ACL Injuries Explained: Anatomy, Common Causes and Rehab Basics
The anterior cruciate ligament, usually called the ACL, is one of the most important stabilisers of the knee, and ACL ligament injuries are among the best known sports injuries. This article explains what the ACL is, how an ACL injury examination works, what guidelines say about rehab, and how to reduce the risk of a torn ACL. It is general information and does not replace an assessment by a qualified health professional.
What Is an ACL in Anatomy?
People often ask what does ACL stand for. It stands for anterior cruciate ligament. In anatomy, it is a strong band of connective tissue inside the knee that connects the thigh bone (femur) to the shin bone (tibia). It works alongside the posterior cruciate ligament (PCL), and the two ligaments cross inside the joint, which is where the word cruciate, meaning cross shaped, comes from.
Where Is the ACL Located in the Knee?
The ACL sits deep in the centre of the knee. It runs diagonally from the inner surface of the outer femoral condyle to the front of the top of the tibia. Because it lies inside the joint, it cannot be felt from the outside, and knee pain alone does not show which structure is injured.
What Does the ACL Do?
The ACL helps control how far the shin can slide forward beneath the thigh bone and how much the tibia can rotate inwards. This stability matters most during cutting, pivoting, landing and sudden stops. The ligament is also thought to contribute sensory information that supports knee control, which is why good rehab addresses movement quality and not just strength.
Can You Explain the Details of an ACL Injury?
An ACL injury is a sprain or tear of the ligament, ranging from a partial tear to a complete rupture. Many ACL injuries happen without contact, for example when an athlete lands awkwardly, sidesteps or changes direction quickly. A direct blow to the knee can also be responsible, and other structures such as the meniscus or other ligaments are sometimes injured at the same time.
Common Symptoms of an ACL Injury
A typical presentation after a high impact sporting incident includes a popping sound, rapid swelling, pain and a sense that the knee is unstable or giving way. Any knee that swells quickly, gives way or cannot take weight should be assessed promptly.
ACL Injury Examination and ACL Tests
An ACL injury examination usually begins with questions about how the injury happened, followed by checks of swelling, range of motion, strength and walking. Hands-on ACL tests then assess how the shin moves in relation to the thigh bone. Imaging such as MRI may be recommended to confirm the diagnosis and look for other injuries.
The Main ACL Tests Used in Clinic
The Lachman test, the pivot shift test and the anterior drawer test have long been the most widely used ACL tests, and the lever sign is a more recent addition. A systematic review and meta-analysis found that a positive pivot shift is particularly useful for ruling in a tear, while the lever sign performed best for ruling one out. Research on the lever sign is mixed, so results are interpreted with caution.
Why ACL Testing Needs the Full Picture
A 2022 systematic review and meta-analysis by Sokal and colleagues reported that the Lachman test is less accurate than earlier reviews suggested, particularly when the knee is examined some time after injury and when the tear is complete. For this reason, ACL testing is best seen as one part of the picture, combined with the injury history, several tests and imaging where needed.
How to Rehab an ACL Injury?
ACL rehab is a structured, progressive process, and it applies whether or not surgery is chosen. Guidelines support a criterion-based approach, where progression depends on meeting strength, movement and confidence milestones rather than on time alone. For people having reconstruction, the Royal Dutch Society for Physical Therapy guideline describes a prehabilitation phase followed by three postoperative phases, with rehabilitation continuing for 9 to 12 months.
The Main Stages of ACL Rehabilitation
Stage 1: Settle the Knee and Restore Movement
Early goals include managing swelling and pain and regaining knee movement. The 2023 Aspetar guideline, based on 140 randomised controlled trials, notes that recovering active end range knee extension by improving quadriceps activation is particularly useful in this phase.
Stage 2: Build Strength and Control
Progressive strengthening of the thigh, hip and calf muscles is combined with balance and landing control. The Aspetar guideline also gives guidance on elements such as eccentric training, plyometrics and cross-education.
Stage 3: Sport-Specific Training and Return to Play
The final stage introduces running, jumping and sport-specific drills. The Dutch guideline recommends using a battery of strength and hop tests, movement quality assessment and psychological readiness measures to guide progression. The Aspetar authors also note that certainty of evidence is low for many rehab components and that no progression criteria have been proven superior, so plans are individualised.
Common ACL Injury Myths and What the Evidence Supports
Myth: A Torn ACL Always Needs Surgery
In the KANON randomised trial of 121 young, active adults with acute ACL tears, rehabilitation plus early reconstruction was not superior to rehabilitation with the option of later surgery at two years or at five years. By five years, about half of the delayed group (51%) had chosen reconstruction. The decision depends on knee stability, activity goals and other injuries, and is best made with your treating team.
Myth: You Can Return to Sport Once the Knee Feels Fine
A knee that feels settled in daily life is not necessarily ready for pivoting sport. Objective strength and hop testing, movement quality and confidence all help guide the decision.
How to Prevent a Torn ACL: What the Evidence Supports
No program removes the risk completely, but the 2023 clinical practice guideline from the Academy of Orthopaedic Physical Therapy and the American Academy of Sports Physical Therapy recommends exercise-based knee injury prevention programs for athletes to reduce knee and ACL injuries. It singles out soccer players, especially women, as a group who should use them.
The guideline also describes how these programs should be delivered. Useful features include:
Training multiple times per week
Sessions longer than 20 minutes, with a weekly volume above 30 minutes
Multicomponent neuromuscular training that combines strength, balance, agility and plyometric work
High compliance, which matters as much as the choice of exercises
Physiotherapy for ACL Injuries in Balmain
At Sydney Allied Health Group, our physiotherapists assess knee injuries, explain what your examination findings mean, and build a rehab plan around your goals, whether that is walking comfortably, returning to sport or preparing for and recovering from surgery.
If your knee has given way, swelled quickly or feels unstable, a physiotherapy assessment is a sensible first step, and you should seek prompt medical review if you cannot take weight through the leg.
📅 Book a physiotherapy session with Sydney Allied Health Group in Balmain and get a treatment plan tailored to your knee and your goals.
References
Sokal, P. A., Norris, R., Maddox, T. W., & Oldershaw, R. A. (2022). The diagnostic accuracy of clinical tests for anterior cruciate ligament tears are comparable but the Lachman test has been previously overestimated: a systematic review and meta-analysis. Knee Surgery, Sports Traumatology, Arthroscopy, 30(10), 3287-3303. https://pmc.ncbi.nlm.nih.gov/articles/PMC9464183
Kotsifaki, R., Korakakis, V., King, E., et al. (2023). Aspetar clinical practice guideline on rehabilitation after anterior cruciate ligament reconstruction. British Journal of Sports Medicine, 57(9), 500-514. https://pmc.ncbi.nlm.nih.gov/articles/pmid/36731908
van Melick, N., van Cingel, R. E. H., Brooijmans, F., et al. (2016). Evidence-based clinical practice update: practice guidelines for anterior cruciate ligament rehabilitation based on a systematic review and multidisciplinary consensus. British Journal of Sports Medicine, 50(24), 1506-1515. https://bjsm.bmj.com/content/50/24/1506.short
Arundale, A. J. H., Bizzini, M., Dix, C., et al. (2023). Exercise-based knee and anterior cruciate ligament injury prevention: clinical practice guidelines. Journal of Orthopaedic & Sports Physical Therapy, 53(1), CPG1-CPG34. https://www.orthopt.org/uploads/content_files/files/jospt.2023.0301.pdf
Frobell, R. B., Roos, H. P., Roos, E. M., Roemer, F. W., Ranstam, J., & Lohmander, L. S. (2013). Treatment for acute anterior cruciate ligament tear: five year outcome of randomised trial. BMJ, 346, f232. https://www.bmj.com/content/346/bmj.f232.abstract
