee flexor muscles (37,39). If the hamstrings are adequately activated at the proper time, they can increase knee flexion and decrease ACL loading. However, at low knee flexion angles, the hamstrings have little ability to protect against ACL loads (26,28,45). In addition, at angles greater than 45°, quadriceps recruitment can aid in the resistance of anterior tibial translation, providing an agonistic role to protect the ACL (1,8). Therefore, it is important to teach increased knee flexion ROM to reduce KAM and allow the hamstrings to provide protective muscular force to the ACL (35). To influence deep knee flexion, the box butt touch exercise (Figure 11) is used, in which a box is placed behind the athlete and the athlete starts with feet shoulder width apart and performs a squat down to the height of the box, softly touches the box without resting, and then ascends up to initial starting position. Once appropriate trunk and lower extremity control is achieved during the pseudodynamic maneuver, the athlete can progress to the box drop off-deep hold series of exercises (Figure 12).

Figure 11

Figure 11

Figure 12

Figure 12

Squat jumps (Figure 13) can also be used to target improved knee flexion range of motion because their execution requires the athlete to go into deep knee flexion angles, past 90°. In addition, the squat jump can help teach the athlete to initiate landing in a more flexed knee position, which decreases the quadriceps ability to load the ACL and improves the ability of the hamstrings to offset anterior shear forces because of their line of pull (1,8,26,28,45). Utilization of the described exercises to influence increased knee flexion ROM during landing may reduce the potential high KAM and ACL injury stemming from an overextended landing position (4,43).

Figure 13

Figure 13

Back to Top | Article Outline

Side-to-Side Differences in Landing Mechanics Contribute to Differential High Knee Abduction Moments

Side-to-side imbalances in muscular strength, flexibility, and coordination have been shown to be important predictors of increased injury risk (2,17,24). In addition, female athletes may generate lower hamstrings torques on the nondominant than in the dominant leg (20). Specific to ACL injury risk prediction, adolescent female athletes demonstrate significant side-to-side differences in maximum knee valgus angle compared with men during a box drop vertical jump (10). Half of the parameters used in a highly sensitive and specific regression model to predict increased KAM and ACL injury were side-to-side differences in lower extremity kinematics and kinetics (17). Leg-to-leg differences in KAM were also observed in injured, but not uninjured, women. Side-to-side KAM difference was 6.4 times greater in ACL-injured vs. the uninjured women (17). Female athletes tend to demonstrate side-to-side differences that are visibly evident for maximum knee valgus angle during a box drop vertical jump (Figure 14) (10). Overreliance on a single limb can put greater stress and torques on the knee increasing KAM and, in turn, increasing the risk for noncontact ACL injury on that limb (17).

Figure 14

Figure 14

Figure 15 presents a subject with side-to-side differences in landing biomechanics that the ACL injury prediction algorithm also delineates with side-side differences in prediction of risk for high KAM. Based on measurements in her left leg (Figures 15A,B), this subject would have a 35% (78.5 points; Figure 15E) chance to demonstrate high KAM during the drop vertical jump. Her actual KAM measure for the presented drop vertical jump that was quantified simultaneously with 3D motion analysis was 15.4 N m of knee abduction load on her left leg. However, when using the frontal plane motion from the right leg (Figure 15C,D; 13 cm) in the ACL injury risk prediction algorithm, this subject would have a 96% (128.5 points; Figure 15F) chance to demonstrate high KAM during the drop vertical jump. Her actual KAM measure for the presented drop vertical jump that was quantified simultaneously with 3D motion analysis was 29.2 N m of knee abduction load on her right leg. The provided example of this important observation indicates that the ACL injury prediction algorithm is both sensitive and specific to high KAM, even between limbs in a single subject. Accordingly, clinicians should evaluate side-to-side differences in frontal plane mechanics, and the largest knee valgus motion measurements should be employed to maximize the utility of the proposed ACL injury prediction algorithm (17).

Figure 15

Figure 15

Back to Top | Article Outline

Targeted Training for Side-to-Side Difference Risk Factor

Before teaching dynamic movements focused to correct side-to-side imbalances, athletes should first be taught proper athletic position (Figure 16). The athletic position is a functionally stable position with the knees comfortably flexed, shoulders back, eyes up, feet approximately shoulder-width apart, and the body mass balanced evenly over the balls of the both feet. The athletic “ready position” should be the starting and finishing position for several of the training exercises and is focused to teach symmetry in weight distribution between limbs.

Figure 16

Figure 16

The majority of the initial dynamic movement exercises should involve both legs to safely introduce the athlete to plyometric training movements such as those portrayed in Figures 6 and 7 (6). Early training emphasis should be on balanced athletic positioning (Figure 16) that can help create dynamic control of the athlete's center of mass (33,41,42). Once bilateral symmetry is gained during a bipedal task, clinicians can incorporate single limb balance exercises on unstable surfaces (Figure 17).

Figure 17

Figure 17

During the tuck jump exercise, some female athletes may unload their weaker limb (unloaded limb positioned anterior), as is visually evidenced by uneven foot placement at landing (Figure 18A) and asymmetrical limb alignment during flight of jumping (Figure 18B). To target limb-to-limb deficits, the single leg hop and hold (Figure 19) and single leg X hop progression are used (Figure 20). Single limb hopping maneuvers should be initiated with submaximal effort during the single limb hop and hold exercise so that they can experience the level of difficulty. Once the initial landing dissipation strategy is mastered, the distance of the broad hop can be progressively increased as the athlete improves her ability to stick and hold the final landing. The athlete should be instructed to keep her visual focus away from her feet to help prevent too much forward lean at the waist. Clinicians should provide real-time feedback to encourage the athlete to gain equal lower extremity biomechanics and ability on both limbs during these exercises to facilitate side-to-side sports-related symmetry.

Figure 18

Figure 18

Figure 19

Figure 19

Figure 20

Figure 20

Back to Top | Article Outline

Targeted Training for Increased Quadriceps to Hamstring Ratio Risk Factor

Female athletes who demonstrate the combination of decreased relative hamstrings and high relative quadriceps strength may be at increased risk for noncontact ACL injury (31). Targeted neuromuscular interventions that increase relative hamstrings muscle strength and recruitment may decrease injury risk and potentially increase performance in this population (20,31,33,40,41). Adequate co-contraction of the knee flexors may help balance active contraction of the quadriceps that can compress the joint and assist in the control of high KAM during deceleration tasks. To influence increased relative hamstring strength and recruitment, the single leg Romanian Deadlift progression (Figure 21) can be employed. In addition, the pelvic bridge progression (Figure 22) can influence synergistic posterior chain recruitment, especially from the gluteal musculature (maximus and medius) that can improve net hamstring recruitment and torque at the knee. End-stage exercise progression targeted to improved hamstring strength and recruitment should incorporate exercises from the Russian hamstring progression (Figure 23). These exercises may help improve dynamic knee stability during multi-planar movements via increased hamstring strength and coactivation to resist anterior tibial translation and KAM, which result from unequal opposition to forceful quadriceps contraction (9,20,46). As indicated in the current ACL injury risk prediction algorithm, increased relative strength and recruitment of the posterior chain musculature achieved through targeted neuromuscular training exercise may provide a mechanism for successful reduction of high KAM and noncontact ACL injury risk in female athletes (15,17,20,31,33,40).

Figure 21

Figure 21

Figure 22

Figure 22

Figure 23

Figure 23

Back to Top | Article Outline

Practical Applications

To achieve the objective of reducing noncontact injury risk in female athletes, identification and treatment of those who preferentially demonstrate high KAM landing mechanics appear salient. The provided ACL injury risk prediction algorithm provides an integrative approach to guide targeted dynamic neuromuscular analysis training to specifically address and correct neuromuscular deficits that increase high KAM risk within the algorithm. Targeted correction of high KAM risk factors is important for both optimal biomechanics of athletic movements that maximize sport performance and ultimately the reduction of knee injury incidence in female athletes.

Although this article attempts to delineate proper training techniques that are specifically targeted for biomechanical deficits, the practitioner should understand the synergistic nature of the suggested training modes. Several of the individual components of the aforementioned training may have positive effects outside of the discrete changes that are referenced. Although it is beyond the scope of this paper to elucidate all the effects of the recommended training, we propose a method by which the practitioner can improve and refine a training program intended to target identified deficits in individual athletes. As coaches and practitioners apply their own knowledge and understanding of how each exercise works both discretely and in concord with the proposed integrative approach, we would expect even further positive adaptive responses from their athletes. Future research focused to determine the injury risk reductions using the proposed methods is warranted.

Back to Top | Article Outline

Acknowledgments

The authors would like to acknowledge funding support from National Institutes of Health/NIAMS Grants R01-AR049735, R01-AR05563 and R01-AR056259. The authors would like to thank Boone County Kentucky, School District, especially the School Superintendent Randy Poe, for participation in this study. We would also like to thank Mike Blevins, Ed Massey, Dr. Brian Blavatt, and the athletes of Boone County public school district for their participation in this study. The authors would also like to acknowledge the Cincinnati Children's Sports Medicine Biodynamics Center Team who have contributed intellectually and physically to the presented research outcomes.

Back to Top | Article Outline

References

1. Andriacchi TP, Andersson GBJ, Fermier RW, Stern D, Galante JO. Study of lower-limb mechanics during stair-climbing. J Bone Joint Surg Am 62A: 749–757, 1980.
2. Baumhauer J, Alosa D, Renstrom A, Trevino S, Beynnon B. A prospective study of ankle injury risk factors. Am J Sports Med 23: 564–570, 1995.
3. Besier TF, Lloyd DG, Ackland TR, Cochrane JL. Anticipatory effects on knee joint loading during running and cutting maneuvers. Med Sci Sports Exerc 33: 1176–1181, 2001.
4. Boden BP, Dean GS, Feagin JA, Garrett WE. Mechanisms of anterior cruciate ligament injury. Orthopedics 23: 573–578, 2000.
5. Brent JL, Myer GD, Ford KR, Hewett TE. A longitudinal examination of hip abduction strength in adolescent males and females. Med Sci Sport Exerc 40: 731, 2008.
6. Chmielewski TL, Myer GD, Kauffman D, Tillman S. Plyometric exercise in the rehabilitation of athletes: Physiological responses and clinical application. J Orthop Sports Phys Ther 36: 308–319, 2006.
7. Claiborne TL, Armstrong CW, Gandhi V, Pincivero DM. Relationship between hip and knee strength and knee valgus during a single leg squat. J Appl Biomech 22: 41–50, 2006.
8. Daniel DM, Malcom LL, Losse G, Stone ML, Sachs R, Burks R. Instrumented measurement of anterior laxity of the knee. J Bone Joint Surg Am 67A: 720–726, 1985.
9. Draganich LF, Vahey JW. An in vitro study of anterior cruciate ligament strain induced by quadriceps and hamstrings forces. J Orthop Res 8: 57–63, 1990.
10. Ford KR, Myer GD, Hewett TE. Valgus knee motion during landing in high school female and male basketball players. Med Sci Sports Exerc 35: 1745–1750, 2003.
11. Ford KR, Myer GD, Hewett TE. Increased trunk motion in female athletes compared to males during single leg landing. Med Sci Sports Exerc 39: S70, 2007.
12. Ford KR, Myer GD, Toms HE, Hewett TE. Gender differences in the kinematics of unanticipated cutting in young athletes. Med Sci Sports 37: 124–129, 2005.
13. Hewett TE, Biro FM, McLean SG, Van den Bogert AJ. Identifying Female Athletes at High Risk for ACL Injury. Cincinnati Children's Hospital, National Institutes of Health (Bethesda, MD), 2003.
14. Hewett TE, Ford KR, Myer GD. Anterior cruciate ligament injuries in female athletes: Part 2, a meta-analysis of neuromuscular interventions aimed at injury prevention. Am J Sports Med 34: 490–498, 2006.
15. 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 27: 699–706, 1999.
16. Hewett TE, Myer GD, Ford KR. Decrease in neuromuscular control about the knee with maturation in female athletes. J Bone Joint Surg Am 86-A: 1601–1608, 2004.
17. Hewett TE, Myer GD, Ford KR, Heidt RS Jr, Colosimo AJ, McLean SG, van den Bogert AJ, Paterno MV, Succop P. 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 33: 492–501, 2005.
18. Hewett TE, Myer GD, Ford KR, Slauterbeck JR. Preparticipation physical exam using a box drop vertical jump test in young athletes: The effects of puberty and sex. Clin J Sport Med 16: 298–304, 2006.
19. Hewett TE, Paterno MV, Myer GD. Strategies for enhancing proprioception and neuromuscular control of the knee. Clin Orthop Relat Res 402: 76–94, 2002.
20. Hewett TE, Stroupe AL, Nance TA, Noyes FR. Plyometric training in female athletes. Decreased impact forces and increased hamstring torques. Am J Sports Med 24: 765–773, 1996.
21. Hodges PW, Richardson CA. Contraction of the abdominal muscles associated with movement of the lower limb. Phys Ther 77: 132–142; discussion 142–144, 1997.
22. Hodges PW, Richardson CA. Feedforward contraction of transversus abdominis is not influenced by the direction of arm movement. Exp Brain Res 114: 362–370, 1997.
23. Ireland ML. The female ACL: Why is it more prone to injury? Orthop Clin North Am 33: 637–651, 2002.
24. Knapik JJ, Bauman CL, Jones BH, Harris JM, Vaughan L. Preseason strength and flexibility imbalances associated with athletic injuries in female collegiate athletes. Am J Sports Med 19: 76–81, 1991.
25. Krosshaug T, Nakamae A, Boden BP, Engebretsen L, Smith G, Slauterbeck JR, Hewett TE, Bahr R. Mechanisms of anterior cruciate ligament injury in basketball: Video analysis of 39 cases. Am J Sports Med 35: 359–367, 2007.
26. Lloyd DG, Buchanan TS. Strategies of muscular support of varus and valgus isometric loads at the human knee. J Biomech 34: 1257–1267. 2001.
27. Markolf KL, Burchfield DM, Shapiro MM, Shepard MF, Finerman GA, Slauterbeck JL. Combined knee loading states that generate high anterior cruciate ligament forces. J Orthop Res 13: 930–935, 1995.
28. More RC, Karras BT, Neiman F, Fritschy D, Woo SL-Y, Daniel DM. Hamstrings-an anterior cruciate ligament protagonist: An in vitro study. Am J Sports Med 21: 231–237, 1993.
29. 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 42: 614–619, 2008.
30. Myer GD, Chu DA, Brent JL, Hewett TE. Trunk and hip control neuromuscular training for the prevention of knee joint injury. Clin Sports Med 27: 425–448, ix, 2008.
31. Myer GD, Ford KR, Barber Foss KD, Liu C, Nick TG, Hewett TE. The relationship of hamstrings and quadriceps strength to anterior cruciate ligament injury in female athletes. Clin J Sport Med 19: 3–8, 2009.
32. Myer GD, Ford KR, Brent JL, Hewett TE. The effects of plyometric versus dynamic balance training on landing force and center of pressure stabilization in female athletes. Br J Sports Med 39: 397, 2005.
33. Myer GD, Ford KR, Brent JL, Hewett TE. The effects of plyometric versus dynamic balance training on power, balance and landing force in female athletes. J Strength Cond Res 20: 345–353, 2006.
34. 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 Musculoskelet Disord 8: 1–39, 2007.
35. Myer GD, Ford KR, Hewett TE. Rationale and clinical techniques for anterior cruciate ligament injury prevention among female athletes. J Athl Train 39: 352–364, 2004.
36. 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 45:238–244, 2011.
37. Myer GD, Ford KR, Khoury J, Succop P, Hewett TE. Clinical correlates to laboratory measures for use in non-contact anterior cruciate ligament injury risk prediction algorithm. Clin Biomech (Bristol, Avon) 25: 693–699, 2010.
38. Myer GD, Ford KR, Khoury J, Succop P, Hewett TE. Development and validation of a clinic-based prediction tool to identify female athletes at high risk for anterior cruciate ligament injury. Am J Sports Med 38: 2025–2033, 2010.
39. Myer GD, Ford KR, Khoury J, Succop P, Hewett TE. Biomechanics laboratory-based prediction algorithm to identify female athletes with high knee loads that increase risk of ACL injury. Br J Sports Med 45: 245–252, 2011.
40. 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 34: 490–498, 2006.
41. Myer GD, Ford KR, Palumbo JP, Hewett TE. Neuromuscular training improves performance and lower-extremity biomechanics in female athletes. J Strength Cond Res 19: 51–60, 2005.
42. Myklebust G, Engebretsen L, Braekken IH, Skjolberg A, Olsen OE, Bahr R. Prevention of anterior cruciate ligament injuries in female team handball players: A prospective intervention study over three seasons. Clin J Sport Med 13: 71–78, 2003.
43. Olsen OE, Myklebust G, Engebretsen L, Bahr R. Injury mechanisms for anterior cruciate ligament injuries in team handball: A systematic video analysis. Am J Sports Med 32: 1002–1012, 2004.
44. Onate JA, Guskiewicz KM, Marshall SW, Giuliani C, Yu B, Garrett WE. Instruction of jump-landing technique using videotape feedback: altering lower extremity motion patterns. Am J Sports Med 33: 831–842, 2005.
45. Pandy MG, Shelburne KB. Dependence of cruciate-ligament loading on muscle forces and external load. J Biomech 30: 1015–1024, 1997.
46. White KK, Lee SS, Cutuk A, Hargens AR, Pedowitz RA. EMG power spectra of intercollegiate athletes and anterior cruciate ligament injury risk in females. Med Sci Sports Exerc 35: 371–376, 2003.
47. Wilson JD, Dougherty CP, Ireland ML, Davis IM. Core stability and its relationship to lower extremity function and injury. J Am Acad Orthop Surg 13: 316–325, 2005.
48. Winter DA. Biomechanics and Motor Control of Human Movement. New York, NY: John Wiley & Sons, Inc., 2005.
49. Zatsiorsky VM. Science and Practice of Strength Training. Champaign, IL: Human Kinetics, 1995.
50. Zazulak BT, Hewett TE, Reeves NP, Goldberg B, Cholewicki J. The effects of core proprioception on knee injury: A prospective biomechanical-epidemiological study. Am J Sports Med 35: 368–373, 2007.
Keywords:

prevention of anterior cruciate ligament injury; knee; drop vertical jump; landing mechanics; young athletes; biomechanics

© 2012 National Strength and Conditioning Association