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).
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.
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).
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.
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).
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.
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.
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.