Hamstring injury rates have increased during recent seasons and now constitute 24% of all injuries in men’s professional football: the UEFA Elite Club Injury Study from 2001/02 to 2021/22

Abstract

Objectives
To: (1) describe hamstring injury incidence and burden in male professional football players over 21 seasons (2001/02 to 2021/22); (2) analyse the time-trends of hamstring muscle injuries over the most recent eight seasons (2014/15 to 2021/22); and (3) describe hamstring injury location, mechanism and recurrence rate.

Methods
3909 players from 54 teams (in 20 European countries) from 2001/02 to 2021/22 (21 consecutive seasons) were included. Team medical staff recorded individual player exposure and time-loss injuries. Time-trend analyses were performed with Poisson regression using generalised linear models.

Results
2636 hamstring injuries represented 19% of all reported injuries, with the proportion of all injuries increasing from 12% during the first season to 24% in the most recent season. During that same period, the percentage of all injury absence days caused by hamstring injuries increased from 10% to 20%. Between 2014/15 and 2021/22, training hamstring injury incidence increased (6.7% annually, 95% CI 1.7% to 12.5%) as did burden (9.0% annually, 95% CI 1.2% to 18.3%). During those years, the match hamstring injury incidence also increased (3.9% annually, 95% CI 0.1% to 7.9%) and with the same trend (not statistically significant) for match hamstring injury burden (6.2% annually, 95% CI −0.5% to 15.0%).

Conclusions
Hamstring injury proportions—in number of injuries and total absence days—doubled during the 21-year period of study. During the last eight seasons, hamstring injury rates have increased both in training and match play.

Full text

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EkstrandJ, etal. Br J Sports Med 2022;0:1–7. doi:10.1136/bjsports-2021-105407
Hamstring injury rates have increased during recent
seasons and now constitute 24% of all injuries in
men’s professional football: the UEFA Elite Club Injury
Study from 2001/02 to2021/22
Jan Ekstrand ,1 Håkan Bengtsson ,1 Markus Waldén ,1 Michael Davison,2
Karim M Khan ,3 Martin Hägglund 1
Original research
To cite: EkstrandJ,
BengtssonH, WaldénM, etal.
Br J Sports Med Epub ahead
of print: [please include Day
Month Year]. doi:10.1136/
bjsports-2021-105407
1Department of Health,
Medicine and Caring Sciences,
Linköping University, Linköping,
Sweden
2FIFA Medical Centre of
Excellence, London, Isokinetic
Medical Group, London, UK
3Family Practice & Kinesiology,
The University of British
Columbia, Vancouver, British
Columbia, Canada
Correspondence to
Professor Jan Ekstrand,
Department of Health, Medicine
and Caring Sciences, Linköping
University, Linköping S- 582 20,
Sweden;
jan. ekstrand@ telia. com
Accepted 2 November 2022
© Author(s) (or their
employer(s)) 2022. Re- use
permitted under CC BY.
Published by BMJ.
ABSTRACT
Objectives To: (1) describe hamstring injury incidence
and burden in male professional football players over 21
seasons (2001/02 to 2021/22); (2) analyse the time-
trends of hamstring muscle injuries over the most recent
eight seasons (2014/15 to 2021/22); and (3) describe
hamstring injury location, mechanism and recurrence
rate.
Methods 3909 players from 54 teams (in 20 European
countries) from 2001/02 to 2021/22 (21 consecutive
seasons) were included. Team medical staff recorded
individual player exposure and time- loss injuries. Time-
trend analyses were performed with Poisson regression
using generalised linear models.
Results 2636 hamstring injuries represented 19% of
all reported injuries, with the proportion of all injuries
increasing from 12% during the first season to 24%
in the most recent season. During that same period,
the percentage of all injury absence days caused by
hamstring injuries increased from 10% to 20%. Between
2014/15 and 2021/22, training hamstring injury
incidence increased (6.7% annually, 95% CI 1.7% to
12.5%) as did burden (9.0% annually, 95% CI 1.2%
to 18.3%). During those years, the match hamstring
injury incidence also increased (3.9% annually, 95% CI
0.1% to 7.9%) and with the same trend (not statistically
significant) for match hamstring injury burden (6.2%
annually, 95% CI −0.5% to 15.0%).
Conclusions Hamstring injury proportions—in number
of injuries and total absence days—doubled during the
21- year period of study. During the last eight seasons,
hamstring injury rates have increased both in training
and match play.
INTRODUCTION
In 1999, the Union of European Football Associ-
ations (UEFA) conceived a research project—the
Elite Club Injury Study (ECIS)—which had the
aim of evaluating the risk of injury for top- level
men’s football players in Europe. Its ultimate
purpose was to reduce football injuries and increase
player safety.1–3 We have previously reported that
hamstring injuries constituted 12–17% of all time-
loss injuries in male professional football,2 4 and
that hamstring injury was the most common recur-
rent injury in football players.5
The incidence of match- related hamstring inju-
ries was stable from 2001/02 to 2013/14, even
though the incidence of training- related hamstring
injuries increased by an average of 4% annually
during that period.6 Researchers have proposed
various interventions to stem the tide of hamstring
injuries, and teams have variably embraced system-
atic hamstring prevention programmes.7 Whether
men’s professional football clubs have flattened the
curve of hamstring injuries since 2014 is unknown.
WHAT IS ALREADY KNOWN ON THIS TOPIC
⇒Hamstring muscle injuries increased in
incidence in men’s professional football from
2001 to 2014.
⇒Since then, players train more intensely and
their match calendar is more crowded. Many
professional teams aim to prevent muscle
injury as part of their strength and conditioning
programmes.
WHAT THIS STUDY ADDS
⇒During the recent eight seasons (2014/15
to 2021/22), the incidence and burden of
hamstring injuries during training and match
play have increased significantly.
⇒The proportion of injuries diagnosed as
hamstring injuries increased from 12% in
2001/02 to 24% in 2021/22.
⇒The proportion of all injury absence days caused
by hamstring injuries has increased from 10%
in 2001/02 to 20% in 2021/22.
⇒Around 18% of all reported hamstring injuries
were recurrences with over two- thirds occurring
within 2 months of the footballer’s return to
play.
HOW THIS STUDY MIGHT AFFECT RESEARCH,
PRACTICE OR POLICY
⇒These data on incidence and burden of
hamstring injuries provide a strong rationale for
teams to keep focusing on preventing first and
recurrent hamstring injuries.
⇒The high rate of recurrent hamstring injury
within 2 months of return to play suggests
this period is a particularly important time for
team clinicians to: (1) carefully monitor players
completing evidence- based rehabilitation; (2)
manage training and match loads; and (3)
maintain preventive programmes.
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2EkstrandJ, etal. Br J Sports Med 2022;0:1–7. doi:10.1136/bjsports-2021-105407
Original research
If hamstring injury incidence and burden have risen in recent
years, it might imply that current preventative measures are
not working. On the other hand, if hamstring injury incidence
and burden have decreased, it could be due to effective preven-
tion strategies. We know of no comparable surveillance studies
of hamstring injuries in any professional sport (not just men’s
football). Additionally, little is known about whether hamstring
injuries occur more commonly in the biceps femoris or the semi-
membranosus/semitendinosus muscles,8 and about the propor-
tion of hamstring injuries that meet the criteria for structural
(muscle fibre) injury.9
The primary objective of this ECIS study was to describe the
hamstring injury incidence and burden in professional men’s
football over 21 seasons (2001/02 to 2021/22). Our secondary
aim was to analyse the time trends of hamstring muscle injuries
over the most recent eight seasons (2013/14 to 2021/22) to see
if previously reported trends have continued. Further objectives
were to compare the prevalence of (1) biceps femoris injuries
and semitendinosus/semimembranosus injuries, and (2) struc-
tural and functional hamstring injuries.
METHODS
The ECIS data collection began in July 2001,10 and the study
recently completed its 21st season (2021/22). The data in this
prospective hamstring sub- study cover the period from 2001/02
(inception) to 2021/22 (21 consecutive seasons). Each year, all
teams that qualify for the UEFA Champions League (UCL) group
stage are invited by UEFA to participate in ECIS. Teams that have
been enrolled for at least one season may continue in ECIS even
if they do not qualify for the UCL group stage during a subse-
quent season.
Study population
Inclusion criteria at team- level: to increase homogeneity in this
hamstring sub- study it was decided to include data from ECIS
teams only during seasons that they participated in the UCL
group stage.11 We did this for two reasons:
1. there could be some important differences between teams
participating in the UCL group stage and teams that did not
(eg, fixture congestion, tactics, playing style, financial con-
ditions, etc)
2. the number of teams that failed to qualify for the UCL group
stage following their initial inclusion in ECIS has increased
during the course of the study.
During the 21 seasons, 54 teams from 20 European countries
(total 323 team- seasons) met this criteria and were included for
analysis.
Inclusion criteria at player- level: all players with a first team
contract within the included teams were invited to volunteer for
the study. A total of 3909 players (total 9728 player- seasons)
who gave their written informed consent were included. Players
who left or joined the team during the season (eg, due to club
transfer) were included during their time on the team. No player
refused to participate in the study.
Study procedure
Included teams assigned a contact person (a member of the medical
staff) who was responsible for registering data. The contact person
was given a manual that provided the methodology and opera-
tional definitions used in the study (table 1). These definitions have
been constant throughout the entire period of ECIS. New variables
were added for the 2011/12 and subsequent seasons, describing
affected muscles (biceps femoris or semitendinosus/semimembra-
nosus) and injury classification according to the Munich muscle
injury classification system (functional or structural).9 Functional
muscle disorders was the term for disorders without macroscopic
evidence of fibre tear on MRI; structural muscle injuries refer to
cases where there was macroscopic evidence of fibre tear on MRI.
Sub- classifications were presented for each type.9
The study was carried out by the Football Research Group
(FRG) on behalf of UEFA. FRG is an international research
team conducting studies on football injuries. Teams were asked
to provide the study group with exposure and injury data each
month. All data were reviewed by one FRG researcher in the
beginning, but lately by two FRG researchers (as the study has
expanded), who performed quality control (ensuring that the
data were complete and complied with the study protocol).
These controllers had been trained in football epidemiology
and held either a PhD (HB, MW, MH, JE, Matilda Lundblad
and Karolina Kristenson) or a Master’s degree (Anna Hallén) in
this field. If any missing or unclear data were identified during
this review process, immediate feedback was sent by the quality
controllers to the club’s contact person to complete or correct
the data. Further, a statistician (Henrik Hedevik) scrutinised the
database after each season, seeking to flush out other possible
data input errors. The study methods were consistent with the
prevailing international consensus statements on football injury
epidemiology; detailed descriptions of the methods,12–14 and the
FRG study group,15 have been published previously.
Table 1 Operational definitions
Training session Team training that involved physical activity under the supervision of the coaching staff
Match Competitive or friendly match against another team
Injury Any physical complaint sustained by a player that resulted from a football match or football training and led to the player being unable to take full
part in future football training or match play.
Hamstring injury A traumatic distraction or gradual onset injury to the hamstring muscle group
Functional muscle injury Acute indirect muscle disorder without macroscopic evidence of muscle tear
Structural muscle injury Acute indirect muscle injury with macroscopic evidence of muscle tear
Injury severity The number of lay- off days caused by the injury between the injury date and the date when the medical team declared the player as ready for full
participation in team training and availability for match play
Recurrent injury Injury of the same type and at the same site as an index injury occurring previously during the same season
Early recurrence Recurrent injury that occurs within 2 months after return to full participation from the index injury
Injury incidence Number of injuries per 1000 player hours ((Σ injuries/Σ exposure hours)×1000)
Injury burden Number of lay- off days per 1000 player hours ((Σ lay- off days/Σ exposure hours)×1000)
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Original research
Exposure
Individual football exposure was registered in minutes on a stan-
dard attendance record. This form included information about
the duration for each exposure (including training and match
play), or whether players were absent due to injury, illness,
national team duty, or other reasons.
Injury
Injury was defined according to time- loss (table 1).12 For each
injury, the contact person was asked to complete an injury card
including a free text diagnosis. Based on the information on the
injury card, members of the study group classified the injury using
the Orchard Sports Injury Classification System (OSICS).16 17
OSICS version 2 was used throughout the entire study period
while OSICS version 10 was added from the 2011/2012 season.
Since the 2011/12 season, injuries were also described using
the Munich muscle injury classification,9 18 which distinguishes
between functional and structural muscle injuries. We have not
changed any element of data collection that might bias how we
measure injury incidence.
Injury severity was defined by the number of lay- off days that
passed between the injury occurrence and return to play, and
categorised in five different categories: slight (0 days), minimal
(1–3 days), mild (4–7 days), moderate (8–28 days), severe
(>28 days).1 2 4
Injuries that did not occur during a specific identifiable event
(eg, overuse/gradual onset injuries) were assigned to the last
activity (training or match) that the injured player completed
before being removed from full participation in team activities.
For match injuries the injury card also contained information
about the match minute in which the injury occurred.
Patient and public involvement
Before the launch of the first season of ECIS, the medical staff
of the participating teams (as per May 2001) were invited to
comment on the study procedures and definitions.12 This
research was done without patient (player) involvement; patients
(players) were not invited to comment on the study design or
to contribute to the drafting of this document. We propose to
include players and health professionals in the knowledge trans-
lation that will follow the publication of this study.
Equity, diversity, and inclusion
This study focused exclusively on male professional football
players. We acknowledge that our author list does not reflect
the diversity of the sport and exercise medicine/rehabilitation
community. A women’s ECIS (WECIS) was launched in July
2017 in collaboration with UEFA with similar data collection
on hamstring injuries. The WECIS is led by Anna Hallén and the
author team includes two female football doctors.
Data analyses
Time- trend analyses were performed with Poisson regression
using generalised linear models (GENLIN) with injury count
data (number of injuries and number of lay- off days) as the
dependent variables, season as covariate, and with natural log
of the exposure variable (total hours, training hours, and match
hours) as offset variable. We used the model- based estimator for
the covariance matrix, and Wald statistics to calculate the p value
and 95% CI. Time- trends in injury incidence and injury burden
are expressed as the annual % change between seasons with a
95% CI. Time- trend analyses were performed (1) for the entire
21- season study period, and (2) for the sub- period 2014/15
to 2021/22 (since the 2001/02 to 2013/14 seasons have been
covered in a previous report).6
Injury incidence was defined as the number of injuries per
1000 hours and described with 95% CI. To compare the injury
incidence between training and match play, a rate ratio (RR) with
a corresponding 95% CI was calculated and tested for statistical
significance using Z- statistics. Injury burden was calculated for
all participating teams in each of the 21 seasons separately and
are presented as the mean injury burden for these seasons with
corresponding SD. Injury severity was expressed as the median
number of lay- off days with the corresponding IQR. The number
of lay- off days was compared between structural and functional
injuries, as well as between biceps femoris injuries and semiten-
dinosus/semimembranosus injuries, using the Mann- Whitney U
test due to skewed data distribution.
The proportion of injuries representing the different severity
categories, injury mechanisms, period of match, and recurrences
were compared between structural and functional injuries,
and between biceps femoris injuries and semitendinosus/semi-
membranosus (both available from 2011/12), using the χ2 test.
The Z- test was used for pairwise comparisons with Bonferroni
correction for multiple comparisons. A one- sample proportional
Z- test was used to compare the proportion of injuries occurring
in different 15 min periods of match halves. Analyses were two-
sided and the significance level was set at p<0.05. All analyses
were conducted in Statistical Package for the Social Sciences
(SPSS) v28.
RESULTS
Time trends during the 21 seasons
During the 2001/02 to 2021/22 seasons, team medical staff
reported 2636 hamstring injuries during 2 131 561 total expo-
sure hours; 922 injuries (34%) during 1 787 823 training hours
and 1714 injuries (66%) during 343 738 match hours. The
proportion of reported injuries that were diagnosed as hamstring
injuries increased from 12% in the first season to 24% in the
last season and constituted 19% of all 14 057 injuries regis-
tered during the 21- season study period. In terms of absence
days, hamstring injuries caused 14% of the total injury lay- off
days, increasing from 10% to 20% between the 2001/02 and the
2021/22 seasons. The hamstring injury incidence was 10 times
higher during match play than training (4.99/1000 hours vs
0.52/1000 hours; RR 9.67, 95% CI 8.93 to 10.47). The median
lay- off following a hamstring injury was 13 days (IQR 7–22).
In general, 20% of players missed training or match play due
to hamstring injury during a season, and a 25- player squad can
expect about eight hamstring injuries per season.
Of all the hamstring injuries, 475 (18%) were recurrences,
and early recurrences (within 2 months, n=325) made up 69%
of these. Recurrences were nine times more likely to occur in
matches than in training (0.88 vs 0.10; RR 9.25, 95% CI 7.67 to
11.15) as were early recurrences (0.61 vs 0.07; RR 9.25, 95% CI
7.37 to 11.60).
Time- trend analyses over all 21 seasons showed no signif-
icant trends for overall football injury incidence (0.7% annu-
ally, 95% CI −0.6% to 1.9%) or injury burden (1.4% annually,
95% CI −0.1% to 3.0%) (figures 1 and 2).
Time-trends during 2014/15-2021/22
In the period 2014/15 to 2021/22, time trend analysis revealed
a significant increase in both training hamstring injury incidence
and training hamstring injury burden. During the same period,
the match hamstring injury incidence also increased with the
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Original research
same trend (not statistically significant) observed for the match
hamstring injury burden (table 2).
Characterisation by injury types during 2001/02 to 2021/22
Injury characteristics of structural/functional and lateral/medial
injuries are presented in table 3. Between the 2011/12 and
2021/22 seasons, 1819 hamstring injuries were reported and
classified according to the Munich muscle injury classification
system (no classification available for 24 injuries). The majority
of these (n=1312, 71%) were classified by the team doctors
as structural injuries. Structural injuries were associated with
longer lay- off than functional injuries (median absence 17 vs
6 days) (table 3). Running/sprinting was the most common injury
mechanism (62% of structural injuries and 51% of functional
injuries). The χ2 test revealed a significant overall difference in
the frequency of injury mechanisms between structural and func-
tional injuries (p<0.001).
Almost 50% of match hamstring injuries occurred during
the last 15 min of the first and second halves, which deviated
from the expected match distribution (p<0.001) (figure 3). A
significant difference (p=0.002) in the frequency of structural
and functional injuries in different match periods was observed
(figure 3).
Characterisation by injury location
Of the 1843 injuries reported since the 2011/12 season, 1319
(72%) were given diagnoses specific to either biceps femoris
or semitendinosus/semimembranosus. Biceps femoris injuries
(n=1054, 80%) were more common, especially in match play
(n=761, 84%).
The χ2 test revealed a significant overall difference in the
frequency of injury mechanisms between biceps femoris and
semitendinosus/semimembranosus injuries (p=0.009), with a
higher proportion of lateral injuries caused by running/sprinting
while fewer lateral injuries were caused by stretching. Lateral
injuries were also associated with longer absence than medial
injuries (p=0.010) with a larger proportion of the medial inju-
ries being minimal or mild (p=0.009).
A significant difference in the frequency of biceps femoris and
semimembranosus/semitendinosus injuries in different match
periods was observed (p<0.001) (figure 3).
DISCUSSION
This unique long- term ECIS dataset allows us to explore 21- year
trends for hamstring injuries in men’s professional football in
Europe.
Figure 1 Development of hamstring injury incidence and injury burden over the study period. Injury incidence is defined as the number of injuries
per 1000 hours of exposure, presented with 95% CI. Injury burden is defined as the number of absence days caused per 1000 hours of exposure
and is presented as the mean of participating teams with SD. The shaded area represents the most recent eight- season period which has not been
previously published.
Figure 2 Development of the proportion of all reported injuries that were diagnosed as hamstring injuries and the proportion of all injury absence
days caused by hamstring injuries over the study period. The shaded area represents the most recent eight- season period which has not been
previously published.
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Original research
►The most important findings were that the proportion of all
injuries diagnosed as hamstring injuries increased from 12%
to 24%, and that the proportion of all injury absence days
caused by hamstring injuries increased from 10% to 20%
during the 21- year study period.
►The most worrying finding was that the injury rates have
increased during the recent eight seasons.
►The most surprising finding was that structural injuries were
more common than functional injuries during the most
recent 11 seasons.
►The most expected findings (extending previous studies)
were that hamstring injuries were: (1) most commonly due
to running/sprinting; (2) more likely to occur in the last
15 min of match halves; (3) affect the biceps femoris rather
than the semimembranosus/semitendinosus muscles; and (4)
predisposed to recur within 2 months in the same location.
Can we explain why hamstring injuries are contributing an
increasing proportion of all injuries?
Over the past 21 years, in professional men’s football, hamstring
injuries have increased substantially as a proportion of the total
number of reported injuries and as a proportion of injury burden.
This is due to a combination of a decrease in other injuries (such
as ligament injuries)2 11 and an absolute increase in the number of
hamstring injuries per unit of exposure. Before we speculate as to
why hamstring injuries have increased, we provide three reasons
to underscore that we feel our data are valid—they are not the
result of a secular trend in reporting (a bias). We note that: (1)
we have not changed our diagnostic criteria; (2) the incidence
data are based on time loss (not a clinician’s impression); and (3)
there has been no increase in ‘all injuries’ within our study. The
latter would be the case if the clinicians who collected data had
become more attuned or motivated to capture injuries.
Can we explain why hamstring injury rates are still high and
even increasing during recent seasons?
This is a prospective epidemiological study revealing significant
associations. Causative factors cannot be evaluated using this
study design; we do not know the reasons for the observations.11
However, after 21 years of monthly contacts with these Cham-
pions League teams, we respectfully propose two hypotheses.
First, the intensity of elite men’s football has increased over
at least a period of the years that are included in the current
study.19 Current football practice includes a large volume of high
intensity football actions.20 21 Professional players now under-
take more high- intensity activities per match than they did previ-
ously and they also run faster than their predecessors.19 In our
study, 61% of hamstring injuries occurred while the player was
running/sprinting. We postulate that the number of hamstring
injuries increased over time due to a greater number of high- risk
activities in later years.
Compounding the pressure on hamstrings associated with
football intensity is the increase in the total amount of inter-
national team travel and matches. This is often referred to as
the problem of the crowded player calendar.22 Professional
players now work year- round apart from a 4–6 week break
between seasons.22 Even during the traditional break between
Table 2 Results from the Poisson regression analyses showing the
seasonal change in hamstring injury incidence between the 2014/15
and 2021/22 seasons
% Annual change 95% CI P value
Training
Hamstring injury incidence 6.7% (1.7 to 12.5) 0.009
Hamstring injury burden 9.0% (1.2 to 18.3) 0.024
Match play
Hamstring injury incidence 3.9% (0.1 to 7.9) 0.045
Hamstring injury burden 6.2% (−0.5 to 15.0) 0.116
Results are expressed as the annual % change with 95% CI. Positive values
represent an increasing trend and negative values a decreasing trend.
Table 3 Hamstring injury characteristics
All injuries
Injury type Injury location
Structural Functional Biceps femoris Semimembranosus/ semitendinosus
Injury lay- off
Lay- off days, median (IQR) 13 (7 to 22) 17 (11 to 25)* 6 (4 to 10)* 16 (10 to 25)* 15 (8 to 23)*
Injury severity
Slight, n (%) 9 (0%) 2 (0%) 1 (0%) 2 (0%) 0 (0%)
Minimal, n (%) 222 (8%) 32 (2%)* 102 (20%)* 27 (3%)* 19 (7%)*
Mild, n (%) 508 (19%) 128 (10%)* 205 (40%)* 122 (12%)* 43 (16%)*
Moderate, n (%) 1514 (57%) 886 (68%)* 187 (37%)* 709 (67%) 164 (62%)
Severe, n (%) 383 (15%) 264 (20%)* 12 (2%)* 194 (18%) 39 (15%)
Total, n 2636 1312 507 1054 265
Injury mechanism
Running/sprinting, n (%) 1230 (61%) 790 (62%)* 232 (51%)* 654 (64%)* 140 (56%)*
Stretching, n (%) 103 (5%) 82 (6%)* 14 (3%)* 52 (5%)* 24 (10%)*
Overuse/gradual onset, n (%) 249 (12%) 60 (5%)* 150 (33%)* 63 (6%) 23 (9%)
Other, n (%) 449 (22%) 334 (26%)* 55 (12%)* 253 (25%) 63 (25%)
Total, n 2031 1266 451 1022 250
Recurrences
Recurrences, n (%) 477 (18%) 234 (18%) 83 (16%) 195 (19%) 45 (17%)
Early recurrences, n (%) 325 (12%) 163 (12%) 47 (9%) 136 (13%) 29 (11%)
Total, n 2636 1312 507 1054 265
Injury mechanisms were registered starting from the 2008/09 season and are thus missing for 605 injuries.
*Significant difference in lay- off days, injury severity proportion, injury mechanism proportion, or recurrence proportion between injury types or injury locations.
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Original research
seasons, players are often required to undertake pre- season tours
which require intercontinental travel.23 This relentless travel
and playing demand is associated with players being limited to
fewer training sessions during the pre- season period23; training
sessions may lower injury risk.23
Why are the hamstring injury rates not decreasing despite
studies showing that the Nordic Hamstring Exercise can
reduce injury rates?
The Nordic Hamstring Exercise programme has been promoted
for injury prevention,24 and may reduce hamstring injuries by
65–70%.25 26 However, the programme has not been widely
adopted in men’s professional football in Europe.24 27 Challenges
to implementing the programme include:
►imited influence by the medical team on coaching practices
►imited time to include preventive exercises in training before
a match
►players indicating that the exercise gives them muscle
soreness.
It is also considered unlikely that a single exercise would be
the whole solution to a multifactorial injury problem.27
Can we explain why structural injuries were reported to be
more common than functional injuries during the recent eight
seasons?
We noted some differences in access to imaging and the quality
of imaging during the 21 years of this study. In the 2007–2011
period, MRI or ultrasound imaging was obtained in 87% of all
hamstring injuries.28 In that study, 13% of hamstring injuries
showed no MRI signal of abnormality and 57% of injuries had
muscle oedema, but no fibre disruption on MRI. These injuries
were functional injuries according to the Munich consensus.9
The remaining 30% of injuries were radiologically classified as
structural.28
The 2007–2011 pattern of imaging contrasts starkly with data
from the recent years (2011–2018) where 72% of all hamstring
injuries were structural, with evidence of muscle tears on MRI.
One difference across the two time periods is the quality of MRI
scanners. The images reported in 2012 were obtained using 1.0
T and 1.5 T scanners. Since then, many football players have
been scanned using 2 T and 3 T magnets which makes the
image more likely to appear as Peetrons29 grade 2 (structural)
than 1 (functional).29 There have been considerable hardware
and software improvements with MR units, irrespective of field
strength. With better coils and resulting improved spatial and
contrast resolution, as well as advances in sequence design, even
the 1.5 T machines are producing much higher quality images
demonstrating more subtle pathology than those of 10 years ago
(B Forster, personal communication, 14 October 2022).
How can our findings be of practical value for players,
clinicians and clubs?
In our opinion, collaboration between medical staff, coaches,
players and directors will provide the best perspective of how
the game of football evolves. Such interdisciplinary discussion is
likely to help find solutions to keep players safe and at reduced
injury risk.11
Our finding that players are exposed to high risk of recur-
rences during the first 2 months after the index hamstring injury
is important for team clinicians. We recommend that clinicians
discuss this crucial point with players and coaches/managers/‘the
football department’ so that appropriate programmes can be
implemented (eg, making all parties aware of the risk, trying
to have players complete their rehabilitation diligently, and
managing load management in training, and where possible, in
matches). We appreciate there is no evidence- based (randomised
controlled trial) programme to prevent hamstring recurrence
yet.
We found the biceps femoris muscle was injured far more
frequently than the semimembranosus/semitendinosus muscles.
The reasons for this finding are not well understood, and provide
an important focus for future research.
METHODS CONSIDERATIONS
A main strength of this cohort study is that its design closely
follows the international consensus statements and reporting
guidelines for epidemiological research in sport.12–14 This allows
for our study to be compared with those that followed similar
methodologies.12–14 Protocol changes are necessary over decades
and where this has been appropriate we clearly described what
we have done—such as when we began obtaining more detail on
the type of hamstring injury and its location.
We took steps to increase the reliability of the collected
data, such as a detailed study manual and close communication
between the study group and all participating football teams and
data collectors.14 We note that the set of teams that contribute
data to ECIS has differed season by season and this could influ-
ence the time- trend analyses. To increase the homogeneity of
the cohort over different seasons, the data reported here only
include players from teams during seasons in which they quali-
fied for the group stage of the UEFA Champions League.
In this study, we used a time- loss injury definition which
means that contact persons were asked to give a date when
injured players were considered ready for full return to
Figure 3 Distribution of different hamstring injury types and locations in different periods of matches. Asterisks indicate significantly different
proportions of biceps femoris to semimembranosus/semitendinosus or functional to structural injuries.
on December 7, 2022 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2021-105407 on 6 December 2022. Downloaded from

7
EkstrandJ, etal. Br J Sports Med 2022;0:1–7. doi:10.1136/bjsports-2021-105407
Original research
participation in all team activities. Return to play is often a
continuum so the exact date when a player is considered ready
to participate fully can be difficult to define. It is also possible
that players may return to full participation even though
they have persisting symptoms that may affect their ability to
perform. These aspects of an injury are not covered using a
time- loss definition and are thus not included in our definition
of injury burden.
Twitter Jan Ekstrand @JanEkstrand, Håkan Bengtsson @HockanB, Markus Waldén
@MarkusWalden, Karim M Khan @KarimKhan_IMHA and Martin Hägglund
@MHgglund
Acknowledgements The authors thank the participating clubs for their
participation in the study. The authors would also like to thank Armin Spreco,
Linköping University, Sweden for statistical advice and Professor Bruce B Forster,
Department of Radiology, UBC Faculty of Medicine, Canada for MRI information. The
Football Research Group has been established in Linköping, Sweden, in collaboration
with Linköping University and through grants from the Union of European Football
Associations, the Swedish Football Association, and the Swedish National Centre for
Research in Sports.
Contributors JE was responsible for the conception of the study. JE, HB, MW
and MH have all been involved in the data collection. HB conducted the analyses
together with the biostatistician. JE drafted the manuscript which was critically
revised by the rest of the co- authors including KK and MD. JE is the study
guarantor.
Funding This study was funded by grants from the UEFA.
Competing interests None declared.
Patient and public involvement Patients and/or the public were not involved in
the design, or conduct, or reporting, or dissemination plans of this research.
Patient consent for publication Consent was obtained directly from the
patient(s)
Ethics approval This study involves human participants and was approved by
Ethics approval 21/06/2014 UEFA. Participants gave informed consent to participate
in the study before taking part.
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement No data are available. Data sets not available due
to confidentiality of players and teams.
Open access This is an open access article distributed in accordance with the
Creative Commons Attribution 4.0 Unported (CC BY 4.0) license, which permits
others to copy, redistribute, remix, transform and build upon this work for any
purpose, provided the original work is properly cited, a link to the licence is given,
and indication of whether changes were made. See:https://creativecommons.org/
licenses/by/4.0/.
ORCID iDs
JanEkstrand http://orcid.org/0000-0002-6092-266X
HåkanBengtsson http://orcid.org/0000-0003-3809-5909
MarkusWaldén http://orcid.org/0000-0002-6790-4042
Karim MKhan http://orcid.org/0000-0002-9976-0258
MartinHägglund http://orcid.org/0000-0002-6883-1471
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