ACL injury: recovery and rehabilitation

In short A torn anterior cruciate ligament (ACL) is one of the best-known knee injuries in sport. Targeted exercise therapy with a physiotherapist is the basis of recovery, with or without surgery. Research shows that many people recover well without surgery, but surgery may be needed if the knee stays unstable. Expect a programme of several months. You return to your sport only when tests show your knee is ready.

What is the anterior cruciate ligament?

The anterior cruciate ligament is a strong band in the middle of your knee. It connects your thigh bone to your shin bone and has two important jobs.

1. Protection under sudden load. In an injury, such as a bad landing or a sudden twist, the ligament stops your lower leg from sliding forward or rotating too far inwards.^[24][25]

2. Guiding how your knee moves. In everyday movement too, the ligament helps set the limits within which your thigh and shin move relative to each other: when bending, straightening and turning. It also contains nerve endings that constantly tell your brain where your knee is and how it is moving, so your muscles can adjust in time.^[23] Even after surgery, this "sense" of the knee often stays reduced for months to years.^[23] That is why rehabilitation trains not only strength, but also control and balance.

[Image: simple drawing of the knee with the anterior cruciate ligament]

How does an ACL injury happen, and who is at risk?

Most injuries happen in sports where you turn, cut or land quickly, such as football, basketball, handball, volleyball and skiing. More than half of ACL injuries in ball sports happen without contact with another player, for example during a bad landing or a sudden turn.^[3]

Women and girls in team sports have a clearly higher risk than men: in a large review, the rate of non-contact injuries was about three times higher.^[3] Most injuries happen during matches rather than in training.^[3]

Do you recognise this?

• You felt or heard a "crack" or "pop" in your knee.
• Your knee swelled up within a few hours.
• You could not continue playing.
• Your knee feels unreliable, as if it "gives way" when you turn.

An ACL injury often comes with damage to the meniscus or other ligaments, so a thorough examination matters.

When to see a doctor or go to A&E straight away?

See your GP or go to the emergency department straight away if you notice any of these signs. In brackets is what the sign may point to; only a doctor can confirm it.

Sign	May point to
Your knee is "locked": you can no longer straighten it fully	A torn meniscus that is caught in the joint
Your foot becomes cold, pale or numb	Damage to a blood vessel or nerve, for example in a dislocated knee. This is urgent.
You cannot put any weight on your leg, or the pain is very severe	A fracture or a more serious injury
Your knee is red, warm and painful and you have a fever	Inflammation or infection of the joint

When should you see a physiotherapist?

As soon as possible, ideally within a few days of the injury, whether or not you later choose surgery. Moving early and safely is important for your recovery.

The swelling after an ACL injury brings three risks in the first weeks:

1. Fear of movement. A swollen, painful knee makes people cautious. Fear of moving is linked to worse knee function and is best tackled early.^[28]
2. Muscle loss through muscle inhibition. Swelling and pain make your nervous system automatically switch down your thigh muscle (quadriceps). This is called arthrogenic muscle inhibition. Your muscle quickly gets weaker, and the effect can last a long time.^[26] Cooling and targeted exercises work best to break through this inhibition.^[27]
3. Instability. A swollen knee with an inhibited thigh muscle almost always feels unstable at first. That says little about how stable your knee will be later (see "Will my knee stay unstable?").

A supportive brace during activities. If a side ligament of the knee is also injured, for example the inner (medial) or outer (lateral) collateral ligament, we recommend a hinged knee brace during activities in the first period. It protects the side ligament from extra stretch while it heals. Keeping moving remains important: immobilising for too long weakens the ligament.^[29] A mild sprain of the side ligament usually needs no brace.^[29]

Do I need a referral?

• After surgery: rehabilitation after an operation still requires a doctor's referral, also after 2027.
• Without surgery: from 1 January 2027 you can see a physiotherapist without a referral for many muscle and joint complaints. Read more about direct access
• If an ACL tear is suspected: we work closely with your GP or orthopaedic surgeon, because a doctor usually confirms the diagnosis and discusses with you whether surgery makes sense.

How do we approach it at Magnus?

1. First consultation. We ask how the injury happened, which sport you play and what your goal is. We examine your knee: swelling, mobility, strength and stability.

2. Choosing together. We discuss with you, and with your doctor where needed, whether you start with rehabilitation or choose surgery straight away.

3. Rehabilitation in phases. We build up your exercises step by step, from settling the knee down to jumping, turning and sport-specific training.

4. Testing before you return. Before you go back to sport, we measure your strength, hop tests and how confident you feel.

What works?

With or without surgery?

This is the most important question people ask us. What research tells us:

• Returning to sport. In a 2025 review, people who had surgery did not return to their sport more often than people who only did rehabilitation. Limited evidence^{¹ [5]}
• Knee function. On average, surgery gives a small improvement in knee function, but no less pain than exercise therapy. Strong evidence^{¹ [6]}
• Rehabilitation first. In a large Dutch study (COMPARE), half of the people who started with rehabilitation did not need surgery later. Those who had surgery straight away scored slightly better after two years, but the researchers do not know whether that difference is noticeable in daily life.^[7]
• If the knee stays unstable. People whose knee keeps "giving way" after rehabilitation benefit more from surgery, according to a large British study (ACL SNNAP).^[8]
• Meniscus and osteoarthritis. After surgery, there is less chance of a later meniscus operation, but surgery does not prevent osteoarthritis (wear) in the long term. Limited evidence^{¹ [9][10]}

What does this mean for you? Both routes are good options. The choice depends on how stable your knee is, which sport you want to do and what matters to you. If you choose surgery, rehabilitation with a physiotherapist is just as important as the operation itself.

Can a torn ACL heal on its own?

For a long time the answer was thought to be no, but that picture is changing.

• Partial tear. A partial tear occurs in about 10 to 27% of isolated ACL injuries. Recovery without surgery is often possible. In young, active athletes, a partial tear can sometimes progress to a complete tear, so good follow-up remains necessary.^[24]
• Complete tear. Spontaneous healing has also been described after a complete tear, especially when the ligament tore close to the thigh bone.^[30][41]
  • In a German study, the ligament was continuous again on an MRI scan after six weeks in 14% of people.^[31]
  • In an Australian study with a special brace that keeps the knee bent for the first weeks, researchers saw signs of healing on MRI after three months in 90%.^[32]
  • These studies are still small, and it is not yet clear who has the best chance of healing. Limited evidence^{¹ [30]}

Will my knee stay unstable?

How unstable a knee feels does not depend only on the tear itself:

• At first almost every knee feels unstable, because of the swelling and the inhibited thigh muscle.^[26]
• Training changes a lot. In a large study from Delaware and Oslo, almost half of the people first classified as "unstable" were rated stable enough after a few weeks of targeted strength and control training.^[33]
• Strength matters more than laxity. In people with a torn ACL, their own rating of their knee was linked to the strength of their thigh and hamstring muscles, not to how much play there was in the knee.^[34]
• How many people still need surgery in the end? In the large studies where people started with rehabilitation, 41 to 50% later had surgery after all, usually because the knee kept giving way.^[7][8]

That is why we only judge the stability of your knee properly once the swelling has gone and your muscles are working as they should.

Rehabilitation: what helps?

• Exercise therapy is the basis, with or without surgery: that is what the guidelines recommend.^[1] There is no single "best" exercise; no type of exercise turns out to work better than another. What matters is a well-structured programme tailored to you. Strong evidence^{¹ [6]}
• Exercising before surgery may help you regain strength and function faster after the operation. Limited evidence^{¹ [11][2]}
• Electrical muscle stimulation of the thigh muscle helps you regain strength in your upper leg, alongside exercises. Strong evidence^{¹ [2]}
• Blood flow restriction (BFR) training. With a band around your upper leg you train with light weights and still build muscle strength and mass. For people who cannot yet use heavy weights, this works better than ordinary light training. Strong evidence^{¹ [35]} After ACL surgery, BFR improved muscle strength, knee function and pain in studies, but the results vary between studies. Limited evidence^{¹ [36][18]} All our practices have the equipment for BFR training.
• Cooling in the first weeks after surgery reduces pain and the need for painkillers. Limited evidence^{¹ [2][1]}
• Attention to fear and confidence. About one in four people do not return to sport after surgery. Fear of a new injury is the most common reason.^[12] Support that addresses this reduces that fear in the short term. Limited evidence^{¹ [13]}

Possible complications, and why the first weeks matter so much

Most people recover well after an ACL injury or surgery. Still, there are complications worth knowing about, because you can do a lot to prevent them yourself.

Stiffness from scar tissue

After an injury or operation, your body produces scar tissue. Sometimes there is too much of it or it gets in the way, so the knee becomes stiff and no longer bends or straightens fully. This is called arthrofibrosis.

• How often? About 1.5% of people need an extra procedure after ACL surgery because the knee stays too stiff.^[42]
• What helps? Early, well-dosed physiotherapy is the most important way to prevent it.^[43]

Read more about scar tissue after surgery.

Cyclops lesion

A cyclops is a nodule of scar tissue that forms at the front of the knee, just in front of the new ligament. It can stop you from straightening your knee fully.

• How often? A cyclops that causes symptoms occurs in about 2 to 11% of people after ACL surgery.^[44]
• Who is at higher risk? People whose knee is still inflamed or does not yet have its full range of movement before surgery, and people whose hamstrings stay tense after surgery.^[44]
• Treatment. If a cyclops causes symptoms, the surgeon usually removes it through keyhole surgery, with good results.^[45]

Muscle inhibition that lingers

In more than half of people with a fresh ACL injury, the nervous system does not activate the thigh muscle properly (arthrogenic muscle inhibition). In almost 8 out of 10 of them, this inhibition disappeared during the first consultation with simple, targeted exercises.^[46] According to French experts, it is better to operate only once this inhibition has gone, because it increases the risk of a stiff knee after surgery.^[47]

Other complications after surgery

• Infection: rare, in about 0.6% of operations.^[48]
• Blood clot with symptoms: in about 0.4%.^[49]
• Pain where the new ligament was taken from: for example at the front of the knee when kneeling. How often this happens depends on the type of tendon the surgeon uses.^[50]

The right balance in the first weeks

In the first weeks after an injury or operation, it is all about the right balance:

• Active exercises to activate your muscles, and passive mobilisation by the physiotherapist to let the knee straighten and bend fully;
• Enough movement to prevent stiffness and scar tissue, but not too much too soon. Putting full weight on the leg too early after surgery was linked in studies to more laxity in the knee.^[51]

Your physiotherapist adjusts this every week to how your knee responds. The same principle applies after other major operations, such as a knee replacement or a meniscus repair. Read more about postoperative physiotherapy.

Popular treatments, but little evidence

Some treatments are often used or asked about. This is what the research says:

Treatment	What does the research say?	Label
Knee brace after ACL surgery, as standard	In an isolated ACL injury, a brace after surgery does not improve knee function, stability or pain. If a side ligament is also injured, a brace is useful (see "When should you see a physiotherapist?").	No evidence¹ [2][14]
Continuous passive motion (CPM) machine after surgery	A machine that moves your knee passively can reduce some pain and swelling and help bending in the first weeks, but the studies are of moderate quality. It does no more than active exercise with your physiotherapist.	Limited evidence¹ [1][2][52]
Kinesio tape	Can reduce pain and swelling slightly in the first weeks after surgery, but does not improve knee function.	Limited evidence¹ [15]
PRP injections (platelets) during surgery	Possibly slightly less pain in the short term, but no better recovery of the ligament or the knee after a year.	Limited evidence¹ [16][17]

"No evidence" does not mean something is harmful. If something feels good and is safe, you may use it, but it does not replace active rehabilitation.

How long does recovery take?

At Magnus we follow our own rehabilitation protocol in five phases. The weeks below apply after surgery and are a guide: you only move on to the next phase when you pass the tests of your current phase.

Phase	When (after surgery)	Goal
Before surgery	Ideally start 2 to 5 days after the injury	Reduce swelling, explain the operation and rehabilitation, mobility, strength, control
1. Recovery from surgery	Weeks 0 to 6	Straighten the knee fully and bend it to about 125°, walk normally, reduce swelling and muscle inhibition
2. Strength and control	Weeks 6 to 15	Balance and strength on one leg, full mobility
3. Running, agility and landing	Weeks 14 to 22	Equal strength and jumping left and right, return-to-sport tests
4. Return to sport	Men usually 6 to 9 months, women 6 to 11 months	Pass sport-specific tests
5. Preventing new injuries	After that	Maintain results and retest regularly

Women need a little longer on average: in a review, female athletes returned to their sport after an average of 10.8 months.^[37]

What can you do yourself?

• Keep moving within what your knee can handle. Complete rest does not help.
• Do your exercises at home. A well-structured home programme, set up with your physiotherapist, works about as well for strength as exercising in the practice.^[2]
• Cool the knee in the first weeks if it is swollen or painful.
• Ask about your fear or doubts. That is a normal part of recovery.

Back to sport

• How many people return? About eight in ten people return to some form of sport after surgery. About two thirds get back to their previous level, and just over half return to competition.^[1]
• Testing before you return. We follow the testing protocol we developed at Magnus for ACL rehabilitation.
  • Strength of your thigh muscle (quadriceps) and hamstrings, measured objectively with a hand-held dynamometer, a portable device we use in all our practices. In research in healthy adults, such devices proved reliable and valid.^[53] We compare your operated leg with your other leg. In the last phase we aim for at least 95% of the strength of your other leg.
  • The strength ratio between quadriceps and hamstrings: we measure this with the same hand-held dynamometer and monitor it as well.
  • A series of single-leg hop tests, forwards, sideways and with rotation: at least 90% of your other leg.
  • Questionnaires about your knee function and how confident you feel.
  • Download our full rehabilitation protocol (pdf)
• Why these tests? A strength difference between left and right often persists for years after surgery.^[39] A combination of strength tests and hop tests with fixed cut-off values is linked to fewer new injuries.^[38] Passing the tests lowers the chance of re-tearing the operated ligament, but the tests are no guarantee. Limited evidence^{¹ [19][38]} The strength ratio between quadriceps and hamstrings alone predicts a new injury poorly; that is why we always look at the whole picture.^[40]
• When can I drive again? Only when you can safely make an emergency stop. After surgery on the right knee that is usually after 4 to 6 weeks, on the left knee after 2 to 3 weeks.^[1]

Can you prevent a (new) ACL injury?

Yes. Prevention programmes with jumping, strength, balance and landing exercises, for example as a warm-up, reduce the risk of an ACL injury by about half. Strong evidence^{¹ [4][20][21]} We are happy to help you, your team or your club get started.

At Magnus near you

Leuven, Naamsepoort
Erasme Ruelensvest 11, 3000 Leuven. Open 8am to 8pm.

Kessel-Lo
Platte Lostraat 278, 3010 Kessel-Lo. Open 8am to 8pm.

Antwerp, Eilandje
Tunnelplaats 1, 2000 Antwerp. Open 8am to 8pm.

Frequently asked questions

Can I play sport again without surgery?
Yes, many people can. In research, people who only did rehabilitation returned to their sport just as often as people who had surgery.^[5] If your knee stays unstable, surgery is still possible later.^[7][8]

Do I need surgery quickly?
Not always. Starting with rehabilitation and only operating if it turns out to be necessary is a good option: in the COMPARE study, half of that group did not need surgery later.^[7] Discuss this with your doctor and physiotherapist.

Do I need a knee brace after my surgery?
For an isolated ACL injury, usually not: after surgery a brace does not improve function, stability or pain.^[2][14] If a side ligament is also injured, a brace is useful in the first period.^[29] Always follow your surgeon's advice.

Can a torn ACL heal on its own?
With a partial tear, that is often possible.^[24] Healing has also been described after a complete tear, but the research is still limited.^[30][31][32]

When can I drive again?
When you can safely make an emergency stop: usually 4 to 6 weeks after surgery on the right knee and 2 to 3 weeks on the left knee.^[1]

Will I get osteoarthritis later?
An ACL injury increases the risk of osteoarthritis. In the long term, about one in three people have osteoarthritis after surgery, and surgery does not reduce that risk.^[10] An international group of experts therefore recommends well-supervised rehabilitation after a knee injury and long-term follow-up of your knee health.^[22]

Is my physiotherapy reimbursed?
Yes, with a referral. Rehabilitation after ACL surgery often falls under the "Fa pathology" list, which means more sessions are reimbursed. Your hospitalisation insurance often covers part of the rest, but usually only for a limited period. Read more about reimbursement

Reviewed by Magnus Kinesitherapie. Last updated: 20 October 2026.

^¹ What do the labels mean? Strong evidence: several good studies or reviews reach the same conclusion. Limited evidence: there are few or small studies, or the results contradict each other; it may help, but it is not certain. No evidence: good studies find no effect greater than a placebo, or there are no good studies yet. No evidence does not mean something is harmful. We base our information on reviews and guidelines published in 2019 or later.

This page gives general information and does not replace an examination by a doctor or physiotherapist.

Glossary

Difficult words on this page, explained in plain language. The explanations are written in our own words, based on the source given.

Sources

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