The Knee - ACL Injuries Part One
What Is an ACL Injury, and Why Does It Happen?
70% of ACL injuries happen with no contact at all — no tackle, no fall, sometimes just a plant and a pivot. Here's what's really going on inside the knee, and why some people are far more at risk than others.
If you've ever watched AFL or netball, chances are you've seen it happen live: a player plants, twists, and goes down clutching their knee. It's dramatic, it's dreaded, and it's one of the most talked-about injuries in sport — the ACL tear.
This is Part One of a three-part series where we unpack everything you need to know about ACL injuries:
● Part One: What is an ACL injury, and how does it happen?
● Part Two: How are ACL injuries diagnosed and treated?
● Part Three: Can you actually prevent one?
Thirty to forty years ago, an ACL tear was close to career-ending. Today, with good diagnosis, skilled management and solid rehab, most people get back to sport and to life — some with surgery, and a growing number without it. More on that trade-off in Part Two.
What actually is the ACL?
The anterior cruciate ligament (ACL) is a strong, cord-like band of connective tissue running through the centre of your knee, connecting your femur (thigh bone) to your tibia (shin bone). It's one of the knee's key stabilisers, resisting rotation and stopping the shin from sliding too far forward under the thigh.
It's made up of two bundles that tighten at different points through the knee's range of movement — a neat bit of engineering that keeps the joint stable whether it's bent or straight. The ACL also has its own artery running through its centre and a rich nerve supply, which is exactly why a tear swells so quickly and hurts so much.
How do people actually injure it?
Here's the surprising part: 70% of ACL injuries are non-contact. There's no other player, no equipment, nothing external at all. The classic mechanism is landing from a height or changing direction, with excessive knee rotation and sideways force, or a hyperextension moment.
If you've got the stomach for it, here are two real examples of non-contact ACL injuries: AFL ACL injury and netball ACL injury.
This is exactly why ACL injuries turn up so often in sports with rapid pivoting, twisting and sudden deceleration — AFL, netball, alpine skiing. The remaining 30% are contact injuries, such as a fall across an opponent's knee or contact in martial arts.
Most ACL injuries come with an audible “pop” and rapid swelling.
They're usually painful, and weight-bearing straight after is genuinely difficult.
Who's most at risk?
Anyone can injure an ACL, but the risk is highest in 16 to 35 year olds, and in females — women are around three times more likely to injure their ACL than men, thanks to a mix of anatomy, hormonal factors, body shape and strength differences.
Physios talk about “neuromuscular and biomechanical” risk factors — really just how strong you are, how fast your muscles react, and how well you control movement. The genuinely good news is that, unlike gender, most of these factors are modifiable through targeted training. We dig into exactly how in Part Three.
You can also manage some of the external risk factors — footwear, playing surface, equipment and even weather conditions.
Strain, tear or rupture — have I “done my knee”?
The ACL can be strained, partially torn, or completely ruptured — what physios call Grade 1, 2 and 3 injuries (more detail in Part Two). It's often injured alongside other structures too, particularly the meniscus or the medial collateral ligament.
Whatever the severity, every ACL injury deserves careful assessment. Some need urgent attention, most need imaging, and many people will still choose surgery — though as we'll cover in Part Two, that's not the only path to a great outcome.
Coming up next
In Part Two, we walk through how ACL injuries are graded and assessed, your treatment options — surgical and non-surgical — and what rehab actually involves. In Part Three, we get into prevention: the modifiable risk factors, and the programs proven to reduce your risk.
ACL Injuries
Frequently Asked Questions
Ready to get on top of your symptoms?
At Movement for Life Physiotherapy, our team across Coconut Grove and Rosebery can assess your knee and diagnose an ACL injury. Direct referral for further investigations and a network of ACL specialist doctors will help you build a personalised, progressive recovery program to help you get stronger, move better, and back to activity sooner.
Get in touch with our team to book an assessment, and let's build a plan that keeps you strong, active, and pain-free today and into the future.
Give us a call now or click on BOOK AN APPOINTMENT to book online.
This article is general information only and does not replace individual clinical assessment. If you're experiencing joint or muscle pain, book an appointment with one of our physiotherapists for a tailored assessment and treatment plan.
Sources
- Arundale, A.J.H., Bizzini, M., Giordano, A., et al. (2018). Physical Therapy Evaluation and Treatment After Anterior Cruciate Ligament Reconstruction: Clinical Practice Guidelines. Journal of Orthopaedic & Sports Physical Therapy, 48(9), A1–A47.
- Dargo, L., Robinson, K.J. and Games, K.E. (2017). Prevention of Knee and Anterior Cruciate Ligament Injuries Through the Use of Neuromuscular and Proprioceptive Training. Journal of Athletic Training, 52(12), 1171–1172.
- Noyes, F.R. and Barber-Westin, S.D. (2021). ACL Injury Prevention Training Programs: The Injury Epidemic and Prevention Approaches. In: ACL Injuries in the Female Athlete. Springer.
- Filbay, S.R., Dowsett, M., Jomaa, M.C., Rooney, J., Sabharwal, R., Lucas, P., et al. (2023). Healing of acute anterior cruciate ligament rupture on MRI and outcomes following non-surgical management with the Cross Bracing Protocol. British Journal of Sports Medicine, 57(23), 1490–1497.
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