The ACL- the ins and outs of the knee’s most commonly injured ligament

The knee joint as an anatomical structure is a notoriously unstable joint, imagine it two bones stacked on top of each other required to take your body weight and move around.

To stabilise the joint there are numerous muscles and ligaments stabilising the joint, the medial and lateral collateral ligaments and then the PCL and ACL there is then the medial and lateral meniscus which forms two shallow cups for the femoral epicondyles to sit in, which provide stability, cushioning and mobility. One of the most important ligaments especially when playing sports or skiing as the ACL reduces extreme tibial rotation more so medially and is also reduces the anterior movement of the tibia on the femur, as well as lateral stresses on the knee.

Before we look into the ACL specifically, an understanding of the knee structure is important to gauge the importance of the ACL and the impact that an injury of this ligament has on the knee’s structure and function.

How is the knee made up?

The knee is comprised of three boney joints; the tibio-femoral, patella-femoral and the superior tibio-fibular joint. All three joints must function properly to allow a full range of motion and provide the required‘scaffolding’. The tibio-femoral joint is a hinge joint meaning that it moves like a hinge of a door, in one plane with small amounts of rotation, assisted by the superior tibio-fibular joint.

Illustration of the front of the knee highlighting the anatomical names and there locations
Illustration of the back of the knee highlighting the anatomical names and there locations

The femur sits on top of the tibia but both are unequal to one another meaning that they don’t sit neatly on top of one another. Therefore this is assisted by two C-shaped ligaments which sit in between the two bones and provide further support and cushioning by cupping around the femur. Additionally,  there are two ligaments called the ACL (Anterior Cruciate Ligament) and the PCL (Posterior Cruciate Ligament) that anchor the tibia to the femur and stop the knee from sheering forward or backwards and limiting the rotation that occurs through the knee. The ACL is a particularly important ligament, especially when playing sports that involve starting and stopping.

There are two other ligaments to note at this stage which also reduce lateral movements of the knee and maintain the movement predominately through a single plane. These are the medial and lateral ligaments which fundamentally brace either side of the knee to allow for flexion and extension. If these ligaments are too loose or tight it puts greater strain on other ligaments or joints. When loose they allow the knee to move more than required. The opposite can occur, as we age or through injury, where the ligament loses its elastic tenancy and movement is reduced.

Now, it doesn’t just stop at the ligaments either. There are thirteen muscles acting over the knee influencing the knees movement each playing a role in how the patella glides over the knee and how forces are transferred through the knee.

One muscle group that is particularly important is the quadriceps. Having the correct amount of strength in the quadriceps allows for the patella to glide over the tibio-femoral joint. If one of the quadriceps muscles is dominating another or if one has become weak from injury then it may cause the patella to work in a different way. If this prolonged it could result in a wear and tear of a joint causing an ache or pain.

The knee also has fat pads called bursa. Bursa act as cushions that are located in high friction areas to simply reduce irritation and provide space for the knee.

Why is the knee unstable and what causes it to give way under extreme loads?

The main reason why the knee is notoriously unstable is that it is a door-hinge joint that promotes only flexion of the knee. It also doesn’t help that the knee joint is composed of two of the longest joints in the body (tibia and femur). Although the main motion of the knee is flexion there are small amounts of torque that are displayed as lateral and medial rotation. This torque is controlled and limited by ligaments such as the ACL and PCL as well as the collateral ligaments, which limit the knee from bending from side-to-side.

All of the ligaments and muscles that overlap the knee are there to control its movements and make the knee relatively stable. However if the stabilising soft tissues become loose due to weakness in the muscles, or due to overstretched or damaged ligaments, then the knee can give way under extreme stress.

When we stress the knee with forces that are unnatural, it can no longer deal with the load and something has to give. Most of the time it is the ACL that gives up under the stress of these forces. This often occurs during activities such as skiing or football where the foot is fixed and an extreme rotational force is propelled through the leg.

The ACL- a knee’s friend or foe?

The main functions of the ACL is to prevent excessive anterior translation of the tibia on the femur (keeping the shin bone from moving too far forward), excessive internal rotation of the tibia (inward rotation of the shin), reducing knee hyperextension and finally reduces excessive valgus forces (bending the knee sideways away from the other leg). ACL injuries usually occur in sports where the demand on the knees is very high. The ACL can be injured if any of the movements mentioned are exherted beyond their normal range Ths is most typical when hyperextension and or valgus forces are stressed on the knee.

Therefore most patients who have suffered an ACL injury often feel like it’s their enemy. It’s so common an injury that it often gets the blame. However, If we were to remove our ACL’s, our knees would be an anterior translated, internally rotated and a hyperextended mess that wouldn’t be much good for any extreme sport. You could still walk around (albeit after months of rehabilitation and reprogramming the knee) but unfortunately, you would have to kiss goodbye to participating in any sports that involve multidimensional movements.

Therefore it’s wise to take care of your ACL. It’s your friend, so do it a favour and help prevent its demise.


So how can you prevent an ACL rupture? My main focus when training the knee is proprioception, this is the bodies awareness of where its joints are in space. A simple example of this is if you can touch your nose with your finger while your eyes are closed.

For proprioception training on your knee, it’s a bit more complicated. We have to stress the knee in a controlled environment while training the muscles, ligaments, bones and mind to become more connected. This will strengthen the identified areas of weakness that playing your particular sport would put on your knee.

This is not a quick process however. Proprioception is something that is acquired over time by doing a particular exercise or gently testing the joint in a controlled way. Repeating these exercises builds up the brains connection to these stabilising muscles until it becomes intuitive. Only at this stage will it reduce the chance of an injury.

Some recent research has pointed out that the best way to improve proprioception is to base it around the sport or activity that you are wanting to do. If it is football, you would aim to focus activities that involve changing direction quickly or skiing you would focus in a semi-squated position but challenge your balance by creating an unstable surface. You will find examples of both below in the images/videos.

So below are my choices for knee prehabilitation

As always, prevention is better than cure. Even if you haven’t had an ACL injury before, it’s better to add these exercises into your training. 

And if you have had an ACL injury, then it’s imperative you seek the correct treatment for rehabilitation.

To begin with we shall start with a simple exercise. Balancing on one leg will start to activate muscles over the ankle and knee. This will show how much proprioception you have, identified by how much your knees and ankles shake/wobble. 

To increase the exercise try it on a Bosu ball make sure you have something nearby to assist you in gaining your balance.

Single Leg- balancing on a stable surface. You can have something there to assist you if it's required
The next stage- on a Bosu ball single leg. Have assistance there to get your balance

Next, we move onto the squat with various levels, start off with a squat on the floor doing it slowly and controlled. 

Then move onto squatting on the Bosu ball which will further challenge balance and increase the difficulty of the muscles used while squatting. Again slowly and controlled. 

Squatting on a stable surface

Keeping the exercise slow and controlled

The next stage onto the Bosu, this can also be done with the Bosu ball the other way. Slow and controlled again

We can then increase the intensity and load that is going through the joints and muscles by doing an assisted pistol squat. I wouldn't recommend going straight in and doing this exercise as it does involve a lot of strength in the legs. 

This can be modified by creating an uneven surface under your feet either using the Bosu ball again or like the left image is showing, on a foam block. 

The pistol squat- approach with caution as it requires a huge amount of strength even with assistance
Pistol squat on a foam block. Work up to this exercise. You could also use a Bosu ball

The Russian deadlift focuses on strengthening the glutes and hamstrings to help stabilise the hips and knees.

Starting position of a Russian Deadlift
Finishing position of the Russian Deadlift

The last collection of images shows compass jump squats being demonstrated. Starting in the centre, jump forward and squat then jump back to the centre and move around to another compass point returning to the centre each time. This is applying some speed and explosiveness to the routine whereas before they have been slow and controlled.  

Compass Jump Squats
Compass Jump Squats
Compass Jump Squats
Compass Jump Squats

Doing these exercises will help avoid a knee injury. Begin with the easier exercises and progress gradually through to the more difficult ones to build strength, proprioception and stability.

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