Strength & Conditioning Journal:
Real-Time Assessment and Neuromuscular Training Feedback Techniques to Prevent Anterior Cruciate Ligament Injury in Female Athletes
Myer, Gregory D PhD, CSCS1,2,3,4; Brent, Jensen L CSCS1; Ford, Kevin R PhD, FACSM1,2; Hewett, Timothy E PhD, FACSM1,2,4
1Cincinnati Children's Hospital Medical Center, Division of Sports Medicine, Cincinnati, Ohio; 2University of Cincinnati, Department of Pediatrics, College of Medicine, Cincinnati, Ohio; 3Rocky Mountain University of Health Professions, Departments of Athletic Training, Sports Orthopaedics, and Pediatric Science, Provo, Utah; and 4The Ohio State University Sports Medicine Center, Departments of Family Medicine, Biomedical Engineering, Cell Biology and Physiology, The Ohio State University, Columbus, Ohio
Gregory D. Myer is the co-director of research for the Division of Sports Medicine at Cincinnati Children's Hospital Medical Center and maintains his primary faculty appointment in the department of Pediatrics in the College of Medicine at the University of Cincinnati.
FIGURE. Caption not ...Image Tools
Jensen L. Brent is a biomechanist in the Sports Medicine Biodynamics, Center Human Performance Laboratory at the Cincinnati Children's Hospital Medical Center and is the head Strength and Conditioning Coach for the Cincinnati Kelts Rugby Football Club.
FIGURE. Caption not ...Image Tools
Kevin R. Ford is co-director of the Human Performance Laboratory and holds a primary appointment as an assistant professor within Sports Medicine at the Cincinnati Children's Hospital Medical Center.
FIGURE. Caption not ...Image Tools
Timothy E. Hewett is professor and director of Research at the Ohio State University Sports Medicine and professor of Pediatrics and director of the Sports Medicine Biodynamics Center at the Cincinnati Children's Hospital Medical Center.
FIGURE. Caption not ...Image Tools
SOME ATHLETES MAY BE MORE SUSCEPTIBLE TO AT-RISK KNEE POSITIONS DURING SPORTS ACTIVITIES, BUT THE UNDERLYING CAUSES ARE NOT CLEARLY DEFINED. THIS ARTICLE SYNTHESIZES IN VIVO, IN VITRO, AND IN SILICO (COMPUTER-SIMULATED) DATA TO DELINEATE LIKELY RISK FACTORS TO THE MECHANISM(S) OF NONCONTACT ANTERIOR CRUCIATE LIGAMENT (ACL) INJURIES. FROM THESE IDENTIFIED RISK FACTORS, WE WILL DISCUSS NEWLY DEVELOPED REAL-TIME SCREENING TECHNIQUES THAT CAN BE USED IN TRAINING SESSIONS TO IDENTIFY MODIFIABLE RISK FACTORS. TECHNIQUES PROVIDED WILL TARGET AND CORRECT ALTERED MECHANICS THAT MAY REDUCE OR ELIMINATE RISK FACTORS AND AID IN THE PREVENTION OF NONCONTACT ACL INJURIES IN HIGH-RISK ATHLETES.
DETERMINATION OF THE POTENTIAL RISK FACTORS FOR ANTERIOR CRUCIATE LIGAMENT INJURY
Most anterior cruciate ligament (ACL) injuries in female sports occur during a noncontact episode, typically during deceleration, lateral pivoting, or landing tasks that are associated with high loads on the knee joint (Figure 1a) (7,58). Sports maneuvers may lead to high external knee loads in both sexes. Why these tasks result in greater incidence of ACL injury in women has, until recently, remained unclear. Three major etiological theories have been proposed to explain the sex disparity observed in ACL injury rates: anatomical, hormonal, and neuromuscular (18, 46). A number of studies of ACL injury risk factors have focused on anatomical or anthropometric measures, such as thigh length (5), joint laxity (55), and femoral notch width (65). However, static anatomical measures, such as Q-angle, often do not appear to correlate with the mechanics associated with the dynamic injury mechanism (47,50). Hormonal factors, particularly those associated with the follicular and ovulatory phases of the menstrual cycle, have also been linked to ACL injury risk (1,19,26,67,74,75,78). However, the precise means by which they may contribute to ACL injury risk and the extent to which these hormonal factors can be modified remains unclear (10).
DETERMINATION OF POTENTIAL MODIFIABLE RISK FACTORS FOR ANTERIOR CRUCIATE LIGAMENT INJURY IN WOMEN
The study of modifiable risk factors has focused on the working hypothesis that ACL injury risks are related to measurable deficits in neuromuscular control in female athletes (11,13,24). Neuromuscular control deficits are defined as muscle strength, power, or activation patterns that lead to increased knee joint and ACL loads (46). Female athletes demonstrate neuromuscular control deficits that increase lower extremity joint loads during sports activities (25).
One neuromuscular deficit, which is operationally termed “ligament dominance,” can be defined as an imbalance between the neuromuscular and ligamentous control of dynamic knee joint stability (46). This control imbalance is demonstrated by an inability to control lower extremity frontal plane motion during landing and cutting. A second modifiable neuromuscular control deficit often observed in female athletes, which is termed “quadriceps dominance,” can be defined as an imbalance between knee extensor and flexor strength, recruitment, and coordination (46). A third neuromuscular control deficit, often observed in female athletes, is “leg dominance,” which can be defined as an imbalance between the 2 lower extremities in strength, coordination, and control (46). The fourth and final neuromuscular control deficit, often observed in female athletes, is “trunk dominance 'core' dysfunction,” which can be defined as an imbalance between the inertial demands of the trunk and control and coordination to resist it.
During landing, pivoting, or deceleration, the motion of the female athlete's trunk is often excessive and directed, to a greater extent, by the body segment's inertia, than by the athlete's core muscle function. This decreased core control and ability to dissipate force result in excessive trunk motion, especially in the frontal plane, as well as high ground reaction forces and knee joint abduction torques (knee load) (22).
These 4 potential neuromuscular control deficits, operationally defined above, are postulated to be important contributors to knee and ACL injury incidence in female athletes (24,43,62,77). ACL injury likely occurs under conditions of high dynamic loading of the knee joint, when active muscular restraints do not compensate for and dampen joint loads adequately (Figure 1a) (6). Decreased neuromuscular control of the joint may place stress on the passive ligamentous structures that may exceed their strength limit and potentially lead to mechanical failure (34,39). External abduction moment (LOAD) is the laboratory-based kinetic measure that is predictive of future injury and likely contributes to the stress on an ACL during injury (Figure 1c and 1d) (24,64). The term “valgus” is used to describe the video-evidenced mechanism of injury (Figure 1a) and is visually associated with a knock-kneed position during dynamic tasks (Figure 1b).
Neuromuscular control of high-load movements is required to maintain dynamic knee stability during landing and pivoting (3,34). Deficits in the active neuromuscular control system likely affect dynamic knee stability and possibly increase the risk of an ACL injury. Methods to identify neuromuscular control deficits during tasks related to ACL injury mechanisms, such as landing, cutting, and decelerating, may offer the greatest potential for the development and application of neuromuscular screening interventions targeted to high-risk populations (7,58).
The authors currently use a 2-pronged approach to the prevention of ACL injuries in young female athletes. We incorporate both risk screening and dynamic neuromuscular training as dual interventions to prevent these devastating injuries. Myer et al. (45) demonstrated that those athletes who subject their knees to higher joint loads benefit to a greater extent from our dynamic neuromuscular training protocols than those who do not. Therefore, we first screen athletes for high-risk status and then intervene with neuromuscular training designed to ameliorate those specific neuromuscular imbalances that those specific athletes demonstrate. This dual approach likely results in greater efficacy of both avenues of intervention.
POTENTIAL MODIFIABLE RISK FACTORS ASSOCIATED WITH DYNAMIC MOVEMENTS
Hewett et al. (25) tested the hypothesis that insufficient neuromuscular control of lower limb biomechanics, particularly the knee joint, leads to high-risk patterns in female athletes during execution of common, albeit potentially hazardous, movements. The results of this study demonstrated that peak landing forces were significantly predicted by knee abduction torques (LOAD) at the knee in women, that women developed decreased relative knee flexor torque during landing compared with men, and that women had greater side-to-side differences in normalized hamstring peak torque compared with men.
Ford et al. (11) reported similar sex differences during the performance of a drop vertical jump (Figure 1b). This study determined that female athletes landed with a greater maximum valgus knee angle and greater total valgus knee motion than male athletes (Figure 1c). Female athletes also demonstrated significant differences between their dominant and nondominant side in maximum valgus knee angle. These differences in valgus measures (ligament dominance) and limb-to-limb asymmetries (leg dominance) reflect neuromuscular deficits that may be indicative of decreased dynamic knee joint control in female athletes (11).
Subsequent studies systematically evaluated neuromuscular control at the hip in female athletes (21,79). The evidence from multiple, potentially high-risk, movements indicated that variables at the hip contributed to dynamic valgus. In addition, electromyographic (EMG) data demonstrated women to men differences in firing patterns of the hip musculature (79). The purpose of these studies was to identify sex differences in hip motion, kinetics, and muscle firing patterns during single leg agility and landing maneuvers. It was hypothesized that female athletes would display increased hip adduction angles and external adduction moments during this multicomponent dynamic maneuver that may contribute to lower extremity valgus.
These results demonstrated that female athletes had greater hip adduction angles and torques than men during multidirectional single-leg landing. These differences were limited to the frontal plane and were not observed in the sagittal plane. EMG patterns showed increased quadriceps and decreased gluteal firing in women (79). Another study examined sex differences during single leg landings from either a medial or a lateral direction (14). In addition to greater knee abduction angles, female athletes also exhibited increased hip frontal plane excursion compared with male athletes during both types of landings (14). The increased hip adduction (hip varus) motion seen in the frontal plane during athletic activities likely contributes to the dynamic valgus knee position that may place the athlete at an increased risk of ACL injury (Figure 1d) (14,21).
PROLIFERATION OF ANTERIOR CRUCIATE LIGAMENT INJURY RISK FACTORS DURING ADOLESCENT GROWTH
Men demonstrate power, strength, and coordination increases with increasing chronological age that correlates to maturational stage, whereas women show significantly smaller changes throughout puberty (4,37). Correlations among height, weight, and neuromuscular performance observed in men are absent in pubescent girls. For example, vertical jump height (a measure of whole-body power) increases steadily in men during puberty but not in women (30,37,63). Musculoskeletal growth during puberty, in the absence of corresponding neuromuscular adaptation, may facilitate the development of certain intrinsic ACL injury risk factors (16,23). These intrinsic risk factors, if not addressed at the proper time, may continue through adolescence into maturity and predispose athletes to ACL injuries.
Neuromuscular training programs have been successful in reducing measures of knee abduction (25,45,53,54). A comprehensive review of ACL injury prevention programs indicates that neuromuscular training appears to decrease ACL injury rates in female athletes (20). These programs typically incorporate plyometric training and technique analysis. Ongoing studies show that knee abduction measures may be useful for the identification of those women who are at an increased risk of ACL injury (24). In a recent longitudinal study, Ford et al. (16) identified that during a landing task, pubertal girls have an increased change in knee abduction motion during a year of adolescent growth. In addition, important reported risk factors of knee abduction motion and torque were significantly greater across consecutive years in young postpubertal female athletes compared with men (16).
Figure 2a-c presents an example athlete whose combination of decreased tibia length and mass before her rapid growth spurt diminish risk to demonstrate high knee LOAD landing mechanics. Conversely, Figure 2d-f provides an example of an athlete whose combination of increased tibia length and mass associated with her rapid growth contribute to her increased risk to demonstrate high knee LOAD landing mechanics when using the clinic-based ACL injury risk prediction algorithm.
DETERMINING THE TIMING OF ATHLETE-SPECIFIC MATURATION
Somatic maturational assessments are traditionally the best way to identify rapid adolescent growth (37). The Khamis-Roche method of estimated adult stature was developed from the Fels Longitudinal Study that collected data on families residing in southwestern Ohio (31). The subject's stature, mass, midparental stature, and age were used to develop regression equations for boys and girls. This equation can be used to estimate the current percentage of height compared with their final estimated height.
Early puberty may be an appropriate time during maturation to institute intervention programs that aim to control knee abduction motion and torque as well as to induce a neuromuscular spurt through targeted neuromuscular training. The pubertal female group in our recent study was approximately 12 years old and estimated to be at 88% of adult stature (16). Therefore, training programs may be most beneficial before timing when girls are growing the fastest during adolescence (peak height velocity), which is near 92% of adult stature.
SCREENING FOR HIGH-RISK ATHLETES
Laboratory-based screening tools demonstrate that altered neuromuscular strategies or decreased neuromuscular control during the execution of sports movements, as evidenced by abnormal lower limb joint mechanics (motions and loads), may underlie the increased risk of ACL injury in female athletes (9,11,15,24,40,53). Prospective measures of external knee abduction moment during landing predict noncontact ACL injury risk in young female athletes (Figure 1a-d) (24).
Calculation of knee injury risk factors, such as knee abduction moment through inverse dynamics, requires complex laboratory-based 3-dimensional kinematic and kinetic measurement techniques. However, a recent report has isolated biomechanical measures that contribute to nearly 80% of the measured variance in knee abduction moment during landing (50). These biomechanical predictors of knee abduction moment, which include increased knee abduction angle, increased relative quadriceps recruitment, and decreased knee flexion range of motion, concomitant with the increased tibia length and mass normalized to body height that accompanies growth, are also measurements that have been related to the increased risk of ACL injury in previous prospective and retrospective epidemiological reports (7,24,59,69). Unfortunately, expensive biomechanical laboratories, with the costly and labor-intensive measurement tools to test individual athletes, are required for these measurements. This restricts the potential to perform athletes' risk assessments on a large scale, in particular limiting the potential to target high injury risk athletes with the appropriate intervention strategies.
To achieve the objective of reduction of noncontact ACL injury risk in female athletes, identification of those athletes who preferentially demonstrate high knee abduction moment landing mechanics appears salient. Therefore, a field-based assessment algorithm was systematically developed and validated that aimed to improve the potential to identify and target injury prevention training to female athletes with an increased knee abduction moment (49,51,52). The validated field-based assessment algorithm delineated 5 biomechanical factors that could be captured in clinic and field settings, which combined to identify high knee abduction moment during landing with high accuracy (49,51,52). This clinic-based assessment tool can be administered in a clinic or field setting, is validated by the highly accurate laboratory-based assessment, and can help to facilitate screening for ACL injury risk on a more widespread basis (Figure 2).
Although the clinic-based assessment tool is valuable to identify athletes who should be targeted with neuromuscular training, it was imperative to develop a “coach friendly” plyometric exercise landing assessment and technique training tool to target high-risk ACL injury mechanisms. For this reason, the tuck jump assessment tool was developed to aid coach's decision making during training sessions (Figure 3) (48).
REAL-TIME FIELD ASSESSMENT AND TRAINING TOOL
The tuck jump exercise may be useful to the strength coach for the identification of lower extremity technical flaws during a plyometric activity (Figure 4) (46). The tuck jump requires a high effort level from the athlete. Because of this, the strength coach may readily identify potential deficits (Figure 3), especially during the first few repetitions. In addition, the tuck jump exercise may be used to assess improvement in lower extremity biomechanics as the athlete progresses through their training (46,48).
Figure 3 provides the coach friendly landing technique assessment tool that strength coaches may use to monitor an athlete's performance of the tuck jump before, during, and after training. Specifically, the athlete performs repeated tuck jumps for 10 seconds, which allows the coach to visually grade the outlined criteria. To further improve accuracy of the assessment, a standard 2-dimensional camera in the frontal and sagittal planes may be used to assist the coach. The athlete's techniques (Figure 3) are subjectively rated as either having an apparent deficit (checked) or not. The deficits are then tallied for the final assessment score. Indicators of flawed techniques should be noted for each athlete and should be the focus of feedback during subsequent training sessions. The athlete's baseline performance can be compared with repeated assessments performed at the midpoint and conclusion of training protocols, to objectively track improvement with jumping and landing technique (Figure 3).
LIGAMENT DOMINANCE CRITERIA OF THE TUCK JUMP ASSESSMENT
For ease of presentation, criteria assessed using the tuck jump tool can be grouped into modifiable risk factor categories (Figure 5). As mentioned previously, ligament dominance is defined as an imbalance between the neuromuscular and ligamentous control of dynamic knee joint stability (46). 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. External abduction moment (LOAD) is the laboratory-based kinetic measure that is predictive of future injury and likely contributes to the stress on an ACL during injury (Figure 1c and 1d) (24,64). The term “valgus” is used to describe the video-evidenced mechanism of injury (Figure 1a) and is visually associated with a knock-kneed position during dynamic tasks (Figure 1b). Figure 6 presents a “ligament dominant” athlete who lacks sufficient frontal plane control of her lower extremity during performance of the tuck jump. Figure 7 presents secondary assessment of ligament dominance with the criteria that assesses if foot placement is maintained shoulder width apart during landing. This athlete's reduced ligament dominant landing mechanics may be driven from the lack of frontal plane control at the hip, which may be improved with targeted training for the trunk and hip (41,42,46,54,61).
QUADRICEPS DOMINANCE CRITERIA OF THE TUCK JUMP ASSESSMENT
A second modifiable neuromuscular control deficit often observed in female athletes, which was termed “quadriceps dominance,” is defined as an imbalance between knee extensor and flexor strength, recruitment, and coordination (46). Landing with the knee at nearly full extension is a commonly associated mechanism of ACL injury (7). Decreased hamstrings strength relative to the quadriceps is implicated as a potential mechanism for increased lower extremity injuries (12,17,32,43,68) and potentially ACL injury risk in female athletes (46). Joint stability through co-contraction of hamstrings and quadriceps may be necessary when the joint experiences high quadriceps activation or when the passive structures are compromised (66,72). Withrow et al. (73) reported that increased hamstrings force during the flexion phase of simulated jump landings greatly decreased relative strain on the ACL.
Another proposed theory related to neuromuscular imbalances and increased ACL injury risk in women is the relatively low knee flexor-to-extensor recruitment that may be reflective of a closed-chain dynamic hamstrings/quadriceps peak torque output (25,38). For example, hamstrings activation can decrease the load on the passive restraints of the knee (36), increase the knee joint compression force, and stabilize the knee from external varus/valgus load (35). Hewett et al. reported that men demonstrated knee flexor moments (measured using inverse dynamics) that were 3-fold higher than those in women during deceleration from a landing (25). This group of women also demonstrated decreased isokinetic hamstrings to quadricep ratio and increased knee abduction (valgus) moments compared with male subjects. The increased knee valgus moment significantly correlated with the peak impact forces during the maneuver and is purported to increase ACL injury risk in female athletes (24,25). Ford et al. (12) reported that women showed an absence of matched increased hamstrings muscle activation relative to quadriceps and overall low amplitude during the landing phase of a jump.
This tendency of female athletes to preferentially activate the quadriceps relative to the hamstrings during high-demand activities may limit their abilities to maintain dynamic knee control during high-risk maneuvers. Athletes who demonstrate quadriceps dominance may increase their risk of ACL injury when they cut and land with low knee flexion angles. An athlete who lands with small knee flexion angles and flat foot position demonstrates characteristics of a quadriceps dominant athlete (Figure 8) and will likely demonstrate excessive landing contact noise. This criterion assessed using the tuck jump tool can be grouped into quadriceps dominance modifiable risk factor category (Figure 5) and if identified with the tuck jump assessment should be targeted with posterior chain and deep knee flexion exercises (43,44,46,53,54).
LEG DOMINANCE/RESIDUAL INJURY DEFICITS CRITERIA OF THE TUCK JUMP ASSESSMENT
A third neuromuscular control deficit, frequently observed in female athletes, is “leg dominance,” which is defined as an imbalance between the 2 lower extremities in strength, coordination, and control (46). Coaches should be cognizant during their risk assessment that side-to-side imbalances in neuromuscular strength, flexibility, and coordination can be important predictors of increased injury risk (2,24,32). Specific to ACL injury risk in female athletes, leg-to-leg differences in dynamic valgus measures were observed in injured but not in uninjured women. Importantly, side-to-side differences in knee LOAD were 6.4× greater in ACL-injured versus the uninjured women. Female athletes tend to demonstrate side-to-side differences in visually evident maximum knee valgus angle during a box drop vertical jump (11). In addition, female athletes often demonstrate leg-to-leg deficits following injury that increase their risk of subsequent injury (60,62). Leg dominance or residual injury deficits may be evident in the grouped tuck jump criteria including thighs not equal side to side during flight (Figure 9), foot placement not parallel (front-to-back) (Figure 10), and foot contact timing not equal (Figure 11). Athletes identified with leg-to-leg deficit should be targeted with integrated training that combines both plyometric and dynamic stabilization to improve leg-to-leg symmetry during dynamic tasks (44,53).
“CORE” DYSFUNCTION DOMINANCE CRITERIA OF THE TUCK JUMP ASSESSMENT
The fourth neuromuscular control deficit, often observed in female athletes, is “core” dysfunction dominance, which may be defined as an imbalance between the inertial demands of the trunk and core control and coordination to resist it. We have developed a concept of trunk and lower extremity function that identifies the body's core as a critical modulator of lower extremity alignments and loads during dynamic tasks (42). The trunk and hip stabilizers may preactivate to counterbalance trunk motion and regulate lower extremity postures (27,28,70). Reduced preactivation of the trunk and hip stabilizers may allow increased lateral trunk positions that can incite knee abduction loads (71).
Decreased “core stability” and muscular synergism of the trunk and hip stabilizers may affect performance in power activities and may increase the incidence of injury secondary to lack of control of the center of mass, especially in female athletes (29,60,62,76). Zazulak et al. reported that factors related to core stability predicted risk of knee injuries in female athletes but not in male athletes (77). Thus, the current evidence indicates that compromised function of the trunk and hip stabilizers, as they relate to core neuromuscular control, may underlie the mechanisms of increased ACL injury risk in female athletes (24,33,58,77).
Core dysfunction imbalance may be evident in the athlete's tuck jump criteria including thighs do not reach parallel (peak of jump) (Figure 12), pause between jumps, and the athlete does not land in the same footprint (Figure 13). Athletes identified with these deficits should be targeted with trunk and hip training to improve core control (41,42).
TECHNIQUE PERFECTION CRITERIA OF THE TUCK JUMP ASSESSMENT
The tuck jump assessment tool can be used to improve high-risk techniques during an exercise that requires a high effort level from the athlete. As suggested earlier, an athlete may place most of his or her cognitive efforts solely on the performance of this difficult jump and demonstrate many technical flaws that are indicative of increased risk for injury. However, if she can improve her neuromuscular control and biomechanics during this difficult jump and maintain control during the entire jump-landing sequence, she may gain dynamic neuromuscular control of the lower extremity and create a learned skill that can be transferred to competitive play (Figure 4). We recommend that athletes who do not improve their scores, or who demonstrate 6 or more flawed techniques, should be targeted for further technique training. These data indicate that the tuck jump assessment is best used for a single coach to reassess athletes to determine changes in technical performance of the tuck jump exercise (56,57).
EFFECT OF TRAINING ON TUCK JUMP ASSESSMENT CRITERIA
Cumulatively, research indicates that ACL intervention training can be used to reduce measured deficits during the tuck jump assessment and may help prevent risk of ACL injury. A pilot study was conducted to test the effects of pre-season and in-season neuromuscular training on the tuck jump assessment scores to determine if both training periods were vital to a training protocol (8). The data from this study indicates that pre-season neuromuscular training should be considered paramount within the yearly strength and conditioning program to help minimize the risk factors associated with ACL injury.
The tuck jump assessment allows a coach or clinician to evaluate an athlete's risk of injury and identify specific deficiencies. By using this test throughout the yearly training cycle, overall progress can be monitored and training can be more thoroughly directed. The results also indicate a potential dose-response to the neuromuscular training targeted to prevent ACL injury training. Preliminary results indicate that additional training throughout the competitive season has added benefits in terms of reducing risk of ACL injury based on improvements to the athlete's tuck jump assessment score. Future research is warranted to determine the relationship of reduced deficits gained from the utilization of the presented techniques with actual reduction of injury in athletes treated with targeted training.
To achieve the objective of reduction of noncontact ACL injury risk in female athletes, identification and treatment of those who preferentially demonstrate “high-risk” landing mechanics appears salient. The provided tuck jump assessment tool provides an integrated approach to guide targeted dynamic neuromuscular analysis training to specifically address and correct neuromuscular deficits that increase high LOAD risk within the algorithm. Theoretically, through identification of female athletes at greater risk for ACL injury, prevention strategies to reduce an ACL injury could be substantially improved. Current nontargeted neuromuscular training programs require application to 89 female athletes to prevent one ACL injury (18).
Female athletes, who demonstrate “high-risk” landing mechanics, are at an increased risk for ACL injury and are more likely to benefit from neuromuscular training targeted to these risk factors (45). Utilization of the developed field-based assessment and training tool may guide the application of appropriate interventions that will have greater potential to reduce their injury risk. Targeted correction of high-risk factors for injury is important for both optimal biomechanics of athletic movements that maximize sport performance and ultimately the reduction of knee ligament injury incidence in female athletes.
The authors thank Boone County Kentucky, School District, especially School Superintendent Randy Poe, for participation in this project. They also thank Mike Blevins, Ed Massey, Dr Brian Blavatt, and the athletes of Boone County public school district for their participation. They also acknowledge the Cincinnati Children's Sports Medicine Biodynamics Center Team who contributed both physically and intellectually to the presented research outcomes.
1. Arendt EA, Bershadsky B, and Agel J. Periodicity of noncontact anterior cruciate ligament injuries during the menstrual cycle. J Gend Specif Med
5: 19-26, 2002.
2. Baumhauer J, Alosa D, Renstrom A, Trevino S, and Beynnon B. A prospective study of ankle injury risk factors. Am J Sport Med
23: 564-570, 1995.
3. Besier TF, Lloyd DG, Cochrane JL, and Ackland TR. External loading of the knee joint during running and cutting maneuvers. Med Sci Sports Exerc
33: 1168-1175, 2001.
4. Beunen G and Malina RM. Growth and physical performance relative to the timing of the adolescent spurt. Exerc Sport Sci Rev
16: 503-540, 1988.
5. Beynnon B, Slauterbeck J, Padua D, and Hewett TE. Update on ACL risk factors and prevention strategies in the female athlete. Presented at: National Athletic Trainers' Association 52nd Annual Meeting and Clinical Symposia;
June: 15-18, 2001, Los Angeles, California.
6. Beynnon BD and Fleming BC. Anterior cruciate ligament strain in-vivo: A review of previous work. J Biomech
31: 519-525, 1998.
7. Boden BP, Dean GS, Feagin JA, and Garrett WE. Mechanisms of anterior cruciate ligament injury. Orthopedics
23: 573-578, 2000.
8. Brent JL, Klugman MA, Myer GD and Hewett TE. The effects of pre-season and in-season neuromuscular training on the tuck jump assessment: A test used to identify risk of acl injury in female athletes. Presented at: National Strength and Conditioning Association Annual Meeting; July 15, 2010; Orlando, FL.
9. Chappell JD, Yu B, Kirkendall DT, and Garrett WE. A comparison of knee kinetics between male and female recreational athletes in stop-jump tasks. Am J Sports Med
30: 261-267, 2002.
10. Chaudhari AM, Lindenfeld TN, Andriacchi TP, Hewett TE, Riccobene J, Myer GD, and Noyes FR. Knee and hip loading patterns at different phases in the menstrual cycle: Implications for the gender difference in anterior cruciate ligament injury rates. Am J Sports Med
35: 793-800, 2007.
11. Ford KR, Myer GD, and Hewett TE. Valgus knee motion during landing in high school female and male basketball players. Med Sci Sports Exerc
35: 1745-1750, 2003.
12. Ford KR, Myer GD, Schmitt LC, Van den Bogert AJ, and Hewett TE. Effect of drop height on lower extremity biomechanical measures in female athletes. Med Sci Sports Exerc
40: S80, 2008.
13. Ford KR, Myer GD, Smith RL, Byrnes RN, Dopirak SE, and Hewett TE. Use of an overhead goal alters vertical jump performance and biomechanics. J Strength Cond Res
19: 394-399, 2005.
14. Ford KR, Myer GD, Smith RL, Vianello RM, Seiwert SL, and Hewett TE. A comparison of dynamic coronal plane excursion between matched male and female athletes when performing single leg landings. Clinic Biomech
21: 33-40, 2006.
15. Ford KR, Myer GD, Toms HE, and Hewett TE. Gender differences in the kinematics of unanticipated cutting in young athletes. Med Sci Sports
37: 124-129, 2005.
16. Ford KR, Shapiro R, Myer GD, van den Bogert AJ, and Hewett TE. Longitudinal sex differences during landing in knee abduction in young athletes. Med Sci Sports Exerc
42: 1923-1931, 2010.
17. Ford KR, van den Bogert AJ, Myer GD, Shapiro R, and Hewett TE. The effects of age and skill level on knee musculature co-contraction during functional activities: A systematic review. Br J Sports Med
42: 561-566, 2008.
18. Grindstaff TL, Hammill RR, Tuzson AE, and Hertel J. Neuromuscular control training programs and noncontact anterior cruciate ligament injury rates in female athletes: A numbers-needed-to-treat analysis. J Athletic Train
41: 450-456, 2006.
19. Hewett TE. Neuromuscular and hormonal factors associated with knee injuries in female athletes: Strategies for intervention. Sports Med
29: 313-327, 2000.
20. Hewett TE, Ford KR, and 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.
21. Hewett TE, Ford KR, Myer GD, Wanstrath K, and Scheper M. Gender differences in hip adduction motion and torque during a single leg agility maneuver. J Orthop Res
24: 416-421, 2006.
22. Hewett TE and Johnson DL. ACL prevention programs: Fact or fiction? Orthopedics
33: 36-39, 2010.
23. Hewett TE, Myer GD, and Ford KR. Decrease in neuromuscular control about the knee with maturation in female athletes. J Bone Joint Surg Am
86-A: 1601-1608, 2004.
24. Hewett TE, Myer GD, Ford KR, Heidt RS Jr, Colosimo AJ, McLean SG, van den Bogert AJ, Paterno MV, and 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.
25. Hewett TE, Stroupe AL, Nance TA, and Noyes FR. Plyometric training in female athletes. Decreased impact forces and increased hamstring torques. Am J Sports Med
24: 765-773, 1996.
26. Hewett TE, Zazulak BT, and Myer GD. Effects of the menstrual cycle on anterior cruciate ligament injury risk: A systematic review. Am J Sports Med
35: 659-668, 2007.
27. Hodges PW and Richardson CA. Contraction of the abdominal muscles associated with movement of the lower limb. Phys Ther
77: 132-142; discussion 142-144, 1997.
28. Hodges PW and Richardson CA. Feedforward contraction of transversus abdominis is not influenced by the direction of arm movement. Exp Brain Res
114: 362-370, 1997.
29. Ireland ML. The female ACL: Why is it more prone to injury? Orthop Clin North Am
33: 637-651, 2002.
30. Kellis E, Tsitskaris GK, Nikopoulou MD, and Moiusikou KC. The evaluation of jumping ability of male and female basketball players according to their chronological age and major leagues. J Strength Cond Res
13: 40-46, 1999.
31. Khamis HJ and Roche AF. Predicting adult stature without using skeletal age: The Khamis-Roche method. Pediatrics
94: 504-507, 1994.
32. Knapik JJ, Bauman CL, Jones BH, Harris JM, and Vaughan L. Preseason strength and flexibility imbalances associated with athletic injuries in female collegiate athletes. Am J Sports Med
19: 76-81, 1991.
33. Krosshaug T, Nakamae A, Boden BP, Engebretsen L, Smith G, Slauterbeck JR, Hewett TE, and Bahr R. Mechanisms of anterior cruciate ligament injury in basketball: Video analysis of 39 cases. Am J Sports Med
35: 359-367, 2007.
34. Li G, Rudy TW, Sakane M, Kanamori A, Ma CB, and Woo SL. The importance of quadriceps and hamstring muscle loading on knee kinematics and in-situ forces in the ACL. J Biomech
32: 395-400, 1999.
35. Lloyd DG and Buchanan TS. Strategies of muscular support of varus and valgus isometric loads at the human knee. J Biomech
34: 1257-1267, 2001.
36. MacWilliams BA, Wilson DR, DesJardins JD, Romero J, and Chao EY. Hamstrings cocontraction reduces internal rotation, anterior translation, and anterior cruciate ligament load in weight-bearing flexion. J Orthop Res
17: 817-822, 1999.
37. Malina RM and Bouchard C. Timing and sequence of changes in growth, maturation, and performance during adolescence. In: Growth, Maturation, and Physical Activity
. Malina and Bouchard, eds. Champaign, Il: Human Kinetics, 1991. pp. 267-272.
38. Malinzak RA, Colby SM, Kirkendall DT, Yu B, and Garrett WE. A comparison of knee joint motion patterns between men and women in selected athletic tasks. Clin Biomech
16: 438-445, 2001.
39. Markolf KL, Graff-Redford A, and Amstutz HC. In vivo knee stability: A quantitative assessment using an instrumented clinical testing apparatus. J Bone Joint Surg
60A: 664-674, 1978.
40. McLean SG, Lipfert SW, and van den Bogert AJ. Effect of gender and defensive opponent on the biomechanics of sidestep cutting. Med Sci Sports Exerc
36: 1008-1016, 2004.
41. Myer GD, Brent JL, Ford KR, and 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.
42. Myer GD, Chu DA, Brent JL, and Hewett TE. Trunk and hip control neuromuscular training for the prevention of knee joint injury. Clin Sports Med
27: 425-448, ix, 2008.
43. Myer GD, Ford KR, Barber Foss KD, Liu C, Nick TG, and 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.
44. Myer GD, Ford KR, Brent JL, and 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.
45. Myer GD, Ford KR, Brent JL, and Hewett TE. Differential neuromuscular training effects on ACL injury risk factors in “high-risk” versus “low-risk” athletes. BMC Musculoskelet Disord
8: 1-7, 2007.
46. Myer GD, Ford KR, and Hewett TE. Rationale and clinical techniques for anterior cruciate ligament injury prevention among female athletes. J Athletic Train
39: 352-364, 2004.
47. Myer GD, Ford KR, and Hewett TE. The effects of gender on quadriceps muscle activation strategies during a maneuver that mimics a high ACL injury risk position. J Electromyogr Kinesiol
15: 181-189, 2005.
48. Myer GD, Ford KR, and Hewett TE. Tuck jump assessment for reducing anterior cruciate ligament injury risk. Athletic Ther Today
13: 39-44, 2008.
49. Myer GD, Ford KR, and 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
50. Myer GD, Ford KR, Khoury J, Succop P, and 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
51. Myer GD, Ford KR, Khoury J, Succop P, and 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.
52. Myer GD, Ford KR, Khoury J, Succop P, and 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.
53. Myer GD, Ford KR, McLean SG, and Hewett TE. The effects of plyometric versus dynamic stabilization and balance training on lower extremity biomechanics. Am J Sports Med
34: 490-498, 2006.
54. Myer GD, Ford KR, Palumbo JP, and Hewett TE. Neuromuscular training improves performance and lower-extremity biomechanics in female athletes. J Strength Cond Res
19: 51-60, 2005.
55. Myer GD, Ford KR, Paterno MV, Nick TG, and Hewett TE. The effects of generalized joint laxity on risk of anterior cruciate ligament injury in young female athletes. Am J Sports Med
36: 1073-1080, 2008.
56. Myer GD, Paterno MV, Ford KR, and Hewett TE. Neuromuscular training techniques to target deficits before return to sport after anterior cruciate ligament reconstruction. J Strength Cond Res
22: 987-1014, 2008.
57. Myer GD, Paterno MV, Ford KR, Quatman CE, and Hewett TE. Rehabilitation after anterior cruciate ligament reconstruction: Criteria based progression through the return to sport phase. J Orthop Sports Phys Ther
36: 385-402, 2006.
58. Olsen OE, Myklebust G, Engebretsen L, and Bahr R. Injury mechanisms for anterior cruciate ligament injuries in team handball: A systematic video analysis. Am J Sports Med
32: 1002-1012, 2004.
59. Padua DA, Marshall SW, Beutler AI, and Garrett WE. Prospective cohort study of biomechanical risk factors of ACL injury: The JUMP-ACL Study. In: American Orthopaedic Society of Sports Medicine Annual Meeting; July 10, 2009; Keystone, CO. 393-395.
60. Paterno MV, Ford KR, Myer GD, Heyl R, and Hewett TE. Limb asymmetries in landing and jumping 2 years following anterior cruciate ligament reconstruction. Clin J Sport Med
17: 258-262, 2007.
61. Paterno MV, Myer GD, Ford KR, and Hewett TE. Neuromuscular training improves single-limb stability in young female athletes. J Orthop Sports Phys Ther
34: 305-317, 2004.
62. Paterno MV, Schmitt LC, Ford KR, Rauh MJ, Myer GD, Huang B, and Hewett TE. Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport. Am J Sports Med
38: 1968-1978, 2010.
63. Quatman CE, Ford KR, Myer GD, and Hewett TE. Maturation leads to gender differences in landing force and vertical jump performance: A longitudinal study. Am J Sports Med
34: 806-813, 2006.
64. Quatman CE, Quatman-Yates CC, and Hewett TE. A “plane” explanation of anterior cruciate ligament injury mechanisms: A systematic review. Sports Med
40: 729-746, 2010.
65. Scoville CR, Williams GN, Uhorchak JM, Arciero RA, and Taylor DC. Risk factors associated with anterior cruciate ligament injury. In: Proceedings of the 68th Annual Meeting of the American Academy of Orthopaedic Surgeons; February 28, 2001; Rosemont IL. 564.
66. Sell TC, Ferris CM, Abt JP, Tsai YS, Myers JB, Fu FH, and Lephart SM. Predictors of proximal tibia anterior shear force during a vertical stop-jump. J Orthop Res
25: 1589-1597, 2007.
67. Slauterbeck JR and Hardy DM. Sex hormones and knee ligament injuries in female athletes. Am J Med Sci
322: 196-199, 2001.
68. Soderman K, Alfredson H, Pietila T, and Werner S. Risk factors for leg injuries in female soccer players: A prospective investigation during one out-door season. Knee Surg Sports Traumatol Arthrosc
9: 313-321, 2001.
69. Uhorchak JM, Scoville CR, Williams GN, Arciero RA, St Pierre P, and Taylor DC. Risk factors associated with noncontact injury of the anterior cruciate ligament: A prospective four-year evaluation of 859 West Point cadets. Am J Sports Med
31: 831-842, 2003.
70. Wilson JD, Dougherty CP, Ireland ML, and Davis IM. Core stability and its relationship to lower extremity function and injury. J Am Acad Orthop Surg
13: 316-325, 2005.
71. Winter DA. Biomechanics and Motor Control of Human Movement
. New York, NY: John Wiley & Sons, Inc, 2005.
72. Withrow TJ, Huston LJ, Wojtys EM, and Ashton-Miller JA. The relationship between quadriceps muscle force, knee flexion, and anterior cruciate ligament strain in an in vitro simulated jump landing. Am J Sports Med
34: 269-274, 2006.
73. Withrow TJ, Huston LJ, Wojtys EM, and Ashton-Miller JA. Effect of varying hamstring tension on anterior cruciate ligament strain during in vitro impulsive knee flexion and compression loading. J Bone Joint Surg Am
90: 815-823, 2008.
74. Wojtys EM, Ashton-Miller JA, and Huston LJ. A gender-related difference in the contribution of the knee musculature to sagittal-plane shear stiffness in subjects with similar knee laxity. J Bone Joint Surg Am
84-A: 10-16, 2002.
75. Wojtys EM, Huston LJ, Lindenfeld TN, Hewett TE, and Greenfield ML.VH. Association between the menstrual cycle and anterior cruciate ligament injuries in female athletes. Am J Sport Med
26: 614-619, 1998.
76. Zatsiorsky VM. Science and Practice of Strength Training
. Champaign IL: Human Kinetics, 1995.
77. Zazulak BT, Hewett TE, Reeves NP, Goldberg B, and Cholewicki J. The effects of core proprioception on knee injury: A prospective biomechanical-epidemiological study. Am J Sports Med
35: 368-373, 2007.
78. Zazulak BT, Paterno M, Myer GD, Romani WA, and Hewett TE. The effects of the menstrual cycle on anterior knee laxity: A systematic review. Sports Med
36: 847-862, 2006.
79. Zazulak BT, Ponce PL, Straub SJ, Medvecky MJ, Avedisian L, and Hewett TE. Gender comparison of hip muscle activity during single-leg landing. J Orthop Sports Phys Ther
35: 292-299, 2005.
anterior cruciate ligament injury; knee; drop vertical jump landing; young athletes
FIGURE. Caption not ...Image Tools
© 2011 National Strength and Conditioning Association
Highlight selected keywords in the article text.