Oct. 1, 2004
Darin Padra, Ph.D.
Assistant Professor of Exercise and Sports Science
University of North Carolina
ACL injuries are common, costly and potentially debilitating. It is estimated that 95,000 new ACL injuries occur each year (1 in every 3000 persons). Those individuals who suffer an ACL injury and undergo surgical intervention face a lengthy rehabilitation process ranging from 6 to 36 months. Unfortunately, surgical intervention does not ensure a return to previous activity levels. Only 75 percent of these individuals return to their previous activity levels. Moderate to severe disability is reported in individuals suffering from ACL injury during walking activities, routine activities of daily living, and sport activities. The outlook is worse for individuals who do not undergo surgical intervention following ACL injury.
Untreated ACL injuries often lead to devastating impairment with two-thirds of these injuries resulting in chronic knee instability, secondary joint damage, and early onset of osteoarthritis. Ultimately, the disability associated with ACL injury leads to reduced physical activity levels and independence. The frequency and potential disability associated with ACL injury make it a significant health concern. Thus, there is a great need to develop training programs that focus on ACL injury prevention.
The rate of ACL injury is greater in females than males, especially in younger individuals. As the number of females participating in competitive and recreational activity continues to increase, the number of females sustaining ACL injury can be expected to increase. Thus, ACL injury prevention programs should target females who are at greatest risk.
The majority of ACL injuries involve no contact with another athlete and these injuries are more common in females. Approximately 70 percent of ACL injuries are non-contact in nature and almost always occur as the body undergoes rapid deceleration while performing either: 1) planting and cutting maneuvers or 2) landing from a jump. In addition, females are at 3-times greater risk for sustaining an ACL injury during non-contact mechanisms when compared to males competing in the same sport. ACL injury prevention programs should focus on preventing non-contact ACL injury, especially in females.
Factors Contributing to Increased Risk of ACL Injury in Females
In June 1999, a consensus conference was held in Hunt Valley, Maryland, organized by NIAMS division of NIH, American Orthopaedic Society for Sports Medicine (AOSSM), National Athletic Trainers' Association (NATA), National Collegiate Athletic Association (NCAA), and Orthopedic Research and Education Foundation (OREF). The purpose of the conference was to form a scientifically based consensus regarding ACL injury risk factors. From this meeting 4 ACL injury risk factors were identified: 1) Anatomical, 2) Hormonal, 3) Environmental, and 4) Biomechanical. (Editor's note: For a greater discussion check out the article on Gender Differences in ACL Injuries)
Role of Non-Biomechanical Factors
Non-biomechanical factors are potential contributors to the greater number of ACL injuries observed in females. However, non-biomechanical factors are not changeable through training programs. Thus, it is important to focus on those factors that are changeable through a training program. Specifically, research is needed to understand ways to effectively modify various biomechanical factors.
The conference reached the consensus that biomechanical factors appear to be the most important contributors to the greater risk of ACL injury in females. Biomechanical factors include such variables as joint motion, joint forces, muscle activation, and muscle strength. These types of biomechanical factors have been demonstrated to directly influence the stress placed on the ACL. It should be noted that research has not yet definitively determined if biomechanical factors do in fact increase the risk of ACL injury. The importance of biomechanical factors on ACL injury risk has been theorized given the association between specific biomechanical factors and ACL loading and deformation. However, experts in ACL injury research agree that biomechanical factors are perhaps the most important contributors to ACL injury risk.
Biomechanical factors are also important from a prevention perspective. Unlike anatomical or hormonal factors, biomechanical factors are modifiable. Individuals can be trained to alter their strength, muscle recruitment and movement patterns. Given the association between biomechanical factors and ACL loading-deformation, by modifying biomechanical factors we may reduce the stress on the ACL. Ultimately this may reduce the incidence of non-contact ACL injury and the associated cost and long-term disability effects. In order to reduce injury risk, ACL injury prevention interventions must minimize the magnitude of ACL loading and deformation during events associated with non-contact ACL injury (e.g. cutting and jumping).
Based on previous research, ACL stress may be reduced through several mechanisms: 1) reduced quadriceps activation, 2) increased hamstrings activation, 3) increased knee flexion, 4) reduced knee valgus, and 5) reduced tibial rotation. Effective ACL injury prevention interventions would be able to achieve these alterations during cutting and jumping maneuvers.
The Influence of Physical Training on ACL Injury
Research investigating ACL injury prevention is limited. Two types of preventive training interventions have been proposed for ACL injury. Balance training exercises were originally proposed as training method to reduce the risk of ACL injury. More recently, plyometric-agility training exercises have been incorporated in ACL injury prevention programs.
Typically, balance training programs involve having subjects attempting to maintain a balanced position while on a single leg. To challenge the individual there are several ways to modify these exercises. First, have the individual perform the single leg balance exercise with their eyes closed instead of open. Second, have the individual perform the exercise on an unstable surface, such as a balance board that is able to wobble in various directions with their eyes open. Third, have the individual perform the exercise with their eyes closed while on an unstable surface.
Balance training has been shown to improve position awareness (proprioception) and balance. Such adaptations are believed to enhance the body's ability to sense potentially injurious movements and facilitate a protective response. In addition, proprioception-balance training has been shown to increase muscle contraction synchrony about the joint and potentially improve joint stability.
Research investigating the effects of balance training on ACL injury risk is mixed. It was originally revealed that balance training was able to significantly reduce the incidence of ACL injuries in male soccer players. However, more recent research that incorporated a similar balance training program has demonstrated contradictory findings in female soccer players. In this study there was no effect of balance training on the number, incidence or type of traumatic leg injuries, including ACL injuries, in female soccer players.
The most recent research on balance training programs has also demonstrated mixed effects of balance training on ACL injury in female handball players. In this study, only elite handball players who complied with the balance training program demonstrated a reduction in ACL injury incidence. However, when including less skilled individuals who complied with the balance training program there was no change in ACL injury incidence. Thus, the effectiveness of balance training is unclear.
Exercises that incorporate jumping, bounding and leaping activities, as well as, rapid changes in direction are often involved with plyometric-agility training programs. These types of training programs have been revealed to improve hamstring muscle strength and reduce impact ground reaction forces during a simulated volleyball block jump, and also improve reflexive and voluntary muscle response times to a controlled anterior tibial translation event.
The effectiveness of plyometric-agility training on ACL injury incidence is also uncertain. The original study investigating plyometric-agility training program was performed in a group female volleyball, soccer, and basketball players. The training group performed plyometric-agility exercises as part of their pre-season conditioning program. It was shown that the untrained-control group had a higher incidence of ACL injury in comparison to the training group. More recent research investigating the effects of a plyometric-agility program has demonstrated similar findings. The more recent research compared the rate of ACL injury between a group of female soccer players that had undergone 12-weeks of plyometric-agility training to an untrained-control group. In this study, the untrained-control group sustained 32 ACL injuries compared to 2 injuries for the training group (88% decrease). Based on these two studies it would appear that plyometric-agility training programs may be an effective method to reduce the risk of sustaining an ACL injury. However, more research is needed to better understand the effects of these types of training programs.
There is evidence that training programs incorporating balance and plyometric-agility exercises may be able to reduce the risk of ACL injury. However, there is a lack of understanding as to why these interventions may be effective. Most of these studies are limited in that no biomechanical analysis has been performed to validate the physical training program's ability to modify factors associated with ACL loading. In order to develop an effective training program, we must know if the proposed training program is effective in modifying factors that are believed to most greatly influence the risk of ACL injury. Therefore, research is needed to determine if proposed ACL injury prevention programs are able to modify key biomechanical factors related to ACL stress, hence ACL injury risk. This will allow for the development of science-based ACL injury prevention training programs. A science-based ACL injury prevention training program may be employed to ultimately reduce the risk of ACL injury as well as reduce the financial costs and long-term disability associated with ACL injury.
Research being performed at the University of North Carolina is currently investigating what are the actual risk factors for ACL injury. Specifically, this research will help to identify the effects of joint movement, muscle strength, posture, and muscle contraction on ACL injury risk. By understanding the role of these factors on ACL injury risk we will be able to develop physical training programs that address specific risk factors for ACL injury. This research will lay the foundation for future studies investigating physical training programs aimed at reducing the risk for ACL injury. It is the long term goal of this research to develop physical training programs that greatly reduce the risk of ACL injury for all individuals.