The 2023-24 season had not yet started when Chelsea centre-back Wesley Fofana and Aston Villa midfielder Emiliano Buendia were ruled out of most of it with anterior cruciate ligament (ACL) tears. By October, four more had been added to the list: Aston Villa’s Tyrone Mings, Arsenal’s Jurrien Timber, Brentford’s Rico Henry and Tottenham Hotspur’s Ivan Perisic.
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This week another name was added to the list. Liverpool confirmed that defender Joel Matip had ruptured his ACL during the 4-3 win over Fulham last weekend.
Last season, there were just four ACL injuries overall (five if you include Chelsea’s Armando Broja, whose injury occurred during the pause for the World Cup), so it’s perhaps unsurprising that this season’s tally has sparked cries of an ‘ACL epidemic!’
Those cries inevitably lead to the search for a cause.
Is it a delayed after-effect of last winter’s World Cup coupled with a rushed return to domestic play? Is it the knock-on effect of a seemingly never-ending season, with UEFA Euro qualifying games taking place in mid-June? Is it a pre-season packed with long-distance travel and matches played on ‘hard’ surfaces? Is it just too many games?
The first consideration is whether what we are seeing is actually anything out of the ordinary.
Three of the seven ACL injuries occurred in August, which, according to Ben Dinnery of Premier Injuries (a website tracking injuries, suspensions and absences in the Premier League) is statistically when we see the highest incidence of ACL injuries occur (around 17 per cent). January is next highest with 13 per cent.
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Looking strictly at in-season numbers, the five ACL injuries so far equal last season’s total of five and is one short of the 2021-22 season when we saw six overall.
However, we still have plenty of the season left to run (having only played 15 of 38 Premier League games to date) so it’s unclear what this season’s grand total will look like come May 2024.
But many are calling for calm. One former Premier League physiotherapist, who asked not to be named to protect relationships, told The Athletic: “We don’t know why people keep talking about the incidence being much higher when it is not, really. The main reason for any increase would be the increased demands on players as the distance covered, sprint distances, decelerations in games continues to rise.
“It’s actually testament to the medical and strength and conditioning input at most clubs that the ACL incidence rate is not rocketing.”
| Season | Number of ACL injuries |
|---|---|
2012-13 | 12 |
2013-14 | 7 |
2014-15 | 9 |
2015-16 | 12 |
2016-17 | 12 |
2017-18 | 5 |
2018-19 | 9 |
2019-20 | 8 |
2020-21 | 5 |
2021-22 | 6 |
2022-23 | 5 |
2023-24 | 4 |
Trying to pinpoint one particular reason for these injuries occurring is a challenge.
For a start, each injury occurs in different ways, and in different circumstances. For example, Perisic’s injury happened during a non-contact training session, while the injuries to Mings and Henry both involved some element of in-game contact. Timber’s appeared to involve no contact but could also have come as a result of a knock earlier in the game and Matip’s was the result of an awkward landing after being knocked off balance by an opposing player.
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One study, published in 2020 analysed 134 ACL injuries across 10 seasons of professional football in Italy and found that 56 per cent of them involved contact of some sort — mostly indirect i.e. to the shoulder, with a small percentage being direct contact to the knee.
“That suggests it might be just an occupational hazard, to a degree,” says Dr Tom Hughes, who spent more than 15 years working as a physiotherapist at Manchester United before leaving this summer to become a clinical fellow at Manchester Metropolitan University.
That leaves 44 per cent of ACL injuries as non-contact injuries (other studies have put this closer to 65 per cent), which is where things get more complicated.
The ACL might only be around four centimetres long, but it takes a lot of force to disrupt it — around 2,000 newtons, according to renowned orthopaedic surgeon Professor Gordon Mackay.
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Usually, the ACL will be protected during decelerations and changes of direction (two manoeuvres that particularly challenge the ligament) by muscles which help to stabilise the joint. But, if anything happens to put a player at a mechanical disadvantage, such as studs sticking in the turf or something inhibiting them muscularly, like a groin strain or tightness in a hamstring, then theoretically they are more vulnerable to suffering a serious injury.
Mackay speculates that the early-season spate (and indeed the high incidence of ACL’s statistically that occur in August) could have its roots in the latter scenario. “The pressure early in the season is you have intensified your training and people are really keen to secure their position and establish themselves and maybe are not entirely transparent regarding niggles and irritations they may be carrying.
“It’s quite challenging for the medical teams to decipher this information. As much as surveillance is very good, for soft tissue injury, tightness or discomfort you’re still very reliant on the player’s interpretation of that, because it’s almost subclinical in a way and hard to objectively assess.”
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In these cases, Mackay says there is a risk of “second injury syndrome”. Meaning, if a player is recovering from a slightly sprained ankle or has an irritation in their Achilles or hamstring, they are probably more susceptible to injuries such as an ACL tear. Though, he admits it is tricky to gather hard evidence to support this.
The Scot also points to the widely accepted notion that strength is absolutely essential to preventing cruciate ligament injuries, warning that: “If there’s an imbalance in muscle strength between your quadriceps and your hamstrings, that may in certain situations leave you more vulnerable to a cruciate injury.”
The most obvious thing to consider with injuries is the ever increasing demands on players. Recent figures from Premier Injuries show that the number of injuries which have caused players to miss at least one game in the Premier League is up 30 per cent from the same stage of last season (196 as of Gameweek 12 compared to 151). If the total number of injuries is up, then perhaps it simply makes statistical sense that the number of ACL injuries we see this season will rise too.
So how much of a risk factor is fatigue specifically for ACL injuries?
“You do have to be careful assuming,” says Mackay. “An ACL injury isn’t like an Achilles tendonitis where essentially your Achilles is fraying from overuse and then tearing. But there are other factors that are really important, like proprioception (the awareness of body parts), deceleration and change of direction, which involves a quite complex, coordinated movement. If you’re fatigued or distracted, then you are a little more vulnerable to injury.”
Others argue that fatigue alone is not a “clear factor” in ACL injuries. One of those is orthopaedic surgeon and member of the global ACL Study Group, Dr Christopher Kaeding who tells The Athletic, “If you look at some of the studies, you probably find ones that go both ways.”
Dr Kaeding has been part of the group for 30 years and says that risk factors for ACL injuries remain one of the key areas of research.
So why do some people suffer ACL tears and some don’t?
In the USA, where Kaeding is based, he says 80 per cent of ACL injuries occur in five sports: soccer (football), American football, basketball, rugby and lacrosse. The common denominator between all these says Kaeding is that they are “an aggressive, competitive, cutting sport activity” (cutting meaning an abrupt change of direction).
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Allied to this, Kaeding notes there are some “genetic predisposition” to ACL injuries, particularly when a first-degree relative has suffered from one. “I know of eight sets of identical twins where both twins have torn their ACL. If you have two or three older siblings and they all love playing soccer or football and they all tore their ACL, it might be wise to find another sport.”
These genetic factors could take many forms.
“All of our collagen (a protein found within the body which helps strengthen tissues) is a little bit different, so is that a factor? Other people think it’s as simple as the size of the ACL you’re born with. Just like your nose, big people can have a small nose and small people can have a big nose. We all have a different size ACL and if you have a smaller ACL and play an aggressive cutting sport like soccer you’re at increased risk.”
If a player has already suffered a non-contact ACL injury, they are statistically at higher risk of having a future injury, either on the same knee or the non-injured knee. Adding hypermobility to the equation increases the risk five times.
The most recent genetic risk factor to come to the forefront is the tibial slope. “When you’re standing totally straight, the top of your shinbone is not exactly parallel to the ground” explains Dr Kaeding. “It has a little slope leaning posteriorly, which means when you bear weight on your leg, there’s a little force pushing your tibia or your shin bone forward out from underneath your femur or thigh bone.
“The higher your slope, the higher the forces pushing your shin bone out from underneath your thigh bone and over the last five to 10 years that’s become a strong predictor: If you have more of a high slope you have a higher risk of tearing your ACL. The magic number seems to be about 12 degrees — more than 12 degrees, you have a higher risk.”
Changing the slope requires a surgery called an osteotomy where the bone is cut, reshaped and redirected. But when is the right time to do it?
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“Should you do it prophylactically i.e. before an injury has even occurred?” asks Dr Kaeding. “In a committed soccer player, should they just go ahead and have it corrected to decrease their risk? That’s part of the current debate.”
Other recognised risk factors, according to Dr Kaeding, are less painful to address. Playing surfaces and shoes are two of those: “The higher the coefficient of friction between your shoe and the playing surface — the stickier that is — the higher the risk.
“If you look at American football games which are played in heavy rain, where it’s muddy and very slippery, there’s very few ACL injuries. If you’re playing on artificial turf or indoors and you’re wearing a shoe or cleat that has a lot of spikes on it, you have greater friction between your shoe and the surface. That’s when we see more ACL injuries.”
But the most obvious risk factor for ACL injuries by far, says Dr Kaeding, is being female.
Although the noise around ACL injuries in women’s football has been loudest in the last couple of years as the prominence of the game has risen, Dr Kaeding says the higher risk factor for female athletes has been known for some time.
“The National Collegiate Athletic Association (NCAA) had an injury database and in the early to mid-90s they noticed that in basketball, the women’s teams had triple the incidence of ACL tears than the men’s teams. As they looked deeper they found that that held true across all high-risk activities.”
Depending on where you read, females are anywhere from two to eight times as likely to suffer an ACL injury than males.
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So far, this season has seen seven WSL players suffer ACL injuries, if you include Arsenal defender Teyah Goldie (whose injury occurred in pre-season) and Liverpool goalkeeper Faye Kirby (who is on loan at Aberdeen), both 19 years old. Statistically, this number is more likely to rise by the end of this season than it is for the men.
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Last season, both Tottenham Hotspur Women and Arsenal Women at one point found themselves with four players out of action at the same time (two of Tottenham’s occurred the previous season but the rehab carried over into 2022-23).
On the international stage, the 2023 Women’s World Cup was missing between 25 and 30 players because of ACL tears.
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If the jury is still out on this being a crisis point for ACL injuries in the men’s game, it is very much in when it comes to the women’s.
Dr Kaeding points out that, prior to puberty, the incidence between males and females is the same (a caveat to that being that it is relatively uncommon prior to puberty, with the vast majority of ACL injuries occurring between the ages of 12 and 25).
It’s often assumed that the increased risk for females comes down to hormonal and anatomical differences.
In 2019, Chelsea Women became the first club in the world to tailor their training programme around players’ menstrual cycles in an attempt to enhance performance and cut down on injuries. Since 2018, only four Chelsea players have ruptured their ACL; Maren Mjelde, Lucy Watson (who was on loan at Charlton), Jorja Fox (who was on loan at Brighton and Hove Albion) and just this week, Aniek Nouwen.
Chelsea Football Club can confirm that defender Aniek Nouwen has sustained an anterior cruciate ligament (ACL) rupture while on international duty with Netherlands Under-23’s squad.
We’re with you all the way, Aniek. 💙
— Chelsea FC Women (@ChelseaFCW) December 7, 2023
The approach was also used by the US Women’s National Team in the lead-up to their World Cup win in 2019.
The theory is that hormonal fluctuations in the menstrual cycle cause ligament laxity, increasing the risk of ACL injuries, and so tailoring training to players’ cycles on an individual basis can help to mitigate this.
But more research into the topic is needed, with some researchers warning that there is not yet sufficient evidence to support universal phased-base responses and that not all women are affected by their menstrual cycle.
Female athletes please do not fear any phase of your menstrual cycle. World records, PBs, world cup finals, etc. have been won by women across all days of the menstrual cycle
We can help with cycle-related symptoms but otherwise there is no blueprint for performance across the MC— Kirsty Sale (@ElliottSale) November 18, 2023
Anatomically, there are suggestions that the way that female athletes tend to land is also a factor.
Dr Kaeding cites a study from the 1990s that looked at male and female basketball players and studied how they landed from a jump off an 18-inch box.
“The males bent their knees deeper and landed in more of a squat — they contracted their quads and their hamstrings at the same time. You have to fire your quads so you don’t fall to the ground and firing the hamstring helps pull the shin bone under your thigh bone so it protects the ACL.
“The female basketballers landed more upright and they did not fire their hamstring nearly as much, so there is some type of neuromuscular firing pattern in how they landed differently.
“They also noticed that a lot of the female players, when they landed, their knees would go into bilateral valgus (their knees would come toward each other). So when they’re landing, rather than keeping the knees well-aligned, where the hip, knee and ankle are lined up, right above each other, they allow the knees to come into the midline.”
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In 2017 FIFA created the FIFA 11+ Kids programme, a warm-up protocol tailored for preadolescent athletes, aged 7–13 years old, with the purpose of injury prevention. Designed to improve attention, coordination, balance, and lower limb and core strength. One study, published in The American Journal of Sports Medicine, found that female football players who participated in the programme were 30 per cent less likely to suffer an ACL injury than those who did not participate in it.
“The reality is that the higher risk [in females] would seem to be related to the mechanics” says Mackay, “and the thing that has been shown to have a significant impact in reducing the risks are those pre-training drills where you’re working on proprioception, landing and turning and decelerating manoeuvres that mechanically start to protect the knee.
“If they become habitual within the female game, then the risk of ACL injury should be reduced significantly.”
Hormones and biomechanics aside, there are a multitude of other factors that have been suggested as potentially playing a role in the increased risk for female athletes — specifically footballers. These largely cut across societal and environmental factors such as traditional attitudes towards strength training and building muscle among females, women’s football boots are still largely just smaller versions of those designed for men (rather than women’s specific) and access to facilities and high-quality coaching.
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In elite youth football, development boys are signed to professional academies from under nine and have access to strength and conditioning coaches and injury surveillance and management. There is no such system for girls until over the age of 12 and even then it is underfunded compared with the boys’ programme.
All these factors represent an emerging direction when it comes to the research into female footballers and ACL injuries, with no hard evidence or answers yet available.
In August, FIFA unveiled their Women’s Health, Wellbeing and Performance project, dedicated to addressing “crucial challenges in women’s health.” And this week, UEFA announced they are taking action by forming a panel of women’s health experts whose mission is to better understand ACL injuries and their prevalence in women’s football. Their first step is the creation of an ACL injury awareness questionnaire addressed to all individuals in the women’s football community, with the aim of gathering more insights.
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The long-term aim is to publish a consensus on ACL injury prevention and management, plus an up-to-date ACL injury prevention programme by the summer of 2024. “The consensus,” say UEFA, “will provide evidence-based guidelines on topics ranging from ACL injury prevention and common risk factors to injury mechanisms and optimal return-to-play strategies, all tailored specifically to women’s football.”
While this season’s numbers in the Premier League look worrying at present, many experts will reserve their concern until May 2024. Even then, deciphering exactly what’s behind those numbers is no simple feat.
“There’s been loads of research done looking at factors that are associated with ACL injuries” says Dr Hughes. “Age, family history of ACL injury, if they’ve had an ACL injury on the opposite knee, any previous history of other injuries, certain anatomical features that people might have, the way they move, boots… It goes on and on.
“These things might be associated with an elevated risk, but we don’t know if they’ve got a direct causal link to any of these ACL injuries. Trying to unpick the different factors is really difficult. At the moment, I don’t think the existing research can give us definite answers about these types of causal factors.”
