The ACL is one of the major ligaments inside the knee. It works with the surrounding ligaments, muscles, menisci and joint surfaces to help keep the knee controlled during daily activities and sport.

When the ACL is injured, the knee may feel unstable or give way during twisting, landing or sudden directional changes. For a broader explanation of symptoms, assessment and management, see our ACL injury guide.

How Does the ACL Stabilise Your Knee?

The ACL runs diagonally through the centre of the knee and connects the femur to the tibia. Its primary mechanical role is to resist excessive forward movement of the tibia relative to the femur.

The ligament also contributes to rotational stability. This role becomes particularly important when your foot remains planted while your body turns, slows down or changes direction.

When Is ACL Control Most Important?

The ACL experiences greater demands during movements that combine speed, deceleration, rotation and a planted foot.

These movements commonly include:

• cutting or side-stepping;
• landing after a jump;
• pivoting on one leg;
• slowing down quickly;
• changing direction during field or court sports; and
• recovering balance after an unexpected movement.

Football, rugby, netball, basketball and skiing often involve these movement demands. However, ACL function also matters when turning quickly, stepping off a kerb, carrying a load or walking on uneven ground.

Does the ACL Work Alone?

No. The ACL forms part of a larger knee-stability system. It works with the muscles around the hip, thigh and lower leg, as well as the joint capsule, menisci and other major knee ligaments.

These ligaments include the:

• posterior cruciate ligament, or PCL;
• medial collateral ligament, or MCL; and
• lateral collateral ligament, or LCL.

Together, these structures help maintain knee alignment, guide movement and manage forces travelling through the joint.

What Happens When the ACL Is Torn?

An ACL tear may reduce the knee’s ability to control forward and rotational movement. Some people notice instability during sport, while others experience giving-way episodes during daily activities.

Possible features of an ACL injury include:

• a popping sensation at the time of injury;
• rapid swelling;
• pain and reduced knee movement;
• difficulty continuing sport; and
• a feeling that the knee is unstable or unreliable.

These symptoms do not confirm an ACL tear on their own. A clinical assessment can examine the injury mechanism, swelling, movement, ligament stability and functional capacity. Read more about the symptoms of an ACL tear.

Can Rehabilitation Improve Knee Stability?

Structured rehabilitation can improve strength, movement control, balance, landing technique and confidence. These changes may help some people return to valued activities without repeated instability.

Rehabilitation commonly addresses:

• knee swelling and movement;
• quadriceps and hamstring strength;
• hip and calf capacity;
• balance and single-leg control;
• running, landing and change-of-direction skills; and
• sport-specific or work-specific demands.

Management decisions vary between people. Some people manage successfully with rehabilitation, while others may consider ACL reconstruction because of repeated instability, associated knee injuries, occupational demands, sporting goals or personal preferences.

Learn more about ACL treatment and rehabilitation options.

Why Do Repeated Giving-Way Episodes Matter?

A knee that repeatedly gives way may place additional stress on the menisci and joint surfaces. Ongoing instability should therefore be assessed, particularly when it limits work, exercise or sport.

An assessment can help determine whether the symptoms relate to ACL instability, another knee injury or a combination of problems. It can also guide activity modification and the next stage of rehabilitation or medical review.

What Should You Do Next?

Consider a physiotherapy assessment if your knee feels unstable after a twisting or landing injury. Your physiotherapist can examine ligament stability, movement, strength and functional tasks before discussing suitable management options.

Early advice may also help you protect the knee, manage swelling and begin appropriate rehabilitation while the diagnosis and longer-term plan become clearer.

Related ACL and Knee Information

ACL Injury

Learn about common injury mechanisms, symptoms, assessment and management options.

ACL Tear Symptoms

Review common signs that may follow a twisting, pivoting or landing injury.

ACL Injury Prevention

Explore strength, balance, landing and movement strategies that may reduce ACL injury risk.

Knee Ligament Injuries

Compare the roles and common injuries of the major knee ligaments.

References

1. Brophy RH, Lowry KJ. American Academy of Orthopaedic Surgeons Clinical Practice Guideline Summary: Management of Anterior Cruciate Ligament Injuries. Journal of the American Academy of Orthopaedic Surgeons. 2023;31(11):531–537. View the AAOS guideline summary on PubMed.
2. Evans J, Mabrouk A, Nielson JL. Anterior Cruciate Ligament Knee Injury. StatPearls. Last updated 17 November 2023. Read the ACL injury clinical overview.
3. Yoo H, Marappa-Ganeshan R. Anatomy, Bony Pelvis and Lower Limb: Knee Anterior Cruciate Ligament. StatPearls. Read the ACL anatomy and function overview.

ACL Function FAQs

What is the main function of the ACL?

The ACL’s primary mechanical function is to limit excessive forward movement of the tibia relative to the femur. It also contributes to rotational knee control.

Does the ACL control twisting?

The ACL contributes to rotational control, particularly when the foot is planted and the body pivots, lands or changes direction. Other ligaments, muscles and joint structures also help control rotation.

Can you walk with a torn ACL?

Some people can walk after an ACL tear, especially once pain and swelling settle. However, walking ability does not rule out an ACL injury, and the knee may remain unstable during turning, sport or uneven-ground activities.

Why does a knee with an ACL tear give way?

Giving way can occur when the knee cannot adequately control forward or rotational movement during a demanding task. Pain, swelling, muscle inhibition and associated knee injuries may also contribute.

Can rehabilitation help without ACL surgery?

Some people regain good strength, control and function through structured rehabilitation without surgery. Suitability depends on instability, associated injuries, activity demands and personal goals.

When should an ACL injury be assessed?

Arrange an assessment after a twisting or landing injury if the knee swells rapidly, loses movement, feels unstable, repeatedly gives way or prevents you from returning to normal activity.