Common, and mostly minorStrong evidence
Across 44 studies of professional men, injuries ran at 8.1 per 1,000 hours, 36 in matches against 3.7 in training; "The thigh was the most commonly injured anatomical region followed by the knee" (1.2 per 1,000 hours), and "Minor injuries (1-3 days of time loss) were the most common". In 43 studies of players aged 19 and under, "In male players, the thigh (1.2 injuries/1000 h) and ankle (0.9 injuries/1000 h) were the anatomical regions where injuries occurred most frequently, whereas the knee (1.5 injuries/1000 h) and ankle (1.5 injuries/1000 h) were the regions where injuries were reported most frequently among females". In women's football, "Slight/minimal injuries (1-3 days of time loss) were the most common".
In 456 Dutch amateur men, "The most commonly injured body parts (n=408) were ankle (19.1%), posterior upper leg (15.9%), knee (15.7%), anterior upper leg (10.5%) and groin (10.5%)", and an umbrella review puts 60% to 90% of football injuries in the legs, especially the ankle, knee and thigh. The NHS says knee pain "can often be treated at home" and "You should start to feel better in a few days". Elite figures come from professionals, who play more and harder than most; grassroots data are thinner.
Sources 1, 2, 19, 20, 21, 3
How long knee problems lastEmerging evidence
In 22,942 injuries in elite men's clubs over 16 years, median days out were 4 for a knee bruise, 6 for synovitis (an irritated joint lining), 7 for patellar tendon pain, 13 for the outer ligament (the lateral collateral ligament, or LCL), 16 for the MCL, 22 for cartilage, 36 for the outer meniscus and 205 for the ACL: "Severe injury diagnoses were shown to be quite uncommon with only two of the 31 most common injury diagnoses causing a median absence of more than 28 days, ACL tears and lateral meniscus injury". ACL tears were 0.9% of all injuries. After a partial meniscus removal, elite players were back in a median 5 weeks for the inner and 7 weeks for the outer meniscus.
Sources 4, 5, 68
Tackles and the inner knee; twists and the meniscusEmerging evidence
"Medial collateral ligament (MCL) injury is the most common knee ligament injury in professional football", 1.31 per 1,000 match hours against 0.14 in training, with an average of 23 days out, and "Three-quarter of the MCL injuries occurred with a contact mechanism", most often being tackled. The American Orthopaedic Society for Sports Medicine (AOSSM) says "Most MCL sprains can be treated without surgery".
The NHS: "A meniscus tear usually happens when you twist your knee while playing sport. But it can also happen from more minor injuries such as twisting when standing up", with pain, swelling that may take hours or days, difficulty straightening and the knee giving way.
Sources 5, 6, 62, 7
The ACL: usually without a direct blowEmerging evidence
The AOSSM: "ACL (anterior cruciate ligament) injuries are primarily non-contact injuries occurring when the leg rotates or pivots while planted on the ground". In video studies of professional men, "Eighty-five per cent of the ACL injuries in male professional football players resulted from non-contact or indirect contact mechanisms", and "The most common playing situation leading to injury was pressing followed by kicking and heading"; in professional women, "88% of injuries involved no direct contact to the knee", and in one German league 7 of 17 non-contact injuries came in the first 15 minutes of a match.
In elite clubs, match play carried about 20 times the training risk, and 58% of tears were non-contact. The American Academy of Orthopaedic Surgeons (AAOS): "Interestingly, most ACL injuries happen without direct impact to the knee".
Sources 69, 8, 22, 23, 24, 70
Women and girls: about twice the rate, but a low absolute riskStrong evidence
Across 28 studies, "The IR of ACL injury among women was 2.2 times higher than the IR of ACL injury among men" (IR is incidence rate), though "Overall, more men sustained ACL injury in football" because more men play. In elite clubs, women's risk was 2.6 times men's (2.4 after allowing for age), and a review of 33 articles found women's risk 2 to 3 times men's, mostly in matches. Among US teenagers across sports, girls' soccer had the highest ACL rate, 0.166 per 1,000 exposures. The AAOS gives a wider range across sports: "Female athletes experience ACL injuries at rates 2 to 8 times higher than males".
The absolute risk is low: about 2 tears per 10,000 sessions in women and 1 in men, and in the authors' words of US college data, "the actual rate of injury remains low and should not be a deterrent to participation in sports". UEFA, European football's governing body, says "ACL injuries continue to affect 1-3% of players during a season" and that "contrary to media reports, there is no epidemic in the women's game".
Sources 9, 24, 25, 26, 10, 70, 66
The front of the knee: kneecap pain and the patellar tendonEmerging evidence
Kneecap pain and jumper's knee build up from running, jumping and kicking: the NHS lists "Pain between your kneecap and shin, often caused by repetitive running or jumping" as tendonitis. In elite men, "Each season, 2.4% of players were affected, with most injuries (61%) resulting in absence up to 1 week or less", 20% came back, and more hours of play went with more risk.
No study links a particular kicking technique to knee pain; knee pain on kicking can be the kneecap, the tendon, or, after a twist, the meniscus.
Sources 3, 11
Growing players: Osgood-Schlatter and Sinding-Larsen-JohanssonMixed evidence
About 1 in 10 young teenagers has Osgood-Schlatter, more with regular sport. In one elite Dutch academy, 17% of boys had it at a given time, and "80% had no time-loss despite presence of clinical symptoms"; but across six Dutch academies Osgood-Schlatter ran at 0.22 per 1,000 hours, peaked at under-13 and under-14, and "has the highest injury severity and burden compared to other types of GRSI" (GRSI is growth-related sports injuries). How much training it costs is conflicting between studies. In Japanese junior players, "Questionnaires were collected from 1162 players of 97 teams, and 547 players (47.1%) complained of pain in the lumbar spine or legs", with Osgood-Schlatter and Sinding-Larsen-Johansson among the growth-plate conditions found.
NHS inform says the pain "usually happens in active young people who are going through a growth spurt" and "Sport or physical activity is not likely to cause any permanent issues or problems". It isn't growing pains, which the NHS describes as aching in the legs in the evening or at night, gone by the morning. Most Osgood-Schlatter settles, but not all: in 36 boys from elite academies, symptoms lasted about 18 days on average, yet knee function scores were lower than in boys who hadn't had it, and in a small hospital follow-up, "This study indicates that OSD may not always be self-limiting" (OSD is Osgood-Schlatter disease).
Sources 28, 12, 27, 71, 65, 64, 30, 29
Former players and arthritisStrong evidence
In 1,207 retired English professionals against 4,085 men from the general population, "The prevalence of all knee osteoarthritis outcomes (knee pain, RKOA and TKR) were two to three times higher in male ex-footballers compared with men in the general population group" (RKOA is X-ray knee arthritis, TKR a knee replacement), and "Knee injury is the main attributable risk factor". A review of 4 studies of former elite players found knee arthritis common, at 40% to 80% depending on the study.
Twelve years after an ACL tear, 51% of 67 women players had X-ray knee arthritis; 14 years after, 41% of men's injured knees had moderate X-ray changes, whether or not they had surgery. Across sport generally, knee arthritis was 7.7% in participants against 7.3% in non-participants, but higher in soccer (odds ratio 3.5), mostly in former elite players, and in a small study of former elite athletes, "The increased risk is explained in part by knee injuries in soccer players". These are mostly professional or elite players; recreational football hasn't been studied in the same way. That is a reason to treat a knee injury properly and keep up the prevention warm-up, not to stop playing.
Sources 18, 55, 56, 57, 58, 67
When a scan or test mattersStrong evidence
Most knee pain after football is diagnosed by examination. The NHS lists "Unstable, gives way when you try to stand, unable to straighten, may hear a popping sound during injury" as the signs of a torn ligament, tendon or meniscus, and the Ottawa knee rule in the warning signs says who needs an X-ray for a broken bone. A magnetic resonance imaging (MRI) scan matters after a pop with fast swelling, locking or giving way, to plan treatment for an ACL or meniscus tear; the AAOS says "Many meniscus tears will not need immediate surgery".
Scans find meniscus changes in many knees that don't hurt, more with age: in middle-aged and older adults, "Sixty-one percent of the subjects who had meniscal tears in their knees had not had any pain, aching, or stiffness during the previous month". Kneecap pain, jumper's knee and Osgood-Schlatter don't need a scan to be diagnosed. In a child with a limp, the hip is examined and X-rayed.
Sources 3, 73, 63
How it usually goesEmerging evidence
Aches and bruises settle within days; the NHS says "You should start to feel better in a few days". Most knee injuries in elite players cost under 4 weeks, and an MCL sprain about 3 weeks. Osgood-Schlatter usually settles over months: the AAOS says rest, ice, pain relief and stretching and strengthening exercises in most cases "will relieve pain and allow a return to daily activities"; NHS inform says "In some cases symptoms can last for 12-24 months so it’s important to find the right balance between pain and activity", and in a Danish programme "At 12 weeks, 16% returned to playing sport, which increased to 69% at 12 months".
After ACL reconstruction, elite men were back in matches after a median 7.4 months, and across all sports "On average, 81% of people returned to any sport, 65% returned to their preinjury level of sport and 55% returned to competitive level sport after surgery". No one can promise a date.
Sources 3, 4, 5, 74, 65, 61, 53, 50
When it isn't football: the causes the warning signs cover
Some knee pain has causes football didn't create, or needs more than a plan, and the warning signs follow the pages behind them. A knee that changed shape, swelled fast or went with a pop needs A&E, in the words of the NHS dislocated kneecap page; a crack with a misshapen knee follows the NHS sprains and strains and meniscus pages; and the Ottawa knee rule, tested in emergency departments, picks out who needs an X-ray. A pop at the front of the knee with a knee you can't straighten can be a torn quadriceps or patellar tendon, as the AAOS describes, and in a child the bone or a sleeve of cartilage can pull off instead, as a case series of 90 children shows. A hot, swollen knee can be a joint infection, as the NHS septic arthritis page says.
A child or teenager with knee pain and a limp may have a hip problem, as the NHS pages on limping and hip pain in children, NHS inform's slipped growth plate page and a national study of slipped growth plates in Great Britain describe. A swollen, throbbing calf can be a blood clot or a burst Baker's cyst, and sudden breathlessness a clot in the lungs, as the NHS pages on deep vein thrombosis (DVT), Baker's cyst and pulmonary embolism explain; fainting while exercising needs a 999 call, as the NHS fainting page says; and pain that travels down from the back is sciatica, in the NHS's words.
A knee that is locked, a tear after a significant twist in a younger person, or a knee that keeps giving way needs to be seen: the NHS meniscus tear page, a BMJ guideline panel, two trials in younger adults, a European consensus on repairing tears that can be stitched and an NHS programme on knee keyhole surgery are behind those lines. The NHS page on why ACL surgery is done, a UK trial of reconstruction against rehab, an international consensus on ACL injuries in children, a US ligament guideline and a review of inner knee ligament injuries say when a loose knee needs a surgeon. NHS inform's kneecap pain exercises say when kneecap pain needs a healthcare professional, Northern Ireland's dislocated kneecap page covers repeat dislocations, tender bumps below a growing player's kneecap are usually Osgood-Schlatter or Sinding-Larsen-Johansson, as the AAOS and two NHS trusts describe, NHS inform says when a child's knee needs a GP, and the NHS bone cancer page describes bone pain that is worse at night.
Sources 3, 84, 85, 7, 86, 87, 88, 89, 90, 91, 92, 93, 94, 113, 114, 95, 96, 97, 98, 99, 100, 49, 17, 101, 102, 103, 104, 105, 106, 107, 108, 74, 109, 110, 111, 112
