What Is the Main Function of the ACL in Your Knee?

The main function of the ACL (anterior cruciate ligament) is to stabilise your knee by limiting forward movement and rotation of the shin bone (tibia) under the thigh bone (femur). This stability is essential for walking, running, pivoting and landing safely.

If the ACL does not function properly due to injury, the knee may feel unstable or give way. For a full overview of ACL injury causes, symptoms and treatment options, see our ACL Injury condition page.

Short Answer

The main function of the ACL is to prevent the tibia from sliding too far forward and to control twisting forces at the knee. This allows the joint to remain stable during cutting, pivoting and landing tasks. When the ACL is torn, knee instability can develop, particularly during sport or sudden directional change.

How the ACL Stabilises the Knee Joint

The ACL sits deep inside the knee joint and connects the back of the femur to the front of the tibia. Because of this positioning, it resists forward translation of the tibia and limits excessive rotation.

The main function of the ACL becomes especially important during:

• Sudden direction changes
• Deceleration and landing
• Pivoting on one leg
• Contact sport movements

Without adequate ACL control, the knee may feel loose, unstable or prone to buckling.

How the ACL Works with Other Knee Ligaments

The ACL works together with other major knee ligaments, including the PCL, MCL, and LCL.

Together these ligaments:

• Maintain knee alignment
• Control multi-directional forces
• Protect cartilage and menisci
• Support safe load transfer during sport

If multiple ligaments are injured, knee stability may be significantly compromised.

When ACL Function Is Compromised

When the ACL is partially or completely torn, the knee may feel unstable, particularly during twisting or pivoting activities. Some people can manage with structured rehabilitation, while others may require surgical reconstruction depending on age, sport demands and instability episodes.

A physiotherapy assessment can clarify whether instability is present and guide appropriate rehabilitation strategies.

Why ACL Stability Matters in Sport and Daily Life

The main function of the ACL becomes even more important during high-speed or unpredictable movement. Sports such as football, netball, rugby and skiing place strong rotational and deceleration forces through the knee. The ACL helps resist these forces and keeps the joint aligned when the foot is planted and the body changes direction.

However, ACL stability also matters in everyday life. Walking downstairs, stepping off a curb, turning quickly or carrying load can all place rotational stress through the knee. When the ACL is not functioning properly, the knee may feel unreliable even during simple tasks. Over time, repeated instability episodes may increase stress on the meniscus and joint cartilage.

For this reason, restoring knee stability through appropriate rehabilitation is important whether or not surgery is required.

What This Means for You

If your knee feels unstable or has given way after a twisting injury, the ACL may be involved. Early assessment can identify instability, swelling patterns and functional limitations. Targeted rehabilitation may improve stability, strength and confidence, and help determine whether further medical review is required.

Related Information

• ACL Injury: Causes, Symptoms & Treatment
• What Are the Symptoms of an ACL Tear?
• Knee Pain & Conditions
• Physiotherapy Services

References

Brophy RH, Lowry KJ. American Academy of Orthopaedic Surgeons Clinical Practice Guideline Summary: Management of Anterior Cruciate Ligament Injuries. J Am Acad Orthop Surg. 2023;31(11):531-537. doi:10.5435/JAAOS-D-22-01020. Available from: https://pubmed.ncbi.nlm.nih.gov/36727995/

For comprehensive research summaries and rehabilitation pathways, visit our main ACL condition page: ACL Injury: Causes, Symptoms, Treatment & Rehabilitation