Recurrence of Achilles tendon injuries in elite male
football players is more common after early return
to play: an 11-year follow-up of the UEFA
Champions League injury study
Division of Community
Medicine, Department of
Medical and Health Sciences,
Football Research Group,
UEFA Medical Committee,
Division of Physiotherapy,
Department of Medical and
Health Sciences, Linköping
University, Linköping, Sweden
University of Salerno, School of
Medicine and Surgery, Salerno,
Queen Mary University of
London, Centre for Sports and
Exercise Medicine, London, UK
Professor Jan Ekstrand,
Solstigen 3, S-589 43,
Received 2 February 2013
Revised 14 April 2013
Accepted 12 May 2013
Published Online First
14 June 2013
To cite: Gajhede-
Knudsen M, Ekstrand J,
Magnusson H, et al. Br J
Sports Med 2013;47:
Background There is limited information about
Achilles tendon disorders in professional football.
Aims To investigate the incidence, injury circumstances,
lay-off times and reinjury rates of Achilles tendon
disorders in male professional football.
Methods A total of 27 clubs from 10 countries and
1743 players have been followed prospectively during 11
seasons between 2001 and 2012. The team medical
staff recorded individual player exposure and time-loss
Results A total of 203 (2.5% of all injuries) Achilles
tendon disorders were registered. A majority (96%) of
the disorder s were tendinopathies, and nine were partial
or total ruptures. A higher injury rate was found
during the preseason compared with the competitive
season, 0.25 vs 0.18/1000 h (rate ratio (RR) 1.4, 95%
CI 1.1 to 2.0; p=0.027). The mean lay-off time for
Achilles tendinopathies was 23±37 (median=10, Q
=24) days, while a rupture of the Achilles
tendon, on average, caused 161±65 (median=169,
=110 and Q
=189) days of absence. Players with
Achilles tendon disorders were signiﬁcantly older than
the rest of the cohort, with a mean age of 27.2±4 years
vs 25.6±4.6 years (p<0.001). 27% of all Achilles
tendinopathies were reinjuries. A higher reinjury risk was
found after short recovery periods (31%) compared with
longer recovery periods (13%) (RR 2.4, 95% CI 2.1 to
Conclusions Achilles tendon disorders account for
3.8% of the total lay-off time and are more common
in older players. Recurrence is common after Achilles
tendinopathies and the reinjury risk is higher after short
The elastic properties of the Achilles tendon are
important for locomotion.
The tendon has a
capacity to withstand loads up to 12.5 times the
but nevertheless many athletes
sustain Achilles tendon disorders.
The cause of
Achilles tendon disorder is unclear,
but it is sug-
gested that the essential lesion of tendinopathy is a
failed healing response.
often manifests with a gradual onset of pain during
tendon load (especially 2–6 cm proximal to the
morning stiffness and some-
times a localised swelling.
Achilles tendinopathies are often difﬁcult to
treat, have a poor prognosis
13 15 16
and a tendency
to chronicity and recurrence,
with long periods
of absence from sports.
Achilles tendon disorders are frequently seen in
sports including running and jumping.
7 8 15 19 20
Achilles tendinopathy or an Achilles tendon rupture
can be career altering or career ending for profes-
but only a few studies have
focused on Achilles tendon disorders in profes-
sional football players.
17 23 24
Woods et al
not report Achilles tendon disorders as a separate
diagnosis, but when evaluating ankle injuries
during two seasons in English professional football,
they found that 1% of all injuries were classiﬁed as
tendonitis or paratendonitis. Nearly one-third of
these occurred during the preseason period.
The present study investigated the incidence of
Achilles tendon disorders and the risk of reinjury in
a large population of elite male football players.
MATERIALS AND METHODS
Participants and study cohor t
The present study was performed within the frame-
work of the prospective injury study UEFA (Union
of European Football Associations) initiated among
male elite level football teams in 2001, the UCL
(UEFA Champions League) study.
This portion of
the study included 27 European professional foot-
ball teams from 10 countries. The teams were fol-
lowed over a varying number of seasons (1–11
seasons) between February 2001 and December
2011 and consisted of 1743 players (in an average
15 teams per season). All contracted players in the
ﬁrst team were invited to participate in the study.
Players who left the team during the season (eg,
because of transfer) were included during their
time on the team. Teams were followed during the
full season, including the preseason and competi-
Data collection and study deﬁnitions
The study design followed the consensus on deﬁni-
tions and data collection procedures in studies of
football injuries outlined in the consensus docu-
and by UEFA
for studies of football. The
validation of the injury and exposure reporting
system and deﬁnitions has been described previ-
To ensure high reliability of data registra-
tion, all teams were provided with a study manual
describing the deﬁnitions used and procedures to
record data, including explanatory examples. To
avoid language problems, the manual and the study
Gajhede-Knudsen M, et al. Br J Sports Med 2013;47:763–768. doi:10.1136/bjsports-2013-092271 1 of 6
forms were translated from English into ﬁve other languages:
French, German, Italian, Russian and Spanish. In addition, the
study group checked all reports each month, and feedback was
sent to the teams correcting any missing or unclear data. The
deﬁnitions applied in the study are shown in table 1.
At the start of each season, player baseline data were col-
lected. Individual player participation in training and matches
(minutes of exposure) was recorded by the club contact person
on a standard exposure form sent to the study group on a
monthly basis. This included exposure with the ﬁrst and the
second teams, as well as any national team exposure, for all
players. The training content was not recorded. The team
medical staff recorded all time-loss injuries on a standard injury
form that was sent to the study group every month. The injury
form provided information about the diagnosis, nature and cir-
cumstances of injury occurrence. All injuries resulting in the
player being unable to fully participate in training or match play
(ie, time-loss injuries) were recorded, and the player was consid-
ered injured until the team medical staff allowed full participa-
tion in training and availability for match selection. Injuries
were categorised under 4° of severity based on the number of
days of absence from training and/or match. All injuries were
followed until the ﬁnal day of rehabilitation.
The present study included all recorded Achilles tendon injur-
ies, including Achilles tendinopathies, partial Achilles tendon
ruptures and total Achilles tendon ruptures. The diagnosis was
based on the clinical examination by the team medical staff, and
no speciﬁc diagnostic criteria were set out in advance.
For the 48 severe injuries (>28 days lost) in 43 players, spe-
ciﬁc data about surgery and location of injuries were collected
retrospectively from the team doctors and the club medical
Anthropometric data (age, height, body weight and body mass
index (BMI)) are presented with mean±SD and analysed with
independent sample t test. Lay-off time is presented with mean
±SD and corresponding median and quartiles (Q
centile) and Q
(75th percentile)). Owing to a skewed distribu-
tion in the lay-off time, group differences were analysed using
the Mann-Whitney U test. Pearson’s χ
test was used to analyse
association between categorical variables. Injury rates are
reported as the number of injuries per 1000 player-hours and
injury burden was calculated as the number of lay-off days per
1000 player-hours. The rate ratio (RR) with 95% CI was used
for group comparisons of injury rates and injury burden, and
signiﬁcance was tested using z statistics. Seasonal trend,
expressed as the average annual percentage of change, was ana-
lysed using linear regression with log-transformed injury rates as
the dependent variables. A 2-year moving average (MA)
approach, by summarising two consecutive seasons, was also
used to smooth out the large seasonal variation. The reinjury
risk after the short (0–10 days) and long (>10 days) recovery
periods was compared using the risk ratio with 95% CI, and sig-
niﬁcance was tested using z statistics. Season injury prevalence
was calculated as the number of injured players during a season/
total number of players in that season ×100. All tests were two-
sided and the signiﬁcance level was set at p<0.05.
The study design underwent an ethical review at Linköping
University, Linköping, Sweden and was approved by the UEFA
Football Development Division and the UEFA Medical
In total, 1 057 201 h of exposure (888 249 h of training and
168 952 h of match play) were recorded. Overall, 8029 injuries
were documented during the study period, 203 (2.5%) of which
were Achilles tendon disorders. One hundred and ninety-four
(96%) disorders were classiﬁed as gradual onset tendinopathies,
and nine (4%) were acute onset partial or total ruptures. The
injury rate was almost 22 times higher in Achilles tendinopathies
compared with Achilles tendon ruptures (0.18 vs 0.01 injuries/
1000 h, RR 21.6, 95% CI 11.0 to 42.1; p<0.001). As the mean
total exposure time for a team of 28 players is about 7000 h per
season, a team can roughly expect one Achilles tendinopathy
every season, and one Achilles tendon rupture every 17th
Between-season and within-season variation of injury rates
The injury rate of Achilles tendon disorders has ﬂuctuated,
between 0.12 and 0.35 injuries/1000 h, over the 11 seasons.
The highest injury rate was found in the February 2001 season
and the lowest notations were found in the 2006 and 2007
seasons. The crude injury rates and the ‘smoother’ 2-year MA
injury rates are illustrated in ﬁgure 1. The MA approach indi-
cated an average annual decrease of 1.8% in the injury rate. The
ﬁtted regression model, however, indicated a non-signiﬁcant sea-
sonal trend in the log-transformed injury rates (R
−0.056, 95% CI −0.131 to 0.020; p=0.132).
The within-season variation, measured as quarterly injury
rates, are shown in ﬁgure 2 and table 2. A higher injury rate was
found during the preseason ( July–August) compared with the
competitive season (September–May), 0.25 vs 0.18 injuries/
1000 h (RR 1.4, 95% CI 1.1 to 2.0; p=0.027).
Table 1 Operational definitions used in the study
Training session Team training that involved physical activity under the
supervision of the coaching staff
Match Competitive or friendly match against another team
Injury Injury resulting from playing football and leading to a player
being unable to fully participate in future training or match
play (ie, time-loss injury)
Rehabilitation A player was considered injured until the team medical staff
allowed full participation in training and availability for
Index injury The first reported injury of each player is assumed to be an
Reinjury Injury of the same type and at the same site as an index
injury occurring not more than 2 months after a player’s
return to full participation from the index injury
Injury causing absence of 0–3 days from training and match
Mild injury Injury causing absence of 4–7 days from training and match
Moderate injury Injury causing absence of 8–28 days from training and
Severe injury Injury causing absence of over 28 days from training and
Traumatic injury Injury with sudden onset and known cause
Overuse injury Injury with insidious onset and no known trauma
Injury rate Number of injuries per 1000 player-hours ((Σ injuries/Σ
Injury burden Number of days of absence per 1000 player-hours ((Σ days
of absence/Σ exposure-hours)×1000)
2 of 6 Gajhede-Knudsen M, et al. Br J Sports Med 2013;47:763–768. doi:10.1136/bjsports-2013-092271
Injury severity and consequences
The severity of injuries and consequences in the form of lay-off
time and injury burden are shown in table 3. The mean lay-off
time for Achilles tendinopathies was 23±37 (median=10,
=4 and Q
=24) days, while a rupture of the Achilles tendon,
on average, caused an almost seven times longer lay-off time,
with 161±65 (median=169, Q
=110 and Q
=189) days of
absence. Achilles tendon disorders account for 3.8% of the total
lay-off time reported in the study. The injury burden was
approximately three times higher in Achilles tendinopathies
compared with Achilles tendon ruptures (4.24 vs 1.37 days lost/
1000 h, RR 3.1, 95% CI 2.9 to 3.3; p<0.001).
Twenty-seven per cent of all Achilles tendinopathies were reinju-
ries (table 3). No early recurrences were seen among the nine
Achilles tendon ruptures. The reported recurrence rate of all
injuries is 12% in the cohort. A higher reinjury risk was found
after short (0–10 days) recovery periods (31%) compared with
longer (>10 days) recovery periods (13%), (risk ratio 2.4, 95%
CI 2.1 to 2.8; p<0.001).
Players with Achilles tendon disorders were signiﬁcantly older
than the rest of the cohort, with a mean age of 27.2±4 years vs
25.6±4.6 years (p<0.001). No differences could be found in
height (182.1±6.2 cm vs 182.3±6.4 cm, p=0.607), body
weight (78.4±7.1 kg vs 78±7.1 kg, p=0.473) or BMI (23.6
±1.3 vs 23.4±1.4, p=0.092).
Location and treatment of severe injuries
Information about surgery and location was provided for 34 of
the 39 severe (>28 days lost) Achilles tendinopathies and infor-
mation about surgery was received for eight of the nine severe
Achilles tendon ruptures.
Twenty-three per cent of the proximal Achilles tendinopathies
were operated on (3/13), two of them with resection of the
plantaris tendon. Forty-ﬁve per cent (9/20) of the insertional
tendinopathies were operated on. One player had surgery after
problems both in the main body and at the insertion. The mean
lay-off time for tendinopathy of the main body was 66±49
=35 and Q
=78) days, and for insertional ten-
dinopathies 79±50 (median=61, Q
=41 and Q
(p=0.320). Surgery was performed on all seven total ruptures.
The partial rupture, from which we received information about
treatment, was not operated on.
A team of 28 players can roughly expect one Achilles tendon
disorder every season, and older players are more frequently
affected. Most absence periods for Achilles tendinopathies were
short (median of 10 days), but the recurrence rate was high
(27%). There was a signiﬁcantly higher risk of sustaining a rein-
jury if the player rested for less than 10 days. Finally, Achilles
tendinopathy was more common during the preseason period.
Achilles tendinopathies are common overuse injuries—
Achilles tendon ruptures are rare
The season prevalence of Achilles tendinopathies in this study
was 2.1–5.1%. Woods et al
did not report Achilles tendon
Figure 1 Seasonal variations in
injury rates of Achilles tendon
disorders in male professional football
Figure 2 Monthly variations in injury
rates of Achilles tendon disorders in
male professional football players.
Gajhede-Knudsen M, et al. Br J Sports Med 2013;47:763–768. doi:10.1136/bjsports-2013-092271 3 of 6
disorders as a separate diagnosis, but when evaluating ankle
injuries during two seasons in English professional football, they
found that 1% of all injuries were classiﬁed as tendonitis or
paratendonitis. Fredberg et al
reported that 15% of the
players reported time loss attributable to Achilles tendinopathy
in Danish professional football players. The difference in injury
prevalence may reﬂect a true difference in injury prevalence or
differences in study methodology, reporting thresholds or
varying treatment regiments (ie, to let players with symptoms
carry on playing or take them out of training). Achilles tendon
ruptures were rare at 0.01 injuries/1000 h of exposure.
Higher risk of Achilles tendinopathy during preseason
The incidence of Achilles tendon disorders was signiﬁcantly
higher during the preseason. All the teams in this cohort fol-
lowed an autumn–spring season with preseason training in July–
August, and a competitive season between September and May.
One team had a longer break between December and February
and contributed half the injuries in January, but the injury inci-
dence was still signiﬁcantly higher in the preseason.
More overuse, especially tendon injuries, occurs during the
Woods et al
reported that 32% of
all Achilles tendon disorders occurred during the preseason, but
this study did not account for exposure, so the high number of
injuries could just reﬂect more training sessions. The intense
preseason follows a period of rest. These sudden changes in
intensity, training load and training type are suggested to be risk
factors for overuse injuries such as Achilles tendinopa-
because the Achilles tendon may not be able
to adapt fast enough to changes in loading pattern.
Varying absence periods for Achilles tendinopathy
The absence period for Achilles tendinopathy in this study
varied, with a median of 10 days and an average of 23 days.
Both Sankey et al
in rugby (mean=30 days) and Oztekin
in Turkish professional football (median=30 days)
reported longer absences. This may reﬂect differences in meth-
odology and in the demands of the sport.
The variations in lay-off time can be explained by the nature
of the injury. Achilles tendinopathy often manifests with a
gradual onset of pain,
and it can be difﬁcult to decide if
and when a player should rest. Many players continue to partici-
pate in training even with symptoms of Achilles tendinopa-
especially if they want to play an important match.
This affects the rehabilitation time needed. The player’ s import-
ance to the team can also explain why some players rest less
Table 2 Achilles tendon disorders in the UCL cohort, comparison of preseason and competitive season in male professional football
Preseason Competitive season p Value
Achilles tendon disorders, n (percentage of total number of injuries) 54 (3.8) 149 (2.3) <0.001
Reinjuries, n (%) 17 (31) 36 (24) 0.306
Lay-off days, mean±SD 32±56 28±45 N/A
Lay-off days, median (Q
)11(4–27) 10 (4–27) 0.964
Injury rate* (95% CI) 0.25 (0.19 to 0.33) 0.18 (0.15 to 0.21) 0.027
Training injuries 0.17 (0.12 to 0.24) 0.14 (0.11to 0.17) 0.302
Match injuries 0.81 (0.54 to 1.24) 0.37 (0.29 to 0.49) 0.003
Injury severity, n (%) 0.821
Slight/minimal (0–3 days) 13 (24) 28 (19)
Mild (4–7 days) 10 (19) 34 (23)
Moderate (8–28 days) 18 (33) 52 (35)
Severe (>28 days) 13 (24) 35 (23)
Injury burden† (95% CI) 8.16 (7.78 to 8.55) 4.97 (4.82 to 5.12) <0.001
*Injury rate expressed as the number of injuries/1000 h of total exposure.
†Injury burden expressed as the number of days of absence/1000 h of total exposure (injury rate×mean absence).
N/A, not applicable; Q
, 25th percentile; Q
, 75th percentile; UCL, UEFA Champions League.
Table 3 Achilles tendon disorders in the UCL cohort, comparison
of Achilles tendinopathies and Achilles tendon ruptures in male
Achilles tendon disorders,
n (percentage of total
number of injuries)
194 (2.4) 9 (0.1)
Reinjuries, n (%) 53 (27) 0 (0)
Lay-off days, mean±SD 23±37 161±65
Index injury 20±34 161±65
Reinjury 32±44 N/A
Lay-off days, median (Q
)10(4–24) 169 (110–189)
Index injury 8 (4–21) 169 (110–189)
Reinjury 14 (4–37) N/A
Injury rate* (95% CI) 0.18 (0.16 to 0.21) 0.01 (0.00 to 0.02)
Injury severity, n (%)
Slight/minimal (0–3 days) 41 (21) –
Mild (4–7 days) 44 (23) –
Moderate (8–28 days) 70 (36) –
Severe (>28 days) 39 (20) 9 (100)
Injury burden† (95% CI) 4.24 (4.12 to 4.37) 1.37 (1.30 to 1.44)
Days of absence/player/season,
Season prevalence,‡ range 2.1 to 5.1 0.0 to 0.5
*Injury rate expressed as the number of injuries/1000 h of total exposure.
†Injury burden expressed as the number of lay-off days/1000 h of total exposure
(injury rate×mean absence).
‡Season prevalence expressed as the number of players with an injury in a season/
total number of players in that season×100.
N/A, not applicable; Q
, 25th percentile (first quartile, 25%); Q
, 75th percentile
(third quartile, 75%); UCL, UEFA Champions League.
4 of 6 Gajhede-Knudsen M, et al. Br J Sports Med 2013;47:763–768. doi:10.1136/bjsports-2013-092271
Achilles tendon ruptures cause long absences from football
The median absence period for the Achilles tendon rupture in
this study was 169 days (mean=161). Both professional rugby
players (mean=185.5 days)
and professional Turkish football
players (median and mean=180 days)
absences. This may result from differences in methodology and
study size, or in rugby, differences between sports. Also, with
only nine ruptures in 11 years, the average is vulnerable to
Six of the nine players sustaining ruptures of the tendon had
been followed in our study before the season when the rupture
occurred, and none of these players had any Achilles disorders
resulting in time loss before their rupture. Symptoms before
ruptures are reported to vary between 10% and 25%.
Since degenerative changes are found in almost all ruptured
it seems plausible that tendons undergo changes
during an asymptomatic period of several months, where the
tendon might be injured but the player does not experience
symptoms, before the pathology results in pain and the changes
Therefore, some tendons might
rupture before signs of Achilles tendinopathy occur.
Short absence time increases the risk of reinjury
The high recurrence rate for Achilles tendinopathy found in this
study (27%) reﬂects the chronic and recurrent character of this
condition. The frequent relapse of symptoms when players
return to football after a short rehabilitation period could be
explained if pain is only the tip of the iceberg. Even if pain
recedes to below the threshold, the pathological changes in the
tendon could still exist and could therefore become symptom-
atic when the player resumes his activity level.
abstained from training and matches for less than 10 days at
their index injury experienced a signiﬁcantly higher risk of rein-
jury than players who rested for more than 10 days (p<0.001).
Inadequate rehabilitation, with a premature return to activity, is
a risk factor for reinjury.
30 39 40
Therefore, it could be suggested
that a longer rehabilitation period at the ﬁrst signs of Achilles
tendinopathy could be beneﬁcial to avoid recurrences. A func-
tional test before returning to play might also reduce
There is little evidence on how to prevent these recurrences.
Fredberg et al
showed that prophylactic eccentric and stretch-
ing exercises did not reduce the frequency of ultrasonographic
abnormalities in Achilles tendons in professional football
players. On the other hand, the lack of effect might be because
the training was not intense enough (2.25 times/week).
Older players sustain Achilles tendinopathy
The players who sustained an Achilles tendon disorder were sig-
niﬁcantly older than the rest of the cohort (27.2 vs 25.6 years),
as reported in other studies.
The incidences of muscle calf
injuries increase with age
and, if the calf muscle is more
exposed to injury, this could affect the risk of injury in the
Achilles tendon as well. Further, it is well established that pro-
longed, strenuous endurance exercise induces muscle damage
and impairs muscle function.
Normally, the muscle tendon
unit is able to repair and adapt, but there might be a limit to the
regenerative capacity and adaptability, resulting in pathology.
If the calf muscles of older players theoretically have impaired
function, there is a risk that the muscles become more easily
fatigued. Fatigued muscles lose their ability to absorb shock,
which theoretically can put a greater stress on the Achilles
Treatment of the severe injuries
In accordance with other studies, all the total Achilles tendon
ruptures in this study were managed surgically.
Achilles tendinopathies can be treated conservatively, but around
25% require surgery.
Of the severe (>28 days lost) tendino-
pathies in the present study, 38% were operated on.
Most severe tendinopathies in this study were located at the
insertion (20 of the 34 we had information about). The inser-
tional lesions also required surgery more often (45% vs 23%)
and caused longer absences from football than the proximal
lesions (mean=79 vs 66 days).
The prospective design and the 11-season study period are the
strengths of the present study, together with the large and
homogeneous sample of professional footballers.
However, this study also has some limitations. First, no treat-
ment details were requested on the general injury card.
Additionally, no substudy speciﬁc injury card was sent to the
clubs as was done for some of our previous substudies.
Distinction between insertional and proximal Achilles tendon
disorders was therefore not possible, which was a limitation in
the study. We did, however, retrospectively collect data about
the location and surgery for injuries causing an absence of
greater than 28 days and since most clubs preserve ﬁles with
objective data such as images and surgery protocols of previous
injuries, we did receive data for most of these injuries.
Second, our study manual did not provide the participating
clubs with speciﬁc diagnostic or return-to-play criteria: it is
therefore possible that the reported subclassiﬁcations, lay-off
times and reinjury rates varied between clubs due to different
diagnostics and treatment algorithms. Third, no reliable history
of injury was collected at player inclusion; it was therefore not
possible to compare the rates of Achilles disorders between
uninjured players and players with prior Achilles disorders.
Fourth, with the current time-loss deﬁnition, the number of
overuse injuries, such as Achilles tendinopathy, captured in this
surveillance study might only be the tip of the iceberg.
Overuse injuries might be underestimated, because many players
with Achilles tendinopathy keep participating in training and
matches despite having problems, and are therefore not cap-
tured in our time-loss deﬁnition.
However, this deﬁnition
was chosen for the general injury surveillance study to capture
the injuries, which had a substantial effect on the players and
the teams’ ability to perform.
What are the new ﬁndings?
▸ Achilles tendon disorders account for 2.5% of all injuries
and 3.8% of lay-off times in male professional football.
▸ Achilles problems are more common in older players.
▸ A majority (96%) of the disorders are gradual onset
tendinopathies, and 4% are acute onset partial or total
▸ The incidence of Achilles disorders is higher during the
preseason compared with the competitive season.
▸ The median lay-off time for Achilles tendinopathies is
10 days and for ruptures it is 169 days.
▸ Twenty-seven per cent of Achilles tendinopathies are
reinjuries and these are more common after short (>10 days)
Gajhede-Knudsen M, et al. Br J Sports Med 2013;47:763–768. doi:10.1136/bjsports-2013-092271 5 of 6
How might it impact on clinical practice in the near
▸ Players with Achilles tendinopathies might need longer
recovery periods since the reinjury risk is signiﬁcantly higher
after recovery periods of less than 10 days.
▸ The team medical and coaching staffs should be aware of
the increased risk of Achilles problems during the preseason
training period and observant of overuse to Achilles
▸ The team medical and coaching staffs should be observant
of the higher risk of older players who suffer from
muscle-tendon problems after repetitive hard training
Acknowledgements The authors would like to thank the participating clubs,
medical staffs and players.
Contributors JE was responsible for the conception and design of the study. HM
has been the statistical advisor and revised the paper. MGK wrote the ﬁrst draft of
the manuscript, which was critically revised by NM, HM and JE. All authors
contributed to the interpretation of the ﬁndings and approved the ﬁnal draft of the
manuscript. JE is the study guarantor.
Funding This study was supported by grants from the Union of European Football
Associations (UEFA), the Swedish National Centre for Research in Sports and
Competing interests None.
Patient consent Obtained.
Ethics approval The study design underwent an ethical review at Linköping
University, Linköping, Sweden and was approved by the UEFA Football Development
Division and the UEFA Medical Committee.
Provenance and peer review Not commissioned; externally peer reviewed.
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