ACL injury: what happens next and how rehab works

What the ACL does and how it gets injured

The ACL runs diagonally through the middle of the knee, connecting the thigh bone to the shin bone. It stops the shin sliding forward and helps control rotation, which is exactly what your knee needs when you cut, pivot, land or stop suddenly.

Most ACL injuries are non-contact. They happen when landing from a jump with the knee collapsing inwards, changing direction at speed, or decelerating sharply with the foot planted. Football, rugby, netball, basketball, skiing and martial arts are common settings, but gym injuries happen too, for example landing awkwardly from a box jump. Contact injuries, such as a tackle to the side of the knee, often damage other structures as well.

Women have a higher rate of ACL injury than men in the same pivoting sports, for reasons that are still being studied, including differences in movement patterns, strength and anatomy. The good news is that injury prevention warm-up programmes that train landing, balance and strength have been shown to reduce ACL injuries.

Typical symptoms

• A pop or snap felt or heard at the moment of injury
• Immediate pain and difficulty continuing to play
• Rapid swelling, often within a few hours, due to bleeding in the joint
• A feeling of the knee shifting, giving way or being unstable, especially when turning
• Difficulty fully straightening or bending the knee in the first days

Pain and swelling often settle over a few weeks, and some people are surprised how normal the knee can feel for walking. The instability tends to show up when you twist, pivot or change direction, which is why an ACL injury should always be assessed properly rather than judged on how it feels walking around.

What an assessment looks at

A physio or doctor will ask exactly how the injury happened and check for swelling, range of movement and ligament stability. Specific tests, such as the Lachman test, check how far the shin moves forward on the thigh bone. They will also check the other ligaments and the menisci, because ACL injuries often come with a meniscus tear or damage to the medial ligament.

An MRI is commonly used to confirm the diagnosis and look for associated injuries, and a referral to an orthopaedic specialist is usually made to discuss options. In the meantime, rehab should start straight away; you do not need to wait for a scan or a surgical decision to begin.

Surgery or rehab alone?

There are two main paths. ACL reconstruction replaces the torn ligament with a graft, usually taken from your own hamstring or patellar tendon, followed by a long rehab programme. Rehab alone means a structured programme to restore strength, control and confidence without surgery, with the option of reconstruction later if the knee keeps giving way.

Research has shown that some people do well with rehab first, and that delaying reconstruction does not necessarily lead to worse outcomes for everyone. Reconstruction is more often recommended for people who want to return to pivoting sports, who have ongoing instability, or who have other injuries such as a repairable meniscus tear. The decision is individual and should be made with a specialist, considering your sport, goals and knee.

Either way, the rehab is long. Our guide to rehab after ACL reconstruction explains each stage after surgery in detail.

What the early weeks involve

Straight after the injury, the priorities are settling swelling, regaining full straightening of the knee, and getting the quadriceps working again. A knee that cannot fully straighten or a quad that will not switch on makes everything that follows harder, including surgery if you have it.

1. Walk with crutches if needed, progressing to normal walking as swelling and control allow.
2. Quad sets: tighten the thigh to push the back of the knee down, hold for 5 to 10 seconds, 10 to 20 times several times a day.
3. Heel slides and gentle bending to regain range.
4. Straight leg raises once the quad can hold the knee straight.
5. Compression, elevation above heart level and ice to manage swelling if they help.

Building back to sport

Once the knee is calm, rehab builds strength progressively with exercises such as the leg press, split squats and hamstring work, then adds balance, running, plyometrics, cutting and sport-specific drills. Progress should be based on meeting strength and movement criteria rather than dates alone.

Return to pivoting sport commonly takes 9 to 12 months after reconstruction, and sometimes longer. Returning too early is linked with a higher risk of re-injury. For people managing without surgery, return to straight-line activities and the gym is often possible within a few months, while pivoting sports need careful assessment. The on-site physio at Lycan Strength supports sports injury and post-operative rehab, with 20% off for members; you can book a physio appointment online.

When to get urgent help

• Call 999 or go to A&E if the knee looks deformed or dislocated, or the foot becomes cold, pale, numb or weak after the injury.
• Go to A&E if you cannot bear weight at all after the injury.
• See a GP or physio promptly for rapid swelling after a twist, or a knee that locks and will not straighten.
• Call NHS 111 or see a GP urgently for a swollen, warm, painful calf, especially after surgery or immobility. Call 999 if you also have chest pain or breathlessness.
• Go to A&E if the knee becomes hot, red and very painful with a fever, particularly after surgery.

Questions