The Anterior Cruciate Ligament (ACL)… Arguably one of the most widely known ligaments in the body – and for all the wrong reasons! We are lucky if we get through a round of footy (both professional and local leagues) without hearing of a player rupturing the dreaded season-ending ACL. So, what is it and why is it so commonly injured?!

Knee ACL


The ACL is a strong ligament in the knee that runs from the femur (thigh bone) to the tibia (shin bone).

The ACL’s function is to prevent anterior translation of the tibia – ie. preventing the lower leg from slipping forwards from underneath the thigh. It also reduces the knees ability to rotate (twist) and prevents hyperextension of the knee.

The ACL is also important in providing feedback via the mechanoreceptors. The ACL is full of receptors that send feedback to the brain on where the knee is and what it’s doing. While it can seem obvious where your knee is and what it’s doing, this feedback is vital and enables the brain to activate the correct muscles in the correct sequence, which allows you to complete movements or activities with ease (ie. walking).



The knee is a poorly designed joint. There are incredible forces that pass through the knee with normal everyday movements…

  • 3-4x our body weight worth of force when walking down stairs,
  • 5-7x our body weight worth of force passes through the knee while running

…and our muscles, ligaments and tendons are expected to manage and control all this force!

If the knee is placed under force that exceeds what it can control, injury is likely to occur. With that said, the forces required to injure the ACL aren’t excessive. Often injuries are caused by loading in a manner that places majority of the load through the ACL rather than spreading force throughout all the structures (ie. muscles, ligaments and tendons) in the knee.


Most ACL injuries occur in non-contact situations (statistics show approximately 70%), often while landing from a jump. Other common mechanisms of injury include:

  • Rapid deceleration,
  • Side stepping or pivoting,
  • Hyperextension of the knee,
  • During a tackle/contact with another player.


In short, yes, some people are more likely to sustain an ACL injury. Factors include (but are not limited to):


  • Females are more likely to rupture an ACL than males.
  • Intercondylar Notch width – The intercondylar notch is the space between the two femoral condyles. A narrower intercondylar notch has been associated with increased risk of ACL damage.
  • Knee alignment – poor/altered knee alignment can put excessive forces through the ACL, making it more susceptible to injury.

Neuromuscular Control:

  • Our ability to use our muscles to control our movements/actions is referred to as neuromuscular control. Altered or ineffective neuromuscular control is known to make a person more susceptible to ACL injuries.


  • Those who participate in sports with a lot of direction change and pivoting (ie. netball, basketball, soccer, skiing, and AFL) are more susceptible to ACL injuries. This is because they are more frequently exposed to the situations where the ACL may be injured.
  • Links have been made between ground conditions and ACL ruptures in AFL players. Dry field conditions and grass type have both been shown to increase the risk of rupture. Both these factors change the amount of shoe-surface traction and increase the risk of non-contact ACL injuries.

Previous Injury:

  • Primary ACL injury rates are 0.6-0.8% in an adolescent population. An adolescent who has had a prior ACL injury has a 23% chance of a second injury (either rupture of the graft or contralateral ACL)


The ACL is a commonly injured ligament that is important in maintaining stability of the knee. It is commonly injured and unfortunately there are some who are more susceptible to injury than others.

Stay tuned for #3 of our ACL series which will talk about surgical options and rehabilitation post-ACL rupture or contact us on (03) 5229 3911 or if you have any questions for our Physiotherapists.

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