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Can ACL Injuries be Prevented?

Anyone who coaches girls' sports, or who has a daughter playing active sports from middle school and older, is aware that many players suffer serious knee injuries each season. What you may not know is that female soccer players are between two and eight times more likely to suffer a serious knee injury than their male counterparts.

A recently distributed post by US Soccer CoachesNet (May 26, 2009) contained references to many of the studies related to the incidence of ACL injuries in soccer players. There is fairly good statistical data available regarding the incidence of injury in NCAA programs since 1994. Specific data prior to that date is somewhat sketchy for college and not readily available for athletes younger than college age.

What data is available indicates that male college athletes have an injury risk of 16.4 injuries per 1,000 "Athlete Exposures," or "A-E." (One athlete exposure is equal to one athlete participating in training or a game. A team of 20 players participating in a practice would be 20 A-E.)

Female college athletes have an injury rate of 5.2 per 1,000 A-E. The rate of ACL injuries is .13 per 1,000 A-E for males and .34 per 1,000 A-E for females. The rates of injury are believed to be higher for girls in the age range of 14-18, but there is insufficient good data to make a definitive statement of the injury rate for players in those ages.

What do these statistics mean? Consider a typical high-school-age soccer player who participates in school and club soccer. With an average of 20 players on a team and approximately 200 A-E per player per year (200 games and training sessions, formal or informal, per player) a team would have 4,000 A-E each year. Applying the rate of .34 ACL injuries in female athletes per 1,000 A-E results in a figure of 1.6 — which means that an average girls' soccer team with 20 athletes can expect to experience 1.6 ACL injuries per year, or roughly three such injuries every two years. A club with six high-school-age girls' teams with approximately the same number of players and the same A-E for each team could expect nearly 10 ACL injuries among its female players each year. At the same time, the same number of boys' teams with the same A-E would be expected to experience only three ACL injuries.

Why are girls more likely to suffer an ACL injury? Tim Brinker of Hillsboro Physical Therapy presented a workshop at the recently completed AGM that highlighted five factors that contribute to the greater incidence of injuries for girls after the age of puberty.

  1. A girl's pelvis is, on average, wider than a boy's, which tends to result in a larger angle between the femur (the long bone in the thigh) and the tibia (the shin bone) for girls than for boys. As a result of the larger angle, rotation of the tibia in relation to the femur puts more stress on the ligaments that maintain the correct alignment of the joint.

  2. The groove in the base of the femur (intercondylar notch) through which the ACL travels to make its connection to the tibia tends to be smaller in women. As a result, the ligament itself tends to be smaller in women than in men.

  3. Ligaments in women tend to be more lax than in men. This gives women more flexibility, but can allow a larger motion in a joint which can increase the pressure on the ACL as the femur moves further past alignment with the tibia.

  4. Imbalance in strength between the quadriceps and hamstring muscles. The quadriceps drives the leg forward and the hamstring pulls the leg back. When a person plants a foot to change direction the hamstring pulls against the leg to help prevent the femur from moving past the tibia. When the strength of the hamstring is not matched to the strength of the quadriceps, there is more force driving the femur past the tibia than there is pulling the femur back into alignment with the tibia.

  5. Poor biomechanics in jumping and changing direction. Many girls tend to maintain an upright posture when jumping and changing direction. This results in increased stress being placed on the ACL as it absorbs the force of the landing or change in direction. Many girls also allow their knees to bend towards the center line of the body when landing from a jump or changing direction. This lateral movement of the femur in relation to the tibia can put significant additional strain on the ACL to keep the knee joint in alignment.

The first three factors are physiological and not subject to change. The good news is that proper training can help with the last two factors.

There are two major programs of warm-up activities that are designed to help reduce the incidence of ACL injuries — the PEP program (Prevent injury, Enhance Performance) and the WIPP program (Warm up for Injury Prevention and Performance).

Both of these programs include activities that provide dynamic stretching, core strengthening, and proprioceptive (awareness of body position) training. Goals of the programs are to help the athlete be aware of proper positioning of the body while playing their sport — including a lowered center of gravity with bent knees, awareness of maintaining a vertical alignment of the femur and tibia when landing from a jump, bending the knees to use the hamstrings to help absorb the force of landing when jumping, and consciously avoiding the knee's turning inward when running, jumping and changing directions. The programs differ primarily in the specific activities that are included.

Both the PEP and WIPP programs have shown a significant reduction in ACL injuries when consistently and properly applied, as compared to a control group that used more typical warm-up activities such as jogging and static stretching. One of the most important parts of using one of these programs is for the coach to understand and enforce use of proper movement mechanics as the athletes perform the program activities.

Details of the PEP program are available at www.aclprevent.com. Details on the WIPP program are available at www.sportsmetrics.net. FIFA has incorporated many of the ideas of the PEP program in the 11+ program. You can obtain the FIFA 11+ program from FIFA's website at http://www.fifa.com/aboutfifa/developing/medical/the11/index.html.

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