What an ACL injury is
The anterior cruciate ligament, or ACL, runs through the middle of the knee from the thigh bone to the shin bone. It controls forward slide and rotation, which is why the knee feels trustworthy when you plant and turn.
About 70 per cent of tears happen with no contact from another player: landing from a jump on a nearly straight knee, a sudden stop, or a cut with the knee falling inward. People describe a pop, a feeling that the knee shifted, and swelling that fills the joint within two to four hours.
The swelling settles in two to three weeks and walking returns to normal. What remains in many people is giving way: the knee buckling on a turn, on uneven ground or stepping off a kerb. Injuries to the meniscus and to the ligament on the inner side of the knee often happen at the same time.
It is most common in football, netball, rugby, basketball and skiing. Women in pivoting sports tear the ACL two to eight times more often than men playing the same sport.
What causes it
The ligament fails when a high load arrives while the knee is close to straight and collapsing inward, faster than the muscles can react. That takes less than a twentieth of a second, which is why strength, landing technique and fatigue matter more than reflexes.
A previous ACL injury is the largest risk factor, for both the repaired knee and the other one.
Risk factors
- A previous ACL tear in either knee
- Pivoting, cutting and jumping sports, and skiing
- Landing or cutting with the knee drifting inward
- Weak hamstrings and glutes relative to the quadriceps
- Fatigue late in a match or at the end of a ski day
- Returning to sport after a lay-off without rebuilding strength
- Being female in a pivoting sport
Time alone is a poor guide to readiness. Two measures do the work. The first is the calendar: for every month that return to pivoting sport is delayed, up to nine months after surgery, the re-injury rate falls by about half. The second is testing: quadriceps strength and hop distance both within 10 per cent of the other leg. Athletes who passed those tests before returning had around 84 per cent fewer re-injuries than those who went back without passing. A knee that feels fine at six months has usually passed neither.
What the evidence says
Surgery is one route among two. In a randomised trial of 121 young active adults, early reconstruction was compared with structured rehabilitation and the option of surgery later. Outcomes at two and five years were the same, and about half of the rehabilitation group never had an operation.
Reconstruction is generally advised for people returning to pivoting sport, for knees that keep giving way despite good rehab, and where a repairable meniscus tear is present.
Rehabilitation before surgery improves the result after it. Full straightening, minimal swelling and good quadriceps strength going in predict better function two years later.
After reconstruction, around four in five people return to some sport, about two in three to their previous level and roughly half to competitive play. Completing rehab to the testing stage is what separates the groups.
In athletes under 25 who return to pivoting sport, close to one in four has a second ACL injury, to the graft or the other knee. Meeting return criteria and continuing an injury prevention programme reduce that.
Neuromuscular warm-up programmes that train landing, cutting and hamstring strength reduce ACL injury rates by around half in team sports.
Recovery timeline
Without surgery, a structured programme runs three to six months before a return to straight-line sport, with a decision point at about three months on whether the knee is stable enough for your goals.
After reconstruction, expect nine to twelve months before pivoting sport. Jogging usually starts at around three to four months, sport-specific drills at six, and full training before any match play.
Desk work is realistic one to two weeks after surgery. Driving returns at about four to six weeks for a right knee, once you can brake hard without hesitation.
| Stage | ✓ Do this | ✗ Avoid this |
|---|---|---|
| Weeks 0–2 | After injury or surgery: get the knee fully straight, bring the swelling down and wake up the quadriceps. Use crutches until you can walk without a limp. | Resting with the knee bent over a pillow |
| Weeks 2–12 | Full range, normal walking, then strength: squats, leg press, hamstring curls, step-downs, single-leg balance. Bike and pool. | Running before the swelling has gone and the knee is fully straight |
| Months 3–6 | Start running. Build single-leg strength to within 20 per cent of the other side. Add jumping and landing drills on two legs, then one. | Twisting and cutting sports, including a kickabout |
| Months 6–9 | Change-of-direction work, sport drills without contact, then full training. Test strength and hops. | Match play because the knee feels normal |
| Return to sport | Nine months or more, tests within 10 per cent, and several weeks of unrestricted training first. Keep a prevention warm-up for good. | Stopping strength work in the first season back |
How we treat it at Kingsway
Rehabbing an ACL
The first session establishes what is injured, where you are on the pathway and what your sport demands, then sets measured targets for each stage. If the knee has not been seen by a doctor, that referral comes first.
- Ligament and meniscus testing, with referral to your GP or a knee specialist for imaging where the findings call for it
- Range, swelling and quadriceps activation measured and recorded
- Manual lymphatic drainage and soft tissue work for swelling and stiffness in the early weeks
- A progressive strength programme written for the gym you use, updated at each stage
- Landing, hopping and change-of-direction training in the later stages
- Strength and hop testing against the other leg before a return to sport
ACL rehab is long, so sessions are spread out: weekly in the first six weeks, then every two to four weeks as the gym work takes over, with a testing session before each step up. Your surgeon sets any restrictions on range, bracing and weight bearing, and the programme follows them.
When to get it checked
See your GP, a minor injuries unit or a knee clinic within a few days of any knee injury that came with a pop, fast swelling or giving way. Book an assessment here to start prehab while you wait, at any stage after a reconstruction, or if you are past six months and unsure whether you are ready to play.
See a doctor or go to A&E if
- A knee that is locked and will not fully straighten
- A cold, pale or numb foot after a knee injury: call 999
- An obvious deformity, or inability to take any weight
- After surgery: a swollen, hot, painful calf, or breathlessness or chest pain
- After surgery: a hot, red knee with fever, or a wound that is leaking
Common questions
Do I need surgery for an ACL tear?
Many people do well without it. In the main trial comparing the two routes, about half of those who started with rehabilitation never needed an operation, with the same results at five years.
Surgery is usually advised if you want to return to pivoting sport, if the knee keeps giving way after three months of good rehab, or if there is a meniscus tear that can be repaired.
How long until I can play again after ACL reconstruction?
Nine to twelve months for football, netball, rugby, skiing and similar sports. Each month you cut from that roughly doubles the re-injury risk.
The date is a minimum. You also need strength and hop tests within 10 per cent of the other leg.
Can I run and go to the gym without an ACL?
Straight-line running, cycling, swimming and most gym training are achievable for many people without reconstruction, once strength is rebuilt.
Sports with cutting and landing are where an ACL-deficient knee tends to give way.
What should I do while waiting for surgery?
Prehab. Get the knee fully straight, clear the swelling and build quadriceps and hamstring strength for four to six weeks or more.
People who go into surgery with a quiet, strong knee have better function two years later.
Will I get arthritis in the knee?
The risk of knee osteoarthritis rises after an ACL injury with or without surgery, and it is higher when the meniscus is damaged too.
Keeping the leg strong, staying at a healthy weight and avoiding repeat giving-way episodes are the factors within your control.