100,000 to 250,000 Anterior Cruciate Ligament (ACL) injuries occur each year in the United States alone. The total cost of these ACL injuries is estimated to be more than $3 billion, with female varsity athletes alone accounting for $650 million of that cost. The average athlete has a one in 60 to one in 100 chance of sustaining an ACL injury. Unfortunately, those odds don’t change much with the mechanism of injury.
Non-contact ACL injuries are three and a half times more likely to occur in females than males. This discrepancy results from differences in landing mechanics and neuromuscular adaptations. Females exhibit decreased neuromuscular control of the trunk and hip musculature which leads to a valgus (knees collapsed inward) position of the knees and results in increased strain on the ACL. Females also display increased knee extension in landing and cutting maneuvers which increases the shear force at the knee and elevates risk of tearing the ACL. Body mechanics in sports proves to be of high importance in preventing injury when 70 to 78 percent of ACL tears occur by non-contact mechanisms. This means the majority of athletes aren’t going down on the aggressive play against the other team’s “enforcer,” it’s happening in the open field without suspicion of foul play.
ACL injuries are most commonly seen in soccer, basketball, football, and volleyball. Once an athlete tears their ACL across any sport, male or female, they will have a long road to recovery including six to nine months of physical therapy. After this time, the athlete may never play at their prior intensity or skill level again. If they do return to sport they may be at increased risk of future ACL injury. Once that athlete has a history of ACL injury their odds of re-injury or injury to the contralateral (opposite) ACL increase by 15 to 25 times. Females are four times more likely than males to suffer a second ACL injury. For two years after surgical repair of the ACL, the athlete will frequently favor the uninjured limb. This produces asymmetrical loading throughout body mechanics and increases risk of re-injury. Aside from re-injury, there are a number of associated consequences which can be debilitating, including chronic knee problems, knee instability, cartilage injuries, meniscus tears, and osteoarthritis. Within 14 years after surgery 41 to 75 percent of patients will develop osteoarthritis. All of these factors place a greater importance on the prevention of ACL injury.
Thankfully, the mechanical risk factors leading to ACL tear are modifiable and such modifications have been shown to be successful in preventing injury. Recent studies have found that an ACL injury prevention program reduced risk of injury by 74 percent in females and by 85 percent in males. It was also concluded that such a program should include 10 minutes of exercise, three times per week. The best way to spend those 30 minutes per week has been highly researched, but we know that landing mechanics can be modified to protective positions that decrease force on the ACL, thus leaving time for exercise after the new behavior is adopted.
Landing mechanics were able to be modified to increase knee flexion by following a program of warm-up, stretching, strengthening, plyometric, and agility drills. During this training the athletes received feedback to improve body mechanics with an emphasis on knee positioning. Feedback can be provided by coaching staff or by teammates after proper training. Athletes should be instructed to jump and land with their knees over their toes, to prevent their knees from caving in upon landing, to have hips and knees sufficiently flexed, and to land softly with more weight on the forefoot. At the end of the program, the athletes displayed improved landing mechanics at the knees and increased energy absorption through the hips at impact. Body mechanics training must be accompanied by strength training in order to maintain joint positions on impact.
The hamstrings should be emphasized in strength training to promote recruitment of this muscle and decrease strain on the ACL at impact, during landing and cutting movements. With a weak and untrained hamstring, the tibia is at increased risk of translating anteriorly (moving forward) on the femur. In this position, the ACL is strained and at greatest risk of tearing. Females typically require more time to recruit maximal hamstring torque and have greater quadricep activation. These two factors further increase risk of females suffering an ACL tear. While hamstrings are the primary concern, accessory muscles shouldn’t be forgotten.
The hip abductors also become an important muscle group to strengthen in athletes that land in the valgus position. Strengthening the hip abductors can restore a neutral position of the knees and decrease risk of injury. Furthermore, core training has been shown to stabilize trunk motion in activities where the ACL is compromised. Core stability and lower extremity proprioception are both risk factors that can be modified through training with a balance board to decrease risk of injury. Perturbations can be included with balance board exercises for a greater challenge. This form of training is important, as decreased core strength against perturbations is an indicator for ACL injury. Lower limb asymmetry should be taken into account in injury prevention training. Core training using a balance exercise board, along with lower limb strengthening have been shown to improve single-limb stability and neuromuscular control. If there is ever a question of lower limb stability in an athlete, it is best tested using unilateral movements, such as single leg hops. An increase of intensity in hopping will progress the movement to a plyometric exercise.
Plyometric training should include high intensity agility drills that focus on footwork and quick explosive movements. This type of training should emphasize power, speed, and proper mechanics as difficulty increases. In early training this can include unperturbed jumping, cutting, and lateral movements. As the athlete progresses perturbations can be incorporated to simulate a game time situation.
While there are many different methods available to train for ACL injury prevention it is best to take a multifactorial approach and adhere to a program that includes muscle strengthening, plyometrics, neuromuscular control, body mechanics and proper landing patterns. These programs should begin six weeks before the start of the season and can even replace the traditional team warm-up.
ACL injuries are common and remain a healthy concern among adolescent and professional athletes alike. While the road to recovery from ACL injury is long without promise of a full return, the saving grace is that ACL prevention can mean as little as altering the way you warm up for your sport. Thirty minutes spent each week priming your body for the unexpected can mean saving your season, scholarship, contract, or better yet championship.
Chad Franche, PT, DPT
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