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Why Are Females More Prone To ACL Injuries?

When video footage of Anterior Cruciate Ligament (ACL) injuries are studied, there are clear components that occur and lead to injuries. Some of these components however are exaggerated in females, resulting in increased vulnerability and higher chances of ACL injuries in sports that involve jumping and change of direction such as Australian Rules Football and Soccer.

We will discuss the common mechanisms in which the ‘at risk’ female athlete lands, contributing to this increased chance of ACL injuries.

Female Footballer Acl Injury

It’s a Physio’s worst nightmare, when a young female athlete clutches at her knee

Dynamic Knee Valgus

Can you picture what it looked like last time you saw an athlete go down with a knee injury? The knee falling or buckling inwards and across the body as they landed from a jump or tried to change direction sharply… this is called a dynamic knee valgus, and it is not great for your anterior cruciate ligament. When this happens, it is likely that the muscles were not able to absorb the ground reaction forces of the activity, so the joint and the ligaments absorbed a high amount of the force instead, increasing the risk for ACL tears.

Acl Tear Vagus

Dynamic Knee Valgus demonstrating the different forces going through the joint as well as anterior cruciate ligament

Newton’s third law of equal and opposite reaction forces is always obeyed… Therefore, when a female soccer player makes a one-footed cut on the pitch, or a female footballer lands from a contested mark, she hits the ground, and the ground hits her back with an equal and opposite reaction force. The reaction force experienced by the athlete is actually significantly greater than her body weight because her body and body segments have created inertia and impart a force to the ground greater than their mass. This leads to forces experienced during landing, cutting, running and jumping performed during sports, adding up in multiples of body mass (sometimes up to and above 10x as much!), which increases the risk of sustaining ACL injuries.

In a perfect world, the big strong muscles of the lower limbs, such as those that make up the posterior chain, would be recruited in order to shock absorb the force and protect the knee joint and its ligaments, preventing ACL tears and injuries.

These posterior chain muscles include the glutes (maximus and medius), the hamstrings, and the calf (both gastrocnemius and soleus).

Unfortunately in females, studies have shown that they are often not strong enough or coordinated enough to play their role and control the correct movements necessary to avoid ACL injuries.

Valgus Knee Stance

Strong stable position vs Vulnerable knee valgus position

Test this out yourself!
Have a look in the mirror or record yourself on your phone – Can you keep a nice stable and straight leg while performing movements such as single leg squats or lunges? If not, don’t panic! By itself, this doesn’t mean you are destined for doom, but it does indicate that a sports medicine strength program, incorporating some jumping and landing technique/control work would be a great idea for future injury prevention! 

Quadriceps Dominance

Maybe that injury you recall wasn’t one where the knee fell inwards and across the body… maybe it locked back into extension as soon as the player’s foot hit the ground? The straight or locked knee version of Anterior Cruciate Ligament ACL injuries relate to a neuromuscular imbalance where the knee is primarily stabilised by the quadriceps muscles. Females appear to prefer to use the quadriceps more than males in order to stiffen and stabilise the knee joint. When a female athlete contracts her quadriceps in order to stabilise the leg however, it may excessively straighten (or hyper-extend) her knee, potentially placing it in the vulnerable positions observed during ACL injuries.

When the quadriceps muscles contract, they pull the tibia (lower leg bone) forward relative to the femur (thigh bone). This results in a biomechanical problem, as the anterior cruciate ligament serves to hold the tibia from sliding forward, and when a female athlete uses her quadriceps to stabilise the joint, she induces an anterior shear stress to the tibia and therefore also to the ACL, increasing the risk of ACL tears. Quadriceps dominance is a result of poor posterior chain activation and strength (is anyone seeing a common theme here…?). If an athlete uses the quadriceps instead of the posterior chain muscles to control the knee, they are using a single muscle for stability and control rather than multiple muscles capable of a lot more strength and support, therefore increasing the risk of injuries.

Jumping Landing Bent Knees Technique

One important muscle group of the posterior chain for ACL injury prevention is the hamstrings. The hamstrings are able to increase flexion (bending) at the knee, which provides a better position and mechanical advantage for using the muscles to absorb force. According to sports medicine, the hamstrings are considered a synergist with the anterior cruciate ligament and are able to pull the tibia posteriorly, counter-acting the quadriceps, thereby decreasing the stress on the ACL and the potential for ACL tears. 

Break the habit!

When looking at your jumping and landing, try to ensure you absorb the force of your landing by bending at the hip, knee and ankle – think squatting. If you need, exaggerate this movement in practice so that you feel comfortable to support yourself without locking at the knees when you are out training and playing– they’ll thank you for it!

Trunk Proprioception

Trunk proprioception is simply defined as the ability to precisely control the trunk in three dimensional space. Athletes who have been shown to not adequately sense the position of their trunk in space, or allow greater movement following disturbances and perturbations (e.g. when getting pushed, or landing on someone else’s foot when coming down from a jump) have a greater risk of knee injuries, in particular ACL injuries. Poor positioning of the trunk is one key difference that exists between men and women. In females, a valgus positioning of the knee (falling inwards) is frequently observed during varied activities where the trunk is pushed off balance, and is considered to be a high risk for ACL tears. These same activities do not produce a valgus knee positioning in their male counterparts however, indicating that the trunk is more of a key element towards risk of injuries in females.

But why do females have more trouble with controlling their trunks (and therefore their knees) relative to males? As females mature, they also increase body mass and carry proportionally more fat body mass than their male counterparts. Their centre of mass is also now higher off the ground and therefore harder to control and balance. After the adolescent growth spurt, males get what is called a “neuromuscular spurt” where they progress in muscular development and muscle strength, and become proportionately more powerful. In females however, the ratio between the size of their body and the power output of their body basically stays the same after a growth spurt and does not adapt to the increased demands of a bigger, more mature body.
 
This now means that whilst moving in the air or when changing direction, the muscles that make up the core (abdominals, hip and pelvic muscles) of a female athlete will struggle to control and coordinate the top half, placing more strain through the lower limbs to hold her upright. With the added pace of a match coupled with fatigue or the gentle nudge of an opponent, we unfortunately see a recipe for ACL disaster.

Relative Hip Knee Strength Recruitment Diagram

Menstrual Cycle

An important point to note is that the literature definitively shows that females are at a greater risk of ACL injury at different stages in their menstrual cycle. The evidence has found that during the ovulatory phase of the menstrual cycle there is a significant increase in ACL laxity (increased risk of injury) when compared to the follicular phase. Whilst this research is of a high quality of a large period of time, more research is needed to better understand how we can implement this in clinical practice to lower the risk of ACL injury.

Recent studies have also found a 20% decrease in ACL injury risk for female athletes who were taking oral contraceptives, although more investigation is necessary to better understand this finding and how it can be applied to prevent injury.

Femoral Notch (Intercondylar Notch)

The intercondylar notch is essentially the groove that the ACL sits in on the femur and typically females have a smaller notch volume or size. The relevance of this is that a smaller intercondylar notch will accommodate a smaller ACL, therefore resulting in an increased risk of ACL tear.

As this is a genetic predisposition, there is nothing that can be done to change this risk factor, however it is best to be aware of the structural differences that can affect injury risk.

Summary

In summary, the 3 types of movement patterns presented each make up a potentially hazardous mechanism in which ACL injuries can occur. Although these same mechanisms can occur in both male and female athletes, the exaggeration of the trunk and lower limb positioning in landing and change of direction is much greater in female, placing them at much higher risks for injuries. For those who are involved in sports such as Australian Rules Football and Soccer, it is a great idea to get an assessment and overall summary of your movement from a sports medicine expert like a physiotherapist. This is because early identification of poor movement patterns and jumping/landing technique could allow for specific strength, athletic performance and training programs to be implemented, in the hope of preventing ACL injuries from occurring throughout your career!

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