There is no question that knee injuries are a common occurrence among athletes. The incident of devasting knee injury, such as ACL tears, has been on the rise for years. Once considered an adult injury, ACL tears are occurring more often in children as reported by orthopedic specialists, estimating that thousands of children and teens suffer an ACL injury each year. According to statistics presented at the American Academy of Pediatrics 2011 annual meeting, over the past decade youth ACL injuries have increased 400% and girls are at eight times the risk of an ACL tear as compared to boys. Clearly, there is a problem with knee injuries among youth athletes. But where is the solution?

Mechanism of Injury and Risk Factors
To identify a solution, we must first identify the problem. In this case, the problem is the mechanism(s) of injury most commonly associated with ACL injury. Keep in mind that about 2/3 of all ACL injuries are non-contacted related. Meaning, the athlete didn’t have someone tackle them or have a player roll-up on their leg, resulting in injury. Non-contact injury opens the door to the possibility that a large percentage of ACL injuries can be reduced or prevented. Here are the most common mechanisms of injury associated with ACL injury:

• Jumping/landing improperly
• Planting followed by cutting or pivoting
• Straight-knee landing
• Stopping or landing with the knee hyperextended (too straight)
• Sudden deceleration of movement

As far as risk factors are concerned, there have been several identified as having an association with ACL injuries, such as:

• Weak hamstring and gluteal (hip) muscles
• Poor neuromuscular control and balance
• Poor dynamic biomechanics (Jumping, landing, cutting, etc.)
• Fatigue
• Female Gender

It is important that all risk factors be evaluated for how they play into an athlete’s risk of knee injury. Currently, there are movement screens available to help in identifying what potential risk factors may predispose an athlete to increased risk of ACL injury. Although these can prove to be valuable, one cannot underestimate the importance of simply watching an athlete move outside of a controlled environment. This means keeping an observant eye on them during training or practice and stressing them to see how their movement changes. You may be surprised by how much you learn about the physical abilities of that athlete from just simple observation.

Understanding the Female Athlete
Now that we have identified some mechanisms of injury and risk factors, we will turn our attention temporarily to the female athlete since they have their own special considerations in preventing ACL injuries. While researchers are continuing to study and gain understanding of the possible causes that may place young females at an increase risk of injury, a number of factors specific to female anatomy and development have been the focus of attention.

Female Hip and Knee Anatomy
Despite many young female athletes experiencing pain in their knees, the root of some of the problem may actually originate in the pelvis/hip structure. There is a growing trend among sports medicine specialists who focus on the pelvis/hip to reduce the incidence of knee pain and injury.

According to the Women’s Health and Fitness Guide (2006), the female pelvis has a number of differences as compared to the male pelvis for the purpose of accommodating childbirth. Among those differences, the female pelvis has a greater forward tilt and more forward facing hip joints.  These features of the female pelvis/hip result in the femur (thigh bone) being positioned with more of an inward angle and internal rotation at the knee as compared to the average male. It is this increased angle of the femur when compared to the vertical position of the tibia (shin bone). This anatomical difference is known as the "Q-angle" and is illustrated below.



What does all this mean? SImply put, it means the female knee is predisposed to having unfavorable forces placed on it and that the core, hip, and thigh musculature must be strong enough to compensate for the increased angle of the femur to the tibia, or else the female athlete may be at a higher risk for experiencing knee pain or injury.

What can be done?
Unfortunately, regardless of gender, there is no such thing as complete injury prevention. However, there are reasonable and appropriate steps that have been implement in programs that are successful in reducing the occurrence of knee pain and ACL injuries:

1. Improve hamstring strength. The hamstring muscles have a critical role in maintaining healthy knees. Proper hamstring training and strengthening must take into consideration how the hamstrings function during the primary sporting movement(s). For example, land-based sports with an emphasis on jumping and sprinting ability will place a high demand on the hip extension action of the hamstring. The hamstrings must be trained accordingly to be able to meet and accommodate the forces generated during sport.
2. Improve hip and core strength. The musculature of the core and hips have a tremendous amount of control on the pelvis and femur, and thus the knee. Poor hip control puts the knee in compromising positions, increasing the risk of injury. When the core and hips are weak, they needs to be a focus of treatment/exercises. This will serve to improve the stability of the knee.
3. Improve Proprioception (Balance) and Neuromuscular Control. Sufficient proprioception and neuromuscular control is the difference between being able to ride a bike and falling on your butt every time you get on a bike. Understand that altered proprioception and neuromuscular control contribute to abnormal motion during dynamic sporting activities, such as cutting and jumping/landing. One study revealed, “Improved joint mechanics during landing were achieved regardless of the individual’s muscle strength, suggesting that strength may not always be a prerequisite for movement re-education.”  This should demonstrate the importance that mental focus and repetitive use of proper movement has on correcting mechanics.
4. Decrease fatigue. There are 2 types of fatigue, peripheral (muscles) and central (brain).  Peripheral refers to exercise induced processes leading to decreased force production (typical muscle fatigue).  Central fatigue relates to a gradual exercise-induced reduction in voluntary muscle activation. Essentially meaning the brain gets fatigued. It is plausible to say injury comes from both, however from an injury prevention stand point; peripheral fatigue is difficult to manage because your muscles will get fatigued.  But targeted training of central fatigue might be the way to go in preventing injury. How does one train central control. As one study put it, “Exposure to more complex or cognitively demanding movement tasks may facilitate improved perception and decision making within the random sports environment.” This is were mental focus and developing an athlete's awareness of their body during drills becomes important. Mental imagery may prove beneficial in developing central control by utilizing “mental reps” to help engrain proper movement and ideal mechanics.

References:

1. Powers  CM, Souza RB. Differences in Hip Kinematics, Muscle Strength, and Muscle Activation Between Subjects With and Without Patellofemoral Pain. J Ortho Sports Physical Thearpy. 2009;39(1):12-19.
2. Powers CM. The Influence of Abnormal Hip Mechanics on Knee Injury: A Biomechanical Perspective. J Orthop Sports Phys Ther. 2010;40(2):42-51. http://www.jospt.org/issues/articleID.2396,type.2/article_detail.asp.
3. Heiderscheit B. Lower Extremity Injuries: Is It Just About Hip Strength? J Ortho Sports Phys Ther. 2010;40(2):39-41. http://www.jospt.org/issues/articleID.2404,type.2/article_detail.asp.
4. Mizner R, Kawaguchi J, Chmielewski T. Muscle Strength in the Lower Extremity Does Not Predict Postinstruction Improvements in the Landing Patterns of Female Athletes. J Orthop Sports Phys Ther. 2008;38(6):353-361. http://www.jospt.org/issues/articleID.1408,type.2/article_detail.asp.
5. McLean SG, et al. Impact of Fatigue on Gender-Based High-Risk Landing Strategies. Medicine & Science in Sports & Exercise. October 2006.
6. McLean SG. Fatigue-Induced ACL Injury Risk Stems from a Degradation in Central Control. Medicine & Science in Sports & Exercise. January 2009.
7. Hilgefort M, Winchester B. Preventing ACL Injuries in Female Athletes.