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Investigating the Anterior Cruciate Ligament (ACL) and Designing a Prevention Programs

Anterior Cruciate Ligament (ACL) injury has become devastatingly common, especially in female athletes. NCAA statistics estimate that the likelihood of a female sustaining an ACL injury is two to eight times greater than males. Research has shown that one of 10 college female athletes and one out of 100 high school female athletes experience an ACL injury. These numbers are staggering and have caused great concern among coaches and sports administrators. This concern has sparked investigation into the possible causes of ACL injury and the development of prevention programs.

Training programs designed to specifically address the risk factors associated with ACL injury have been successfully implemented across the country. The programs are a combination of common exercises with modifications in technique and instruction and novel exercise designed specifically for this purpose. Coaches at all levels can assist in the prevention of ACL injuries by including components of these programs into their team workouts. This article is part of a two-part series that will discuss the evidence for the design of prevention programs and the theory for the techniques used. The second part will provide sample exercises that can be included in any basketball workout.

There are two types of ACL injuries, those that occur as a result of direct contact with another player or object, and those that have no associated contact. These injuries are referred to as non-contact and include situations like landing from a jump, cutting and stopping. It has been estimated that 70% of ACL injuries do not involve contact. While the exact mechanism for them is not known, several potential risk factors for non-contact injury have been identified: environmental (e.g., equipment, shoe-surface interactions), anatomic (e.g., knee angle, laxity, notch size), hormonal, and biomechanical (e.g., muscular strength, body movement, skill level, neuromuscular control). Of the identified factors, only the biomechanical risks can be affected by training.

One of the biomechanical factors is a muscle strength imbalance between the hamstrings and the quadriceps muscles of the thigh. When the quadriceps contracts, it pulls the lower leg forward, putting stress on the ACL. The stress increases as the knee becomes more extended thereby increasing the risk of injury. Hamstring contraction counteracts the force of the quadriceps on the ACL by pulling the lower leg rearward, protecting the ACL from stress. Female athletes tend to have imbalances between the strength of the quadriceps and hamstrings- with the quadriceps dominating in strength. Hence, when the quadriceps contracts forcefully, as during a sudden stop or cut, the hamstring is unable to counter the force to protect the ligament. Additionally, females demonstrate less gluteal activation when cutting, which can result in decreased hip control and improper knee position during movement. Designing a strength-training program that emphasizes the hamstrings to balance out the knee stabilizer strength is believed to be an essential component to preventing ACL injury.

Another factor is the technique used when landing, planting and cutting. Females tend to perform these activities in a more upright position resulting in greater hip and knee extension. When the knee is more extended, the ACL is under increased tension making it more susceptible to injury during forceful quadriceps contractions. In addition, analysis of video has shown that females tend to perform planting, cutting and landing movements by turning the knees inward. This position can exaggerate anatomical risk factors such as Q angle stress (the angle formed where the femur meets the tibia). Increased Q angles can lead to more rotational forces on the ACL when the knee rotates during movement and a greater chance for injury. Jump training (e.g., plyometrics), with an emphasis on technique, can provide a skill development opportunity so that female athletes can learn to land, plant and cut with a lower center of gravity and more hip and knee flexion. In addition, the inward turn of the knee can be eliminated to reduce the exaggeration of the Q angle and associated increased risk of injury.

In addition to the inward turn of the knees, video was used to examine foot position at ground contact. Ground contact in the "flat foot" position was noted in the majority of the athletes who sustained an ACL injury. An explanation for this could be that this foot position may place the athlete's center of gravity behind the knee. When the center of gravity is behind the knee, the quadriceps may be used to aid the hip flexors in bringing the trunk forward. This can lead to an increase in the intensity of the quadriceps contraction and ACL stress. Landing on the toes makes it difficult to have the center of gravity behind the knee and thereby reduces the need for large quadriceps forces to be generated to assist with trunk position.

Training players to make movements on the balls of their feet instead of using a flat foot position is essential to reduce the risk associated with improper foot contact. Landing, jumping and cutting motions should all be performed from the balls of the foot with the center of gravity over the knee and not behind it. Successful maintenance of this position throughout a game or practice may require additional training of the strength and endurance for the abdominal, hip flexor and calf muscles.

A final factor that should be trained in a prevention program is proprioception. Proprioception is an unconscious sense of body movement that is used by the nervous system to make very rapid adjustments to position in order to maintain stability and prevent injury. Proprioception can be used both before and during a movement to fine tune position and muscle action. These minor adjustments may help to reduce the risk of ACL injury by improving knee position and adjusting contractions according to the knee position. Exercises that challenge balance help to train proprioception and have been shown to reduce the incidence of ACL injury. Progressively increasing the difficulty of the exercises challenges the proprioceptive system in order to train it to become sensitive to the necessary joint changes and the nervous system learns how to make the necessary adjustments.

A comprehensive training program for the prevention of ACL injury should include the following components:

1) Strength training with an emphasis on the hamstrings in order to balance the quadriceps/hamstring strength ratio.
2) Muscular strength and endurance training of the gluteals for hip and trunk control.
3) Muscular strength and endurance training of the abdominal, hip flexor and calf muscles to promote proper foot contact and to reduce the fatigue of this position during participation.
4) Technique training for landing, planting and cutting. Emphasize knee and hip flexion during movement and reduction of the inward turn of the knees.
5) Technique training for foot contact so that players learn to land and move off the balls of their feet instead of a flat foot.
6) Balance training to improve proprioception.

The next installment will include specific exercises to address each of the six components above. Specific instructions for the development of proper technique will be given to assist coaches in the implementation of an ACL prevention program.

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