What is the ACL?
The anterior cruciate ligament also known as the ACL is one of the four main ligaments inside the knee joint.
Ligaments: tough, rope-like bands of connective tissue that connect bones together and keep joints stable.
The ACL provides approximately 85% of the restraint against excessive forward movement of the tibia.
The ACL specifically connects the femur (thigh bone) to the tibia (shin bone). It runs diagonally through the center of the knee, crossing with the posterior cruciate ligament (PCL).
The ACL is one of the most frequently injured ligaments in the knee.
What does the ACL do?
The ACL has three critical jobs:
Control
Controls rotational stability, meaning it stops the knee from twisting inward during cutting, pivoting, and landing movements.
Prevents
Prevents the tibia from sliding too far forward relative to the femur.
Limits
Limits knee hyperextension (the knee straightening/extending too far)
The ACL contains mechanoreceptors:
- Sensory nerve endings that detect changes in joint position, speed, and direction of movement.
- Mechanoreceptors provide information about the joint position and movement (proprioceptive information) to the central nervous system, helping the brain coordinate muscle activation and joint stability.
- When the ACL is torn, many mechanoreceptors are also damaged, which reduces proprioceptive feedback and may contribute to re-injury.
The Knee: Surrounding Structures That Support the ACL and Their Role in Injury
The ACL does not work alone. It is supported by:
- Hamstring muscles (back of the thigh) – act as co-stabilizers of the knee and aid in resisting forward tibial movement
- Quadriceps muscles (front of the thigh) – the primary knee extensors; imbalance between quads and hamstrings increases ACL risk
- Gluteus medius and maximus (hip muscles) – control inward collapse of the knee during landing
- Calf muscles – assist with force absorption during landing and help stabilise the knee and ankle
- Menisci (cartilage discs inside the knee) – often damaged at the same time as an ACL tear
Neuromuscular training programs that include strengthening the surrounding muscles improve knee stability and have been shown to significantly reduce ACL injury risk.
What is an ACL Tear?
Learn about the anatomy, function, and injury mechanics of the anterior cruciate ligament in athletes.
An ACL tear occurs when the ligament is partially or fully torn from the forces placed on it. A full tear is the most common type in athletes and often requires surgical reconstruction followed by 9–12 months of rehabilitation.
The majority of ACL tears (around 70%) are non-contact injuries.
The most common injury situations include:
- Sudden deceleration followed by a change of direction (e.g., defender planting to cut)
- Landing from a jump with a stiff, straight-legged position and knees collapsing inward (valgus collapse)
- Pivoting or rotating around a planted foot
- Awkward sidestep cutting motions
High risk position: landing or pivoting with the knee in ‘valgus collapse’ (knee caving inward, hip internally rotating, with little bend in the knee.) This dramatically increases force on the ACL.
Treatment after an ACL
Re-injury statistics:
- About 1 in 5 athletes experience another ACL injury after returning to high-risk sports following ACL reconstruction.
- Female soccer players who return to soccer after ACL reconstruction have about a 2-5× higher rate of new knee injury compared to players who had never torn their ACL.
Return to competitive sport typically takes 9–12 months
About 65% of athletes return to their pre-injury level of sport
Athletes under 25 have a higher risk of re-rupture
ACL injuries are associated with increased risk of developing knee osteoarthritis later in life
Understand Your ACL Injury and Recovery Options
Discover how understanding ACL anatomy and injury prevention can help you stay active and reduce risks. Learn about treatment and rehabilitation choices tailored for athletes.