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Copyright © 2016 Korean Society of Exercise Rehabilitation http://www.e-jer.orgpISSN 2288-176X
eISSN 2288-1778 134*Corresponding author: Kjell Svensson
http://orcid.org/0000-0001-9478-2792
CIFU/Capio Artro Clinic, Sophiahemmet, Box 5605, 114 86 Stockholm, Sweden
Tel: +46-8-406-2700, Fax: +46-48-44-8280, E-mail: [anonimizat]
Received: January 26, 2016 / Accepted: May 20, 2016The correlation between the imaging characteristics of
hamstring injury and time required before returning to
sports: a literature review
Kjell Svensson1,*, Marie Alricsson2,3, Mattias Eckerman1, Theofilos Magounakis4, Suzanne Werner1,5
1Stockholm Sports Trauma Research Centre, Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden
2Swedish Winter Sport Research Centre, Department of Health Sciences, Mid Sweden University, Östersund, Sweden
3Department of Sports Science, Linnaeus University, Kalmar, Sweden
4Clinic of Radiology, Gällivare Hospital, Gällivare, Sweden
5Capio Artro Clinic, Stockholm, Sweden
Injuries to the hamstring muscles are common in athletes. Track and
field, Australian football, American football and soccer are examples of sports where hamstring injuries are the most common. The purpose of this study was to investigate whether there is a correlation between a hamstring injury prognosis and its characteristics of imaging parame
–
ters. The literature search was performed in the databases PubMed and CINAHL, and eleven articles were included. Seven out of the 11 ar
–
ticles showed a correlation between the size of the hamstring injury and length of time required before returning to sports. Different authors have reported contrasting results about length of time required before returning to sports due to location of injury within specific muscle. Ma
–
jority of the articles found hamstring strain correlated to an extended amount of time required before returning to sports.
Keywords: Athletes, Magnetic resonance imaging, Muscle, Sonogra
–
phy, Sport, Strain
INTRODUCTION
Injuries to the hamstring muscles are common in athletes
(Alonso et al., 2009). Track and field (Bennell and Crossley, 1996),
Australian football (Orchard et al., 2013) and American football
(Feeley et al., 2008) are examples of sports where hamstring inju –
ries are the most common. Soccer, one of the largest sports in the
world (Federation Internationale de Football Association [FIFA ],
2006), is a sport where hamstring injuries frequently occur. Ac –
cording to Ekstrand et al. (2011b), muscle injuries accounted for
31% of all injuries in football with hamstring injuries being the
most common. They also reported that a football team of 25 play –
ers usually have about ten muscle injuries per year (Ekstrand et
al., 2011a). According to Carling et al. (2011), the risk of a mus –
cle strain in soccer is 4.3/1,000 hr exposure time, with hamstring muscles most often being affected.
A careful review of the injury incident followed by a clinical ex –
amination is required in order to diagnose a muscle injury. Clini –
cal examination of a muscle injury consists of inspection, palpa –
tion, testing of active and passive range of motion and manual
muscle tests of the involved muscle. Minor injuries are more diffi –
cult to diagnose than more extensive ones; therefore, ultrasound
or magnetic resonance imaging (MRI) have been suggested
(Järvinen et al., 2007; Slavotinek, 2010). Ultrasound is considered
the first choice of imaging, as it is dynamic, straightforward and
rapid compared with MRI and also considerably more inexpensive
(Connell et al., 2004; Järvinen et al., 2007; Peetrons, 2002). With
ultrasound and MRI, it is possible to detect the location of the
damage, fluid retention and discontinuity of muscle fibres. Fur –
thermore, it is possible to measure the size of the injury (Peetrons, http://dx.doi.org/10.12965/jer.1632558.279Review Article
Journal of Exercise Rehabilitation 2016;12(3):134-142

http://www.e-jer.org 135 http://dx.doi.org/10.12965/jer.1632558.279Svensson K, et al. • Hamstring injuries and returning to sports
2002; Slavotinek, 2010). According to Peetrons (2002), a muscle
injury can be classified into four different degrees. Grade 0 indi –
cates no visible tissue damage, grade 1 means oedema but without
fibre damage, grade 2 fibre damage and grade 3 a total muscle or
tendon rupture.
Other parameters to assess the extent of a muscle injury are to
measure the size of the total cross-sectional area as well as the
length and volume of the hamstring injury. The image that is
chosen to determine the cross-sectional area is taken from where
the extent of the damage is greatest. The ratio of the extent of
damage to the entire muscle bundle cross-sectional area is the per –
centage cross-sectional area that is damaged. This area is therefore
considered to represent the proportion of myofibrils that are de –
stroyed. For this to be assessed, the image should be taken in the
transverse plane (Slavotinek, 2010). Thus, it will be possible to
measure the extent of damage where it is the largest in both the
transverse (T) and anterior-posterior (AP) direction. In longitudi –
nal images, i.e., where the damage is seen from the side, it is pos –
sible to measure the length of the injury in cranio-caudal (CC) di –
rection. To calculate the volume, the formula ( [π/6]×AP×T×CC)
is used, assuming the damage to form an ellipse (Slavotinek et al.,
2002). Sensitivity to fluid retention has been shown to be equal in
both MRI and ultrasound (Koulouris and Connell, 2005). The as –
sessment of the volume of fluid in the muscle has even proven to
be better using ultrasound than MRI (Thorsson et al., 1993). MRI
examination is the most accurate method for assessing a muscle
injury (Hayashi et al., 2012), but the sensitivity to detect the
healing process is not as high as with ultrasound examination
(Connell et al., 2004). However, it is important that the radiolo –
gist is experienced in order to make an accurate assessment (Douis
et al., 2011). Depending on the muscle group, the images should
be taken 2 hr to 5 days after the injury occurs (Peetrons, 2002).
Imaging can follow the healing process of a muscle. Retractions of
muscle damage, acute effects, the development of scar tissue and
detection of complications are parameters that can be assessed
(Slavotinek, 2010).
Hamstring muscle injuries account for a large part of the time
that an athlete is injured (Ekstrand et al., 2011b). There is often a
high priority to return to playing sports as quickly as possible af –
ter a muscle injury, which means that the risk for reinjury is high
(Ekstrand et al., 2011b). Maybe the injury characteristics could
determine the length of time before returning to sports, which is
important in order to have accurate prognosis and improve the re –
habilitation process.
The purpose of this study was to investigate whether there is a correlation between a hamstring injury prognosis and its charac –
teristics of imaging parameters. The objectives are threefold: to
study whether there are any correlations between (a) muscle that
is injured, (b) size of the injury, (c) injury location and length of
time before being able to return to sports, respectively.
MATERIALS AND METHODS
Systematic literature review was performed. The keywords used
were hamstring injury, hamstring strain, return to sports, time to
recovery, return to play, imaging, magnetic resonance and sonog –
raphy. In total, 388 articles were found. First, the abstract of each
article was reviewed; second, the whole article was reviewed to de –
termine if it met the inclusion criteria presented below.
Inclusion criteria
– Hamstring injured athletes
– Assessment of injury with MRI or sonography
– Length of time before returning to sports after injury as out –
come variable
– Prospective design
– Publications in English in scientific journals from 1990
– Original articles
– Human studies
Exclusion criteria
– Case studies
– Other muscle groups than hamstrings
– Articles where only total hamstring ruptures or avulsion frac –
tures were included
Database search
The literature review search was performed using the databases
PubMed and CINAHL with different keyword combinations (Ta –
ble 1). All searches were carried out until the 28th of November
2015. Eleven articles (Askling et al., 2007a; Askling et al.,
2007b; Askling et al., 2008; Comin et al., 2013; Connell et al.,
2004; Ekstrand et al., 2012; Gibbs et al., 2004; Hallén and Ek –
strand, 2014; Moen et al., 2014; Petersen et al., 2014; Silder et
al., 2013) qualified for inclusion. These eleven articles were re –
viewed according to the aims of the study, and four articles (Com –
in et al., 2013; Connell et al., 2004; Ekstrand et al., 2012; Hallén
and Ekstrand, 2014) addressed the question about the specific
muscle injury. All 11 articles addressed the question about the
size of the injury, and six articles related to the question about in –

http://dx.doi.org/10.12965/jer.1632558.279Svensson K, et al. • Hamstring injuries and returning to sports
136 http://www.e-jer.orgjury location within the muscle (Askling et al., 2007a; Askling et
al., 2007b; Askling et al., 2008; Comin et al., 2013; Connell et
al., 2004; Moen et al., 2014) (Table 2). The purpose of each in –
cluded article is presented in Table 3.
RESUL TS
Item presentation
Table 2 presents the articles that relate to each question. Also, an overview of each item is presented in Table 3.
A summary of the included studies is provided in Table 4. Be –
low is a presentation of the results from the issues.
Is there a correlation between size of injury and length of time
before returning to sports?
The answer to the question was based on nine out of the 11 ar –
ticles (Askling et al., 2007a; Askling et al., 2007b; Askling et al.,
2008; Connell et al., 2004; Ekstrand et al., 2012; Gibbs et al., Table 1. Keywords and selected articles in databases
Database Issue No. of items Total selected studies Study
PubMed Hamstring injury return to play 130 5 C omin et al. (2013); Ekstrand et al. (2012); Hallén (2014); Moen et
al. (2014); Petersen et al. (2014)
PubMed Hamstring injury return to sport imaging 23 5 A skling et al. (2007a); Askling et al. (2007b); Askling et al. (2008);
Connell et al. (2004); Silder et al. (2013)
PubMed Hamstring injury return to play imaging 36 0 –
PubMed Hamstring strain return to sport magnetic resonance 7 0 –
PubMed Hamstring injury time to recovery imaging 29 1 Gibbs et al. (2004)
PubMed Hamstring strain return to sport sonography 0 0 –
CINAHL Hamstring injury return to play 28 0 –
CINAHL Hamstring injury return to sport imaging 81 0 –
CINAHL Hamstring injury return to play imaging 10 0 –
CINAHL Hamstring strain return to sport magnetic resonance 11 0 –
CINAHL Hamstring injury time to recovery imaging 16 0 –
CINAHL Hamstring strain return to sport sonography 17 0 –
Table 2. Study presentation and the issues they address
Study TitleResponds issue,
number
1 2 3
Askling et al. (2007a) A cute first-time hamstring strains during high-speed running: A longitudinal study including clinical and magnetic resonance
imaging findings× ×
Askling et al. (2007b) A cute first-time hamstring strains during slow-speed stretching: Clinical, magnetic resonance imaging (MRI) and recovery char –
acteristics× ×
Askling et al. (2008) P roximal hamstring strains of stretching type in different sports: Injury situations, clinical and magnetic resonance imaging char –
acteristics and return to sport× ×
Comin et al. (2013) Return to competitive play after hamstring injuries involving disruption of the central tendon × × ×
Connell et al. (2004) Longitudinal study comparing sonographic and MRI assessments of acute and healing hamstring injuries × × ×
Ekstrand et al. (2012) Hamstring muscle injuries in professional football: The correlation of MRI findings with time required before return to play × ×
Gibbs et al. (2004) T he accuracy of MRI in predicting recovery and recurrence of acute grade one hamstring muscle strains within the same season
in Australian Rules football players×
Hallén and Ekstrand (2014) Return to play following muscle injuries in professional footballers × ×
Moen et al. (2014) Predicting return to play after hamstring injuries × ×
Petersen et al. (2014) The diagnostic and prognostic value of ultrasonography in soccer players with acute hamstring injuries ×
Silder et al. (2013) C linical and morphological changes following 2 rehabilitation programmes for acute hamstring strain injuries: A randomised
clinical trial×
1, Is there a correlation between size of injury and length of time before returning to sports?; 2, Is there a correlation between the injured muscle bundle and length of time be –
fore returning to sports?; 3, Is there a correlation between the injury location within the muscle and length of time required before returning to sports?

http://www.e-jer.org 137 http://dx.doi.org/10.12965/jer.1632558.279Svensson K, et al. • Hamstring injuries and returning to sports
2004; Hallén and Ekstrand, 2014; Petersen et al., 2014; Silder et
al., 2013). In five of these (Askling et al., 2007a; Connell et al.,
2004; Ekstrand et al., 2012; Gibbs et al., 2004; Hallén and Ek –
strand, 2014), there was a correlation in that the more extensive
the injury, the longer the time required before returning to sports.
Askling et al. (2007a) reported clear links between volume
(r=0.608) and cross-sectional area (r =0.695), as measured by
MRI four days after the injury, and a longer time required before
returning to sports ( P<0.05). Connell et al. (2004) showed that
at baseline, the longitudinal length of the hamstring tear on MRI
had the highest correlation with recovery (r =0.58, P<0.0001)
and was the best radiologic predictor of amount of time required
before returning to sports. Multivariate analysis of MRI showed
that the injury incidence of the biceps femoris and the length of
the injury were factors resulting in an extended amount of time
required before returning to sports (r =0.62). An analysis of ultra –
sound images showed that the incidence of an injury to the biceps
femoris, percent of cross-sectional area and presence of hematomas
were predictors of length of time required before returning to
sports (r =0.58, P<0.05). Ekstrand et al. (2012) reported that the
greater the extent of the injury, using Peetrons (2002) classifica –
tion, the longer the time required before returning to sports
(P<0.001). Gibbs et al. (2004) found that an increased length (r=0.84) and per cent of the cross sectional area (r =0.78) of the
lesion increased the amount of time required before returning to
sports. Silder et al. (2013) found that a greater cranio-caudal
length of the injury was positively correlated with longer time re –
quired before returning to sports (r =0.41, P=0.04). Hallén and
Ekstrand (2014) observed a significant association between lay-off
days and MRI grading ( P<0.001) (grades 1–3 according to Peet –
rons (2002). In another two studies by Askling et al. (2007b;
2008), they did not find any correlation between the extent of the
injury and an extended amount of time required before returning
to sports (r =0.008 to 0.625, P=0.055 to 0.981, respectively).
Furthermore, Petersen et al. (2014) reported that there was no cor –
relation between the length of the injured area and extended time
required before returning to sports (r =0.19, P=0.29). Moen et al.
(2014) found no correlation between MRI parameters in grades 1
and 2 of the hamstring injuries and amount of time required be –
fore returning to sports ( P=0.54, P>0.83, respectively).
Is there a correlation between the injured muscle bundle and
length of time before returning to sports?
In four out of the 11 articles (Comin et al., 2013; Connell et al.,
2004; Ekstrand et al., 2012; Hallén and Ekstrand, 2014), it was
possible to find an answer to this question. Using univariate anal -Table 3. Study and their purpose
Study Purpose or hypothesis
Askling et al. (2007a) In vestigate acute, first-time hamstring strains in sprinters with respect to the occurrence and progression of both clinical and MRI signs of injury during first 6
weeks after injury, as well as the extent of correlation between clinical and magnetic resonance imaging (MRI) findings and their association with time required
before return to preinjury level
Askling et al. (2007b) In vestigate acute, first-time hamstring strains in dancers with respect to injury mechanism, location, and extent of the injury as well as possible relationships
with clinical and MRI findings and time required before return to preinjury level
Askling et al. (2008) In vestigate the generalisability of our earlier findings (Askling et al., 2007b) of specific injury location and long recovery times for stretching-type hamstring inju –
ries in dancers
Comin et al. (2013) E valuate the use of a novel qualitative MRI parameter, the presence of disruption of central tendon fibres, for prognostication of hamstring injuries in a group of
elite athletes
Connell et al. (2004) C ompare the characteristics of sonography with MRI in assessing both the acute and healing phases of hamstring injuries. Also, to investigate whether MRI and
sonography characteristics identified at baseline could serve as clinically useful prognostic factors to determine whether professional football players can return
to full competition (Australian football)
Ekstrand et al. (2012) E valuate the use of MRI as a prognostic tool for lay-off time after hamstring injuries in professional football players. A further aim was to investigate use of MRI
in hamstring injuries in elite level football teams and to study the association between MRI findings and injury circumstances
Gibbs et al. (2004) C omparing the estimated time of return to sport based on clinical diagnostics or MRI with the actual recovery time as well as to find out the degree of agreement
between clinical diagnosis and MRI examination for the presence or absence of injury
Hallén and Ekstrand
(2014)H ypothesis; imaging would provide detailed data that would assist the persons working on the football field in answering the common question, ‘When can the
player return to ordinary training and matches?’
Moen et al. (2014) Assess the prognostic value of clinical and MRI parameters for length of time required before returning to play after acute hamstring injury
Petersen et al. (2014) (1) Investigate the characteristic sonographic findings of acute hamstring injuries in soccer players, (2) compare the mean injury severity (time required before re –
turn to play) in injured players with and without sonographically verified abnormalities, and (3) correlate the length of the injured area and absence from soccer
play (time required before return to play) to investigate if ultrasonography can be used as a prognostic indicator of length of time required before return to play
Silder et al. (2013) A ssess differences between a progressive agility and trunk stabilisation rehabilitation programme and a progressive running and eccentric strengthening rehabil –
itation programme in recovery characteristics following an acute hamstring injury, as measured via physical examination and MRI

http://dx.doi.org/10.12965/jer.1632558.279Svensson K, et al. • Hamstring injuries and returning to sports
138 http://www.e-jer.orgTable 4. Summary of the included studies
StudyInvestigation
method and
measured
variablesNo. of
included
injuriesSportWhen a participant
was considered
to have returned
to sportStandardized
rehabilitation
protocolQuestion No. 1 Question No. 2 Question No. 3Time before
return to
sport
Askling et al.
(2007a)MRI. Muscle
bundle, distance
to tuber
ischiadicum,
location, length,
width, depth,
area18 Sprint Perform at a similar
level as before the
injuryNo The greater the
extent of the injury,
the longer time
required before
return to sports- The closer to the
ischial tuberosity the
injury was located,
the longer the time
required before
return to sports.
Location in the
proximal tendon
meant more time
required before
return to sportsMedian: 16
weeks (range,
6–50 weeks)
Askling et al.
(2007b)MRI. Muscle
bundle, distance
to ischial
tuberosity,
location, length,
width, depth15 Dance Perform at a similar
level as before the
injuryNo No association
between extent of
injury and length of
time before return
to sports- No correlation
between distance
from ischial
tuberosity to injury
and length of time
before return to
sports. All injuries in
semimembranosus
were involved with
proximal tendonMedian: 50
weeks (range,
30–76 weeks)
Askling et al.
(2008)MRI. Muscle
bundle, distance
to ischial
tuberosity,
length30 Twenty-
one
different
sportsPerform at a similar
level as before the
injuryNo No correlation
between the length
and time of the
injury and amount
of time required
before return to sports- No association
between distance
from the ischial
tuberosity to injury
and length of time
before return to sportsMedian: 31
weeks (range,
9–104 weeks)
Comin et al.
(2013)MRI. Muscle
bundle, central
tendon, presence
of absence of
disruption.62 Australian
footballReturn to play
arbitrary and varies
on different factorsNo Positive correlation
between central
tendon disruption
and length of time
before return to sportsNo significant
differences in
recovery time for
the different
hamstrings musclesPositive correlation
between central
tendon disruption
and length of time
before return to sportsMedian
recovery
time: 21 days
(range, 14–42
days)
Connell et al.
(2004)MRI &
sonography.
Muscle bundle,
location, area,
length,
prevalence
of hematoma60 Australian
footballMatch completed No Positive correlation
between injury
length and % cross-
sectional area and
longer time
required before
return to sportsPositive correlation
between injuries in
biceps femoris and
longer time required
before return to sportsPositive correlation if
the injury was not
located at the
junction between
muscle and tendon,
and longer time
required before
return to sportsMedian: 21
days (range,
4–56 days)
Ekstrand et al.
(2012)MRI & US
Muscle bundle,
extent
(Peetrons, 2002)207 Soccer The medical team
allowed full
participation in
training,
availability for
matchesNo The greater the
injury (Peetrons, 2002),
the longer the time
required before return
to sportsNo difference in time
to return to sport in
respect of which
muscles are effected- Average,
19 days
Gibbs et al.
(2004)MRI. Muscle
bundle, length,
area17 Australian
footballFull participation
in trainingYes Positive correlation
between injury
length and % cross-
sectional area and
longer time required
before return to sports- – Average,
20.2 days
Hallén and
Ekstrand
(2014)MRI. Muscle
bundle, location,
severity
classification
and location249 Soccer The team´s medical
staff allowed full
training and
declared available
for match selectionNo Significant
association between
lay-off days and
MRI gradingNo difference in lay-
off days was detected
between the
hamstring muscles – Average,
21 ± 19 days
(Continued to the next page)

http://www.e-jer.org 139 http://dx.doi.org/10.12965/jer.1632558.279Svensson K, et al. • Hamstring injuries and returning to sports
yses, Connell et al. (2004) found an injury to the biceps femoris to
be correlated with an extended time before returning to sports
(r=not reported). Multivariate analysis of MRI showed that an in –
jury to the biceps femoris and the length of the injury were factors
requiring an extended amount of time before returning to sports
(r=0.62), while an analysis of ultrasound images showed that the
incidence of injury to the biceps femoris, per cent of cross section –
al area and the presence of hematoma were predictors of length of
time required before returning to sports (r =0.58) ( P<0.05). This
is in contrast with the findings of Comin et al. (2013), who did
not find any difference in terms of recovery time for the different
hamstring muscles ( P=0.33). In a study by Ekstrand et al. (2012)
and Hallén and Ekstrand (2014), no correlation could be seen be –
tween any of the three injured hamstring muscles and an extend –
ed amount of time before returning to sports ( P=0.79, P=0.83,
respectively).
Is there a correlation between the injury location within the
muscle and length of time required before returning to sports?
Six out of the 11 articles (Askling et al., 2007a; Askling et al.,
2007b; Askling et al., 2008; Comin et al., 2013; Connell et al.,
2004; Moen et al., 2014) contributed to an answer to this ques –
tion. Askling et al. (2007a) reported that the closer the injury to
the ischial tuberosity, the longer the time needed before returning
to sports ( r=0.5440 to 0.705; P=0.005 to 0.044). In cases where the proximal tendon was involved, it took 35 weeks (median) be –
fore being able to return to the sport, while it took 13 weeks (me –
dian) if the proximal tendon was not involved (Askling et al.,
2007a). Askling et al. (2007b) did not find any correlation be –
tween the distance of the injury from the ischial tuberosity and
extended time before returning to sports (r =0.008 to 0.625,
P=0.055 to 0.981). In contrast, when the whole injury was locat –
ed close to the proximal tendon of the semimembranosus, the
time required before returning to sports was the longest compared
to all studied articles (median, 50 weeks). Askling et al. (2008)
did not find any correlation between the distance of the injury
from the ischial tuberosity and extended amount of time before
returning to sports (r =-0.198). Connell et al. (2004) reported that
an injury located in the junction between the muscle and tendon
was not linked to an extended amount of time before return to
sport (r =not shown). Comin et al. (2013) reported that hamstring
injuries involving disruption of the central tendon at any point
along its length have a significantly worse prognosis than injuries
in other parts of the muscle. Specifically, median 21 days (inter –
quartile range [IQR ]), 9–28) recovery time for those without cen –
tral tendon disruption and 72 days (IQR, 42–109), respectively
(P<0.1). Moen et al. (2014) did not find any correlation between
the distance of the injury from the ischial tuberosity and length of
time before returning to sports (95% confidence interval, -1.2 to
0.06; P=0.075).StudyInvestigation
method and
measured
variablesNo. of
included
injuriesSportWhen a participant
was considered
to have returned
to sportStandardized
rehabilitation
protocolQuestion No. 1 Question No. 2 Question No. 3Time before
return to
sport
Moen et al.
(2014)MRI. Muscle
bundle, location,
distance ischial
origin to fibula,
tibia, cross-
sectional area74 Six different
sportsThe physiotherapist
allowed after full
rehabilitation
program included
sport specific
testingYes MRI parameters in
grades 1 and 2
hamstring injuries
are not associated
with length of time
required before
return to sports- No association
between distance to
ischial tuberosity and
length of time before
return to sportsAverage,
44 ± 18 days
Petersen et al.
(2014)US. Muscle
bundle, location,
area, length,
intramuscular,
prevalence of
hematoma51 Soccer The medical team
allowed full
participation in
training,
availability for
matchesNo No correlation
between the length
of time required
before return to
play and the
injured area- – Average,
25.4 ± 15.7
days
Silder et al.
(2013)MRI. Muscle
bundle, tendon,
location, cross-
sectional area25 High-speed
sportsNormal physical
strength and
function after
rehabilitation
programmesYes Positive correlation
between cranio-
caudal length of
injury and longer
time required before
return to sports- – Median, 23
days (range,
13–28 days)
MRI, magnetic resonance imaging; US, ultrasound.Table 4. Continued

http://dx.doi.org/10.12965/jer.1632558.279Svensson K, et al. • Hamstring injuries and returning to sports
140 http://www.e-jer.orgDISCUSSION
The results show that there is a correlation between the size of
the injury and an extended amount of time required before re –
turning to sports. Data suggest that the closer to the ischial tuber –
osity the hamstring injury is located, the longer the amount of
time required before returning to sports. However, the results are
contradictory. It seems doubtful whether the same relationship
exists in terms of the specific muscle bundle that has been injured,
with the conflicting results shown in the included studies.
The study by Comin et al. (2013) highlighted the difference in
amount of time required before being able to return to competi –
tive play, if the central tendon of the biceps femoris was ruptured,
which required significantly longer time for recovery. In their
study, it took four times longer to return to play when the central
tendon was injured.
The outcome variable for length of time required before being
able to return to sports was much larger in studies by Askling et
al. (2007a; 2007b; 2008) and Comin et al. (2013) than in the oth –
er studied articles. Time required before being able to return to
sports varied from 19 days to 50 weeks. One explanation may be
that the reporting of length of time required before returning to
sports differed from the way it was done by Askling et al. (2007a;
2007b; 2008), where the patients themselves reported when they
were able to perform sporting activities at a similar level as before
they were injured. Another explanation could be that Comin et al.
(2013) collected data retrospectively from the club medical re –
cords. In Askling et al. (2007b), the subjects performed similar
sprint times in order to return to playing sports, while in the oth –
er two studies by Askling et al. (2007b; 2008) the performance
was reported based on the patient ʹs own opinion. In the studies by
Ekstrand et al. (2012), Hallén and Ekstrand (2014) and Petersen
et al. (2014), the medical teams, in consultation with the injured
players, decided when the players could return to play. Another
explanation for the varying time lengths before return to play
could be different requirements for the hamstring muscles in the
different sports. A further explanation could be that the character –
istics of the injury may differ between studies, although this has
not been studied in the present investigation. However, for exam –
ple, 70% of MRI examinations in a study by Ekstrand et al. (2012)
were normal. This particular study from 2012 was also the one
with the shortest amount of time before the player could return to
sports, and the subjects had an average return after 19 days.
A limitation of the present literature review was that in the ar –
ticles included in this review, besides the one by Gibbs et al. (2004) and Silder et al. (2013), the training programmes of the
athletes were not described in detail. This could be a confounder
since it has been reported that various rehabilitation protocols in –
fluence the time required before returning to sports (Askling et
al., 2013b; Sherry and Best, 2004 ).
Another problem is that the authors of the different articles
have used different methods for assessing when the player is ready
to return to sports. Today, there is no golden standard for how to
assess when the player can return to sports (Orchard et al., 2005),
which may be due to the fact that different sports require different
demands of the muscles, for instance. It is impossible to know for
sure if the athletes returned to their sport at the right time. The
injured athletes were deemed able to return to sports when they
were able to participate in sporting activities at a similar level as
before their injury (Askling et al., 2007a; Askling et al., 2007b:
Askling et al., 2008). Suggestions for this differ between studies
according to the following: when a match was completed (Connell
et al., 2004), when the medical team allowed full participation
(Ekstrand et al., 2012; Hallén and Ekstrand, 2014; Petersen et al.,
2014), when the physiotherapist allowed participation (Moen et
al., 2014) or when the athlete was back to full training (Gibbs et
al., 2004; Silder et al., 2013). According to Orchard and Best
(2002), the risk of suffering a reinjury is high and could therefore
be a problem when an athlete returns to sports. The analysis of the
reinjury rate was not the scope of the present study; therefore, it
has not been mentioned in the included studies. This is important
to take into consideration since reinjury rate is an equally import –
ant outcome measure as the recovery time. More research is thus
desired within this area. Askling et al. (2010) have developed a
specific clinical test in order to assess when it is appropriate to re –
turn to sports after a hamstring rupture. In supine position, the
athlete performs a hip flexion as fast and as high as possible while
maintaining the knee joint fully extended. Thereafter, the athlete
value recorded on a visual analogue scale. If this estimation is dif –
ferent from zero, the athlete is not allowed to return to their sport.
The test has shown good results in terms of a reduced risk of rein –
jury in soccer players (Askling et al., 2013a). The test needs, how –
ever, to be further evaluated.
Another factor that may influence the timing of when the player
can return to sport is the support that the athlete receives from the
medical team, both in acute care but also during rehabilitation.
Regarding the four studies with the shortest amount of time be –
fore returning to the sport, it is tempting to think that the athletes
received better care when it came to sports performed at a high
level with large financial resources unlike the other seven studies.

http://www.e-jer.org 141 http://dx.doi.org/10.12965/jer.1632558.279Svensson K, et al. • Hamstring injuries and returning to sports
When the length of the injury is important for amount of time
required before returning to sports, it is positive from the point of
view that it is easier and less time-consuming to measure the
length of injury instead of the cross-sectional area (Gibbs et al.,
2004). According to the present study, there was a correlation be –
tween the size of injury and an extended amount of time before
returning to sports. However, it is unclear whether the same rela –
tionship exists for length of time required before to returning to
sports and the cross-sectional area.
Moreover, according to Askling et al. (2013b) and Comin et al.
(2013), the amount of time required before returning to sports
might be extended, if the injuries involve the proximal muscle
tendon unit and the central tendon. The reason may be that the
blood flow in a tendon is much less extensive than in the muscle,
which could mean fewer opportunities for healing. However, it
should be pointed out that this argument is quite weak for this is –
sue, since a correlation between the size of injury and an extended
amount of time before being able to return to sports is not known.
There may also be other influencing factors not investigated in
this study. Among other things, the impact of the injury mecha –
nism at the time of returning to sports is not clear (Askling et al.,
2013a). Further research is needed to better understand the most
optimal way of evaluating athletes with hamstring injuries before
considering a safe return to the sport.
Majority of cases with hamstring rupture is associated with an
extended amount of time before being able to return to sports.
More controlled research, including improvements in the rehabil –
itation protocol and methods for evaluations before returning to
sports are needed.
CONFLICT OF INTEREST
No potential conflict of interest relevant to this article was re –
ported.
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