Review Methodological approaches and rationale for training to prevent anterior cruciate ligament injuries in female athletes Gregory D. Myer1, Kevin R. Ford1, Timothy E. Hewett1,2 1Sports Medicine Biodynamics Center and Human Performance Laboratory, Division of Molecular Cardiovascular Biology, Cincinnati Children���s Hospital Research Foundation, Cincinnati, OH, USA, 2College of Medicine, University of Cincinnati, Cincinnati, OH, USA Corresponding author: Gregory D. Myer, MS, CSCS, Cincinnati Children���s Hospital, 3333 Burnet Avenue, MLC 10001, Cincinnati, OH 45229, USA. Tel: 11-513-636-4366, Fax: 11-513-636-0516, E-mail: greg.myer@cchmc.org Accepted for publication 08 June 2004 Female athletes have a four- to sevenfold increased risk of anterior cruciate ligament (ACL) injury compared with their male counterparts playing at similar levels in the same sports. The elevated risk of ACL injury in females coupled with the geometric increase in female sports parti- cipation in the last 30 years has led to a rapid rise in these injuries. This large increase in ACL injury incidence has fueled studies into both mechanisms of injury and interven- tions to prevent injury. A review of published multidiscip- linary approaches demonstrates that several training proto- cols have utilized multiple components targeted toward injury prevention training and were able to reduce injury incidence in female athletes. Similar training techniques may also be used to gain improvements in measures of performance. The purpose of this review is to highlight training components that may reduce ACL injury risk and assess their potential for combined use in performance- oriented protocols. Anterior cruciate ligament (ACL) injuries in the female athlete Female athletes have a four- to sevenfold increased risk of ACL injury compared with their male coun- terparts playing at similar levels in the same sports (Malone et al., 1993 Arendt & Dick, 1995 Mykle- bust et al., 1998). The elevated risk of ACL injury in females coupled with the 900% increase in high school (NFHS, 2002) and 500% increase in collegiate (NCAA, 2002) sport participation in the last 30 years has led to a rapid rise in these injuries. This large increase in ACL injury incidence has fueled studies into both mechanisms and interventions to these debilitating sports injuries (Hewett et al., 1996, 1999 Wedderkopp et al., 1999 Soderman et al., 2000 Besier et al., 2001b Malinzak et al., 2001 Chappell et al., 2002 Ford et al., 2003 Myklebust et al., 2003). This paradigm shift of focus away from treatment and rehabilitation, toward injury mechanism and prevention is warranted, as reports from Scandinavia and the United States show that ligament injuries, specifically injury to the ACL are costly (de Loes et al., 2000), with conservative estimates of surgical and rehabilitative costs of $17 000 in US dollars per injury (Hewett et al., 1999). This is in addition to the potential loss of entire seasons of sports participa- tion, scholarship funding, lowered academic perfor- mance (Freedman et al., 1998), long-term disability (Ruiz et al., 2002) and up to 105 times greater risk of radiographically diagnosed osteoarthritis to come in the future (Deacon et al., 1997). It would appear that the use of ACL injury prevention techniques would provide a sound time and financial investment for the coach, athlete, and sports medicine staff as the long- term costs of ACL injury may be significantly greater than the short-term costs. The recent approaches to the design of neuro- muscular training to prevent lower extremity injury have been diverse, utilizing many different types of training components. Review of the multidisciplinary approaches reported in the literature demonstrates that several training protocols have incorporated one or more components targeted toward injury-preven- tion training and were able to reduce injury incidence (Hejna et al., 1982 Caraffa et al., 1996 Lehnhard et al., 1996 Hewett et al., 1999 Heidt et al., 2000 Myklebust et al., 2003). ACL injury-prevention studies Hewett et al. (1996) initiated a program design that focused on correction of dynamic movement patterns and muscle imbalances that utilized technique train- ing and lower body plyometrics with supplemental Scand J Med Sci Sports 2004: COPYRIGHT & BLACKWELL MUNKSGAARD 2004 Printed in Denmark . All rights reserved DOI: 10.1111/j.1600-0838.2004.00410.x 1

strength training. These authors showed that female athletes who participated in a neuromuscular training program demonstrated greater dynamic knee control than females who had not undergone training. They were able to reduce knee varus/valgus moments with this type of neuromuscular training. An epidemiology study was also conducted with the purpose of pro- spectively evaluating the effects of the same neuro- muscular training program on serious knee injury rates in female athletes. The incidence of serious knee injury was 0.43 in untrained females, 0.12 in trained females, and 0.09 in males (injuries/1000 exposures). Untrained females had a significantly higher in- cidence of serious knee injury than trained females and males. Training reduced non-contact ACL in- juries to an even greater extent (Hewett et al., 1999). These results may indicate that the plyometric com- ponent of an exercise program, which trains the muscles, connective tissue and nervous system to effectively carry out the stretch-shortening cycle and that focus on proper technique and body mechanics, can reduce serious ligamentous injuries. Training protocols that focused on resistance training alone have not been shown to reduce ACL injuries. However, there is inferential evidence that resistance training may reduce injury based on the beneficial adaptations that occur in bones, ligaments and tendons following training (Fleck & Falkel, 1986 Kraemer et al., 1998). Lehnhard et al. (1996) were able to significantly reduce injury rates with the addition of a strength-training regimen to a men���s soccer team. They monitored injuries for 2 years without training and 2 years with the strength-train- ing treatment added. While they did not specifically find a reduction of ACL injuries, they did report a decrease in percentage of injuries that were ligament sprains. The significant reduction of ligament sprains may have been related to reduced knee injury (43%) reported in the 2nd year of post-trained competition (Lehnhard et al., 1996). Additionally, Cahill and Gri���th (1978) incorporated weight training into their pre-season conditioning for football teams. They found a reduction in reported knee injuries, and knee injuries that required surgery over four competitive seasons in the trained groups (Cahill and Gri���th, 1978). Protocols that supplement plyometric and technique training with strength training may significantly reduce ACL injuries in female athletes (Hewett et al., 1999). Thus, it appears that resistance training is effective at reducing knee injuries when combined with other training components however, the e���cacy of a single-faceted resistance training protocol on ACL injury prevention is yet to be demonstrated in the literature. Caraffa et al. (1996) prospectively evaluated the effect of balance board exercises on non-contact ACL injury rates in male athletes. The training consisted of approximately 20 min of balance board exercises divided into five phases. They compared athletes who participated in proprioceptive training prior to their competitive seasons vs. controls and found a significantly decreased rate of ACL injuries in the trained group (Caraffa et al., 1996). Similarly, a 5.9-time decrease in overall injury risk was achieved with balance training (Wedderkopp et al., 1999). Conversely, others have examined the effects of similar progressive balance board training and found no similar reduction of ACL injuries in female athletes (Soderman et al., 2000). Myklebust et al. (2003) examined the effects of a more comprehensive and dynamic neuromuscular training program. Their program elaborated on the balance board protocol of Caraffa et al. (1996) by adding a focus to improve awareness and knee control during standing, cutting, jumping, and landing. They demonstrated a reduc- tion on the incidence of ACL injury in women���s elite handball division over two competitive seasons (Myklebust et al., 2003). Others have shown that this type of proprioceptive and balance training can improve postural control and that lack of postural control, and stability were also related to increased risk of ankle injury (Tropp et al., 1984 Tropp & Odenrick, 1988 Holm et al., 2004). Likewise, im- provement in single-leg stability can be gained with a neuromuscular training program that incorporates perturbations into balance training on unstable surfaces (Paterno et al., 2004). Balance training has also been shown to improve maximum lower extre- mity strength and decrease side-to-side imbalances in stabilometric measures (Heitkamp et al., 2001). Side-to-side imbalances in lower extremity measures has been shown to be a risk factor for ACL injury (Knapik et al., 1991). The literature discus- sed above supports integrating proprioceptive stability and balance training in ACL prevention protocols. The effects of training, specifically targeted for speed enhancement on ACL injury risk reduction, have not been demonstrated. Heidt et al. (2000) were able to gain injury prevention effects through a speed-and-foot agility protocol. They were able to reduce lower extremity injuries in the trained female athletes by 19% when compared with the athletes who did not go through training. However, they did not find a statistically significant difference in ACL injuries between groups. They suggest that training protocols aimed at preventing injuries should focus on conditioning of the lower extremity in sport- specific activities (Heidt et al., 2000). Single-faceted sagittal plane treadmill training and conditioning protocols that do not incorporate sport-specific cut- ting maneuvers will not provide similar levels of external varus/valgus or rotational loads that are seen during sports competition (Lloyd & Buchanan, Myer et al. 2