Including the Nordic hamstring exercise in injury prevention programmes halves the rate of hamstring injuries: A systematic review and meta-analysis of 8459 athletes

Abstract

Research question
Does the Nordic hamstring exercise (NHE) prevent hamstring injuries when included as part of an injury prevention intervention?

Design
Systematic review and meta-analysis.

Eligibility criteria for selecting studies
We considered the population to be any athletes participating in any sporting activity, the intervention to be the NHE, the comparison to be usual training or other prevention programmes, which did not include the NHE, and the outcome to be the incidence or rate of hamstring injuries.

Analysis
The effect of including the NHE in injury prevention programmes compared with controls on hamstring injuries was assessed in 15 studies that reported the incidence across different sports and age groups in both women and men.

Data sources
MEDLINE via PubMed, CINAHL via Ebsco, and OpenGrey.

Results
There is a reduction in the overall injury risk ratio of 0.49 (95% CI 0.32 to 0.74, p=0.0008) in favour of programmes including the NHE. Secondary analyses when pooling the eight randomised control studies demonstrated a small increase in the overall injury risk ratio 0.52 (95% CI 0.32 to 0.85, p=0.0008), still in favour of the NHE. Additionally, when studies with a high risk of bias were removed (n=8), there is an increase of 0.06 in the risk ratio to 0.55 (95% CI 0.34 to 0.89, p=0.006).

Conclusions
Programmes that include the NHE reduce hamstring injuries by up to 51%. The NHE essentially halves the rate of hamstring injuries across multiple sports in different athletes.

Trial registration number
PROSPERO CRD42018106150.

Full text

1
van DykN, etal. Br J Sports Med 2019;0:1–10. doi:10.1136/bjsports-2018-100045
Including the Nordic hamstring exercise in injury
prevention programmes halves the rate of hamstring
injuries: a systematic review and meta-analysis of
8459athletes
Nicol van Dyk, 1 Fearghal P Behan, 2 Rod Whiteley3
Review
To cite: van DykN,
BehanFP, WhiteleyR.
Br J Sports Med Epub ahead
of print: [please include Day
Month Year]. doi:10.1136/
bjsports-2018-100045
1Aspetar Injury and Illness
Prevention Program (ASPREV),
Aspetar Orthopaedic and Sports
Medicine Hospital, Doha, Qatar
2Research Department, Aspetar
Orthopaedic and Sports
Medicine Hospital, Doha, Qatar
3Department of Rehabilitation,
Aspetar Orthopaedic and Sports
Medicine Hospital, Doha, Qatar
Correspondence to
DrNicol van Dyk, Aspetar
Orthopaedic and Sports
Medicine Hospital, Doha, Qatar;
nicol. vanDyk@ Aspetar. com
Accepted 27 January 2019
© Author(s) (or their
employer(s)) 2019. No
commercial re-use. See rights
and permissions. Published
by BMJ.
ABSTRACT
Research question Does the Nordic hamstring
exercise (NHE) prevent hamstring injuries when included
as part of an injury prevention intervention?
Design Systematic review and meta-analysis.
Eligibility criteria for selecting studies We
considered the population to be any athletes
participating in any sporting activity, the intervention to
be the NHE, the comparison to be usual training or other
prevention programmes, which did not include the NHE,
and the outcome to be the incidence or rate of hamstring
injuries.
Analysis The effect of including the NHE in injury
prevention programmes compared with controls on
hamstring injuries was assessed in 15 studies that
reported the incidence across different sports and age
groups in both women and men.
Data sources MEDLINE via PubMed, CINAHL via
Ebsco, and OpenGrey.
Results There is a reduction in the overall injury risk
ratio of 0.49 (95% CI 0.32 to 0.74, p=0.0008) in favour
of programmes including the NHE. Secondary analyses
when pooling the eight randomised control studies
demonstrated a small increase in the overall injury risk
ratio 0.52 (95% CI 0.32 to 0.85, p=0.0008), still in
favour of the NHE. Additionally, when studies with a high
risk of bias were removed (n=8), there is an increase
of 0.06 in the risk ratio to 0.55 (95% CI 0.34 to 0.89,
p=0.006).
Conclusions Programmes that include the NHE reduce
hamstring injuries by up to 51%. The NHE essentially
halves the rate of hamstring injuries across multiple
sports in different athletes.
Trial registration number PROSPERO
CRD42018106150.
INTRODUCTION
It is in sports clinicians’ DNA to prevent injuries. But
is there evidence to answer the seemingly innocuous
clinical question—‘If I prescribe preventive exercise
will it reduce injuries?’ Given the substantial burden
of hamstring injuries,1–4 we interrogated the liter-
ature to answer one question: Does a hamstring
prevention exercise—The Nordic hamstring exer-
cise (NHE)—prevent hamstring injuries.
Hamstring muscle injury is the most common
muscle injury across a range of different sports.5–7
A number of intervention studies that used eccen-
tric strengthening reduced hamstring injuries.8
Three large prospective trials (two randomised and
one non-randomised) reduced injuries by approxi-
mately 70% by implementing the NHE in a team’s
training regime.9–11
So why then do we need a systematic review of
this type of intervention? Goode et al12 performed a
comprehensive systematic review but included just
four studies, most likely due to selection criteria
that only allowed for the inclusion of randomised
control trials, and excluded articles not written in
English. The most recent systematic review analysed
the effectiveness of injury prevention programmes
that included the NHE to reduce hamstring injuries
in football while monitoring athlete workload.8 The
results from the meta-analysis suggested that teams
using the NHE (in isolation or as part of a larger
injury prevention programme) reduced hamstring
injury rates up to 51%. However, due to the exclu-
sion of studies that did not provide workload data
(training and match exposure) and sports other than
football, this meta-analysis omits many studies that
also included the NHE. The omission of relevant
studies in both these previous reviews might lead to
a biased estimation of the effect when including the
NHE in an injury prevention programme. Further
exclusion of studies that used an observational or
cross-sectional design, multiple exposure groups,
reporting compliance, and language, limits the
generalisability of these findings. The basic clin-
ical question is perhaps not best answered in this
manner.
We, therefore, carried out an inclusive, compre-
hensive systematic review and meta-analysis on
the effectiveness of injury prevention programmes
that included the NHE in reducing the number of
hamstring injuries.
METHODS
Search strategy and study selection
This review has been registered in the PROSPERO
database (CRD42018106150). The PRISMA state-
ment for systematic reviews was utilised to direct
the reporting and formatting of this review.13 Rele-
vant articles were identified following a search of
the electronic databases: MEDLINE via PubMed,
CINAHL via Ebsco, and OpenGrey. Database
entries were searched from the earliest reported
date (January 1950 for Medline) to August 2018.
Search terms were mapped to relevant MeSH
terms. Search terms were entered into the database
as the keywords ‘Nordic’ and ‘Russian’, which were
grouped with the OR operator. These keywords
on 26 February 2019 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2018-100045 on 26 February 2019. Downloaded from

2van DykN, etal. Br J Sports Med 2019;0:1–10. doi:10.1136/bjsports-2018-100045
Review
were then combined with the operator AND to the keywords
‘hamstring’ and ‘Injur*’, which produced the search strategy and
the final yield. The details from the Medline search can be found
in the online supplementary file 1. To supplement the electronic
database search, the reference lists of relevant papers were also
cross-checked. Publication details from all studies identified in
the literature search were exported to the bibliographic soft-
ware. Once all search results were collated, titles and abstracts
were screened for eligibility. All relevant articles were identified
for full-text review and inclusion. The study selection process is
presented in figure 1.
Eligibility criteria
For this investigation, we considered the population to be
any athletes participating in any sporting activity, the inter-
vention to be the NHE or any programme that included the
NHE, the comparison to be usual training or other prevention
programmes, which did not include the NHE, and the outcome
to be the incidence or rate of hamstring injuries. We did not
include studies without a comparison or control group, thereby
excluding case series and case studies. Studies were not excluded
based on gender, age or level of competition. There were no
language or time limits set.
Data extraction
Two investigators (NVD and RW) conducted the initial search,
duplicates were removed and articles were excluded if they did
not meet the eligibility criteria. A third investigator (FPB) facili-
tated group consensus when disagreements were identified.
Data from the selected full-text articles were independently
extracted by two investigators (NVD and RW). For each study,
outcome data extracted included the number of participants and
injury rates (or a number of hamstring injuries). The outcome
data were imported into Review Manager (RevMan) V.5.3
(Copenhagen, Denmark: The Nordic Cochrane Centre, The
Cochrane Collaboration, 2014) where all further analyses were
performed.
Assessment of methodological quality and risk of bias
The methodological quality of each study was independently
assessed by two investigators (NVD and FPB), with the third
investigator (RW) resolving any discrepancies. The quality and
risk of bias were evaluated according to the Cochrane risk of
bias tool,14 where six domains of bias are assessed: selection bias,
performance bias, detection bias, attrition bias, reporting bias
and other bias. A value of high, low or unknown risk of bias was
provided for each domain.
Data analysis
The meta-analysis was performed according to the Cochrane
methodology.14 The risk ratio with 95% CI was determined for
the outcome of hamstring injury, and calculated as: risk of an
injury in the intervention group/risk of an injury in the control
group. A risk ratio of 1 indicates no difference, and a risk ratio
Figure 1 Flow chart of study selection for the analysis of the effect of prevention programmes including the NHE related to hamstring injury
rates.NHE,Nordic hamstring exercise.
on 26 February 2019 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2018-100045 on 26 February 2019. Downloaded from

3
van DykN, etal. Br J Sports Med 2019;0:1–10. doi:10.1136/bjsports-2018-100045
Review
Table 1 Characteristics of the included studies
Study Location Sex Age Study design
Multiple injuries
included Compliance (%) Study duration
Level of
competition Sport Intervention group
Gabbe et al16 Australia Male 18–35 Randomised control trial No 47 1 season Amateur division Australian
football
NHE
Brooks et al33 United Kingdom Male 18–40 Cohort study Yes Not reported 2 seasons 1st division Rugby Strengthening, stretching,
and NHE
Arnason et al34 Iceland and Norway Male Not reported Non-randomised control trial Yes 48 4 seasons 1st division Football Warm up; NHE
Soligard et al35 Norway Female 13–17 Randomised control trial Not reported 77 1 season Youth Football FIFA 11+ (incl NHE
programme)
Engebretsen et al17 Norway Male 17–35 Randomised control trial No 21 1 season 1–3rd division Football NHE
Petersen et al9Denmark Male 18–40 Randomised control trial Yes 91 1 season 1st–5th division Football Regular training and NHE
Owen et al36 UK Male 18–40 Cohort study Not reported Not reported 2 seasons Professional Football Proprioception, strength
(incl NHE)
Grooms et al37 USA Male 18–22 Cohort study Not reported 100 2 seasons Collegiate Football FIFA 11+ (incl NHE)
Sebelien et al38 Norway Male 18–39 Randomised control trial Not reported Not reported 1 season 3rd–4th division Football NHE
Seagrave et al39 USA Male 18–40 Cohort study No 31 1 season All teams (MLB) Baseball NHE
van der Horst et al10 Netherlands Male 18–40 Randomised control trial No 91 1 season Amateur Football NHE programme
Silvers-Granelli et al40 USA Male 18–25 Randomised control trial Not reported Stratified 1 season Collegiate Football FIFA 11+ (incl NHE)
del Ama Espinosa et al41 Spain Female 19–23 Randomised control trial Not reported 76–80 1 season 1st–2nd division Football NHE; EBE
Nouni-Garcia et al42 Spain Male 18–40 Cohort study Yes Not reported 2 seasons Amateur Football FIFA 11 (incl NHE)
González43 Spain Male 18 Cohort study Not reported Unknown 2 seasons Youth Football Strengthening and NHE
For all studies, the control group performed what would be considered regular training.
EBE, elastic band exercise;, ;FLS, frontal leg swing;MJ, multiple jumping; MLB, Major League BaseballOrganisation; NHE, Nordic hamstring exercise; SLS, side leg swing.
on 26 February 2019 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2018-100045 on 26 February 2019. Downloaded from

4van DykN, etal. Br J Sports Med 2019;0:1–10. doi:10.1136/bjsports-2018-100045
Review
of <1 indicate a positive intervention effect. In this meta-anal-
ysis, a random-effects model was selected, based on the assump-
tion that the studies included diverse populations and different
contexts. The NHE intervention also varied in terms of dose and
test procedure.
Sensitivity analyses were performed to determine the differ-
ence in outcome when (1) only including ‘high’-quality studies
(randomised control trials), (2) removing studies at high risk
of allocation, detection or attrition bias, and (3) examining the
pooled effect when excluding each study individually.
RESULTS
Identification of studies
The initial database search yielded a total of 1590 potentially
relevant studies. After reviewing the titles and abstracts, 15 full-
text articles were retrieved for analysis. Searching the reference
lists and the authors’ personal databases revealed one additional
relevant study for inclusion. Following communication with one
author of two eligible studies, one of the two shortlisted studies
was excluded, as there was overlapping data between the studies.
Consequently, the final number of studies for inclusion was 15.
Characteristics of the included studies
The characteristics of the 15 studies included in the anal-
ysis are summarised in table 1. Eight controlled trials (seven
randomised and one non-randomised) and seven cohort studies
were included. The studies represent different regions, including
North America (n=3), UK and Europe (n=7), Scandinavia (n=5)
and Australia (n=1). Two studies were performed in female
athletes, while the remaining 13 studies included male athletes.
The investigations were performed mainly in football (soccer),
with one investigation in rugby, baseball and Australian football,
respectively. The definition of injury and re-injury (or recurrent
injury) varied somewhat across the 15 included studies. Seven
Table 2 The training protocol using the NHE across different studies (n=14)
Week Sessions per week No of repetitions
Gabbe et al16 1–12 Not specified
5 sessions over 12 weeks
12×6 (10 s rest between repetitions, 2–3 min
rest between sets)
Arnason et al34 1–5 Not specified Introduction to load
Preseason 3 3 sets, 12-10-8 reps
Competitive season 1–2 3 sets, 12-10-8 reps
Engebretsen et al17 1 1 5+5
2 2 6+6
3 3 3×6–8
4 3 3×8–10
5–10 3 12+10 + 8
Petersen et al,9 Gonzalez43 1 1 2×5
2 2 2×6
3 3 3×6–8
4 3 3×8–10
5–10 3 3 sets, 12-10-8 reps
10+ 1 3 sets, 12-10-8 reps
Van der Horst et al10 1 1 2×5
2 2 2×6
3 2 3×6
4 2 3 sets, 6-7-8
5–10 2 3 sets, 8-9-10 reps
6–13 2 3 sets, 10-9-8 reps
Soligard etal35 During warm-up prior to training
(FIFA 11+)
Silvers-Granelli et al40 3–4 (Level 1) 1×3–5
Nouni-Garcia et al42 5–8 (Level 2) 1×7–10
Grooms et al37 9–12 (Level 3) 1×12–15
Owen et al36 1–10 2 2×4
11–20 2 2×6
21–30 2 2×8
31–40 2 3×6
41–58 2 3×8
Del Ama Espinosa et al41 1–8 (Phase 1) 1 1×5
9–15 (Phase 2) 1 1×5
16–23 (Phase 3) 1 1×5
Sebelien et al*38 Preseason 3 2×5, increase to 3×12
Competitive season 2 2×5, increase to 3×12
Seagrave et al39 Entire season Not specified Average of 3.5 repetitions
*Players were encouraged to increase the speed (at the beginning of the movement) and resistance (‘pushed’ by partner) of the exercise progressively. Brooks et al15 did not
report the training protocol used.
NHE,Nordic hamstring exercise.
on 26 February 2019 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2018-100045 on 26 February 2019. Downloaded from

5
van DykN, etal. Br J Sports Med 2019;0:1–10. doi:10.1136/bjsports-2018-100045
Review
studies do not report whether multiple injuries were included,
four studies reported on recurrent injuries and four studies
included only index injuries. Compliance is not reported in five
studies, while four studies observed compliance of <50%. The
remaining six studies all reported compliance of over 70%. The
level of sport varied across the different studies and included
the highest level of competition, subdivisions (second to fifth)
in football, all competition levels in major league baseball,
including collegiate, youth and amateur players. The players
were aged 18–40 years, apart from one study in youth players
(13–18 years). The NHE programme was used in isolation in six
studies, while four studies included the NHE as part of the FIFA
11+ programme. All the other investigations included the NHE
together with other strength, flexibility or warm-up exercises.
The prescription and training volume of the NHE protocol used
for each study are presented in table 2.
Methodological quality and risk of bias
The risk of bias assessment is summarised in figure 2. A high risk
of performance and detection bias was identified, with moderate
selection bias. Attrition and reporting bias was low across the
included studies. The individual assessment of high, low or
unknown risk of bias for each individual study is presented in
table 3.
In the examination of the funnel plots, there is slight asym-
metry, indicating that risk of publication bias may not be present
(figure 3). No adjustment of the overall point estimate was
warranted.
Meta-analysis
The pooled data for 8459 individuals including 525 hamstring
injuries were analysed from the 15 included studies. Expo-
sure data were not available for six studies. In the remaining
nine studies, the incidence of hamstring injuries (weighted by
sample size) was 0.1/1000 hour for players exposed to NHE,
and 0.2/1000 hour in those players who continued with usual
training.
The pooled results show a 51% overall reduction in hamstring
injury in the intervention group that included the NHE
compared with the control group (RR 0.49, 95% CI 0.32 to
0.74, p=0.0008) (figure 4). Statistical heterogeneity across the
different studies was large (I2=74%).
Sensitivity analysis
The pooled data for only the randomised control trials were
analysed and the point estimate changed by 0.03 (RR 0.52,
95% CI 0.32 to 0.85, p=0.0008) (figure 5). Additional sensi-
tivity analyses were performed by removing studies with high
risk of allocation and detection bias, as well as one study with a
high risk of attrition bias (figure 6), with a change in the point
estimate of 0.06 (RR 0.55, 95% CI 0.34 to 0.89, p=0.006). The
overall shift in effect size when systematically removing each
study individually was small (figure 7).
DISCUSSION
In this systematic review and meta-analysis, including 8459
athletes and 525 hamstring injuries, the primary outcome was
the overall hamstring injury rates when introducing the NHE as
a preventative measure. The results indicate a statistically signif-
icant and clinically meaningful reduction of 51% in hamstring
injuries for all athletes competing at different levels of competi-
tion and across multiple sports. These results support the use of
the NHE in prevention programmes.
It is important to compare the results of this systematic review
with previous findings from similar investigations. In 2010,
Goldman and Jones reported in a Cochrane review on interven-
tions aimed at reducing hamstring injuries.15 Of the six studies
included, only two investigated the NHE as an intervention,16 17
and the results were inconclusive as to the effectiveness of the
interventions. In 2014, Goode et al performed an intention to
treat analysis and in their study, eccentric strengthening, with
good compliance, seemed to be successful in hamstring injury
prevention.12 However, due to strict inclusion criteria, only four
studies were included, one of which involved a form of eccen-
tric training other than the NHE.18 In a recent meta-analysis of
injury prevention programmes including the NHE, hamstring
injuries were reduced by up to 51% compared with teams that
did not use any intervention.8
Different methodological approaches meant that previous
meta-analysis only included studies in football and that reported
Figure 2 Risk of bias summary. The authors’ judgements about each risk of bias item is presented as percentages across all included studies.
on 26 February 2019 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2018-100045 on 26 February 2019. Downloaded from

6van DykN, etal. Br J Sports Med 2019;0:1–10. doi:10.1136/bjsports-2018-100045
Review
exposure. The present meta-analysis included an additional 10
studies, 2 of which were published after the previous meta-anal-
ysis reported in 2016. There is an overall pooled-effect of 51%
reduction of hamstring injuries when the NHE is implemented.
This confirms the previous findings, irrespective of a large
amount of heterogeneity identified between the studies in the
present analysis. However, the use of the NHE in practice and its
implementation as a prevention measure is limited.19
Systematic reviews should inform clinical practice where they
can
Staying up to date with the latest scientific evidence is chal-
lenging for all clinicians. A recent educational review advocated
three key steps in making quality decisions about evidence in
practice: (1) systematically searching and assessing the quality
of published literature, (2) combining quality research evidence
with quality clinical evidence, and (3) considering the feasibility
of use in the practical setting.20 Systematic reviews promise a
high-quality, comprehensive summary of the research regarding
an intervention, such as NHE for preventing hamstring injuries.
And while we might expect systematic reviews to be accurate,
they are bound by certain decision-making in how they are
conducted. To ensure methodological rigour, most reviews and
meta-analyses impose strict selection criteria.21 22 The purpose
is to ensure methodological quality, allowing the subsequent
analysis to be performed with greater precision and accuracy.
However, readers must stay alert to publications with provoca-
tive titles but clinical conclusions that are not fully supported by
the study results.23
In the case of the NHE, the strict methodological selection
criteria might disguise the simple clinical question: regardless
of gender, sport or age, will the NHE reduce hamstring inju-
ries if I prescribe them? Many clinicians are faced with a similar
reality—managing male and female athletes from a range of
different sports and age groups. Therefore, we pragmatically
included more clinically heterogeneous studies in this analysis.
Apart from the clinical diversity represented in these studies
(different participant characteristics performed in different
settings), the importance of methodological and statistical
Table 3 Assessment of methodological quality for each study
Risk of bias
domain Gabbeetal16 Brooksetal33 Arnasonetal34 Soligardetal35 Engebretsenetal17 Petersenetal9Owenetal36 Groomsetal37 Sebelienetal38 Seagraveetal39
Van der
Horstetal10
Silvers-
Granellietal40
del Ama
Espinosaetal41
Nouni
Garciaetal42 González43
Random sequence
generation
(selection bias)
− + + − − − + + − + − − − + +
Allocation
concealment
(selection bias)
− + + − − − + + − + − − − + +
Blinding of
participants
and personnel
(performance
bias)
+ + + + + + + + + + + − + + +
Blinding of
outcome
assessment
(detection bias)
+ + + − + + + + + + + − + + +
Incomplete
outcome data
(attrition bias)
− − − − − − ? − + − − − − − −
Selective
reporting
(reporting bias)
− − ? − − − − − − − − − − − −
Other bias − − − − − − − − − − − − − − ?
Positive signs indicate ahigh risk of bias, negative signs indicate alow risk of bias and question marks indicate anunknown risk of bias.
Figure 3 Funnel plot based on SE and log risk ratio of the studies
in assessing publication bias. The diagonal lines represent the pooled
injury risk ratio (RR) which is the summary measure of the study. The
vertical tips of the diagonal lines are the overall effect and the lateral
ends on the x-axis are the associated CIs.
on 26 February 2019 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2018-100045 on 26 February 2019. Downloaded from

7
van DykN, etal. Br J Sports Med 2019;0:1–10. doi:10.1136/bjsports-2018-100045
Review
heterogeneity, and how it may influence results must not be over-
looked. Grindem et al suggest different measures of assessing
statistical heterogeneity, such as a funnel plot or the I2 statistic.24
Although studies were purposefully included that would increase
the heterogeneity, the risk of bias assessment (figure 2) and funnel
plot (figure 3) suggest that it was acceptable in this analysis. To
account for methodological heterogeneity, a further sensitivity
analysis was performed. When only randomised controlled trials
were included, considering these studies to represent the highest
level of evidence for this type of intervention, the overall pooled
effect changes only minimally (figure 5). And after removing
studies at high risk of bias (all studies that were determined to
present a high risk of allocation and detection bias, and one
study due to attrition bias), there is again no substantial change
in the overall effect (figure 6).
It is worth highlighting that there was no significant reduc-
tion of hamstring injuries in the two studies investigating the
effect of the NHE in female populations. Similarly, one study in
Australian rules football demonstrated no significant reduction
in hamstring injuries. However, none of the studies included in
the analysis has a dramatic outcome on the overall effect of the
meta-analysis (figure 7), which suggests that no individual study
grossly influenced the overall findings. Based on these results,
clinicians are encouraged to include the NHE in their prevention
efforts to reduce the number of hamstring injuries, regardless of
sporting code, gender or age.
Study biases and limitations
Moderate selection bias was present in the studies included, and
a high risk of performance bias (figure 2). However, blinding the
assessor or the participant to the intervention is likely impossible
in this type of study, as it would be difficult to introduce an NHE
placebo. A high risk of detection bias is present, and blinded
assessors would have reduced the potential bias when outcomes
are measured.
Substantial variability is present in the training protocol across
different studies (table 2). It is not clear what the ideal prescrip-
tion of the exercise is, although recent studies have demonstrated
similar tissue adaptation and strengthening when comparing low
Figure 4 Primary analysis of overall hamstring injury rates in NHE prevention programmes compared with control intervention. M-H, Mantel-
Haenszel; NHE, Nordic hamstring exercise.
Figure 5 Secondary analysis of overall hamstring injury rates in NHE prevention programmes compared with control intervention when only
including randomised control trials. M-H, Mantel-Haenszel; NHE, Nordic hamstring exercise.
on 26 February 2019 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2018-100045 on 26 February 2019. Downloaded from

8van DykN, etal. Br J Sports Med 2019;0:1–10. doi:10.1136/bjsports-2018-100045
Review
and high volume training regimes.25 Regardless, we observe
a strong overall effect even with a large amount of variability
between studies.
The risk of an index hamstring injury is different from a recur-
rent hamstring injury.3 5 6 Only four studies in our meta-analyses
focus on index injury, while the risk reduction is greater in the
four studies reporting on recurrent injuries. Seven studies do not
report on whether the injuries included were either index inju-
ries or included recurrent injuries as well; therefore, a conclusive
recommendation is not possible.
Clinical implications
A large amount of evidence now supports the use of the
NHE to prevent hamstring injuries. The overall effectiveness
of this exercise has been demonstrated repeatedly, although
the mechanisms by which the NHE provides a protective
effect is not yet fully understood. The NHE may increase
fascicle length, leading to morphological changes that may
protect the hamstring muscle from injury.26 Harøy et al have
demonstrated that when performing the NHE as part of a
prevention programme, an increase in strength is observed27;
increasing eccentric strength may reduce the risk associated
with a hamstring injury. The value of eccentric training and
the NHE is recognised at elite level football,28 but the adop-
tion of the exercise into regular training programmes is poor,19
and the overall use of the NHE in other sports is not known.
The need for better understanding the factors surrounding
the implementation of prevention programmes in sport has
been highlighted through the Translating Research into Injury
Prevention Practice framework.29
Perhaps a key component to ensuring greater success in our
implementation efforts is understanding the context in which
the intervention is being introduced.30 31 This would require the
involvement of key stakeholders in the process of injury preven-
tion, including the medical team, the coach and the player.32
Only when we design our prevention programmes to address
hamstring injuries, and perhaps all sports injuries, in such a
comprehensive manner will we be able to translate the research
evidence into actual clinical practice.
CONCLUSION
This systematic review and meta-analysis demonstrate that the
NHE is effective in reducing hamstring injuries. There is, unsur-
prisingly, a large amount of heterogeneity across multiple sports
and differences such as age and gender between athletes, but
the overall effectiveness of the NHE remains unchanged, and
clinicians are encouraged to include the NHE in their prevention
programmes.
Figure 6 Secondary analysis of overall hamstring injury rates in NHE prevention programmes compared with control intervention when excluding
studies at high risk of allocation, detection and attrition bias. M-H, Mantel-Haenszel; NHE, Nordic hamstring exercise.
Figure 7 Sensitivity analysis demonstrating the change in effect by
systematically removing individual studies included in the meta-analysis.
The effect size shown is the pooled effect when a single (named) study
is removed, with the overall effect listed as a reference.
on 26 February 2019 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2018-100045 on 26 February 2019. Downloaded from

9
van DykN, etal. Br J Sports Med 2019;0:1–10. doi:10.1136/bjsports-2018-100045
Review
Contributors NvD and RW: concept, design and analysis, writing and editing of
the manuscript. FPB: analysis, writing and editing of the manuscript.
Funding The authors have not declared a specific grant for this research from any
funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement Full dataset and/or statistical code are available from
the corresponding author.
RefeRences
1 Bahr R, Clarsen B, Ekstrand J. Why we should focus on the burden of injuries and
illnesses, not just their incidence. Br J Sports Med 2018;52:1018–21.
2 Ekstrand J. Keeping your top players on the pitch: the key to football medicine at a
professional level. Br J Sports Med 2013;47:723–4.
3 Hägglund M, Waldén M, Ekstrand J. Injury incidence and distribution in elite football-a
prospective study of the danish and the swedish top divisions. Scand J Med Sci Sports
2005;15:21–8.
4 Eirale C, Tol JL, Farooq A, et al. Low injury rate strongly correlates with team success
in qatari professional football. Br J Sports Med 2013;47:807–8.
5 Ekstrand J, Hägglund M, Waldén M. Injury incidence and injury patterns in
professional football: the UEFA injury study. Br J Sports Med 2011;45:553–8.
6 Hägglund M, Waldén M, Ekstrand J. Risk factors for lower extremity muscle injury in
professional soccer: the UEFA Injury Study. Am J Sports Med 2013;41:327–35.
7 Opar DA, Williams MD, Shield AJ. Hamstring strain injuries. Sports Medicine
2012;42:209–26.
8 Al Attar WSA, Soomro N, Sinclair PJ, et al. Effect of injury prevention programs that
include the nordic hamstring exercise on hamstring injury rates in soccer players: a
systematic review and meta-analysis. Sports Med 2017;47:907–16.
9 Petersen J, Thorborg K, Nielsen MB, et al. Preventive effect of eccentric training on
acute hamstring injuries in men’s soccer: a cluster-randomized controlled trial. Am J
Sports Med 2011;39:2296–303.
10 van der Horst N, Smits DW, Petersen J, et al. The preventive effect of the nordic
hamstring exercise on hamstring injuries in amateur soccer players: a randomized
controlled trial. Am J Sports Med 2015;43:1316–23.
11 Arnason A, Sigurdsson SB, Gudmundsson A, et al. Risk factors for injuries in football.
Am J Sports Med 2004;32:5–16.
12 Goode AP, Reiman MP, Harris L, et al. Eccentric training for prevention of hamstring
injuries may depend on intervention compliance: a systematic review and meta-
analysis. Br J Sports Med 2015;49:349–56.
13 Moher D, Liberati A, Tetzlaff J, et al. Preferred reporting items for systematic reviews
and meta-analyses: the PRISMA statement. Ann Intern Med 2009;151:264–9.
14 Higgins JPT, Green S. Cochrane handbook for systematic review of interventions
version 5.1. 0: The Cochrane Collaboration. 2011.
15 Goldman EF, Jones DE. Interventions for preventing hamstring injuries: a systematic
review. Physiotherapy 2011;97:91–9.
16 Gabbe BJ, Branson R, Bennell KL. A pilot randomised controlled trial of eccentric
exercise to prevent hamstring injuries in community-level Australian Football. J Sci
Med Sport 2006;9:103–9.
17 Engebretsen AH, Myklebust G, Holme I, et al. Prevention of injuries among male
soccer players: a prospective, randomized intervention study targeting players with
previous injuries or reduced function. Am J Sports Med 2008;36:1052–60.
18 Askling C, Karlsson J, Thorstensson A. Hamstring injury occurrence in elite soccer
players after preseason strength training with eccentric overload. Scand J Med Sci
Sports 2003;13:244–50.
19 Bahr R, Thorborg K, Ekstrand J. Evidence-based hamstring injury prevention is not
adopted by the majority of champions league or norwegian premier league football
teams: the nordic hamstring survey. Br J Sports Med 2015;49:1466–71.
20 Ardern CL, Bizzini M, Bahr R. It is time for consensus on return to play after injury: five
key questions. Br J Sports Med 2016;50:506–8.
21 Hopewell S, Clarke M, Mallett S. Grey literature and systematic reviews: Publ Bias
Meta-Anal Prev Assess Adjust, 2005.
22 Slavin RE. Evidence-based education policies: transforming educational practice and
research. Educational Researcher 2002;31:15–21.
23 Weir A, Rabia S, Ardern C. Trusting systematic reviews and meta-analyses: all that
glitters is not gold!. Br J Sports Med 2016;50:1100–1.
24 Grindem H, Mansournia MA, Øiestad BE, et al. Was it a good idea to combine the
studies? Why clinicians should care about heterogeneity when making decisions
based on systematic reviews. Br J Sports Med 2018:bjsports-2018-099516.
25 Presland JD, Timmins RG, Bourne MN, et al. The effect of Nordic hamstring exercise
training volume on biceps femoris long head architectural adaptation. Scand J Med
Sci Sports 2018;28:1775–83.
26 Timmins RG, Bourne MN, Shield AJ, et al. Short biceps femoris fascicles and eccentric
knee flexor weakness increase the risk of hamstring injury in elite football (soccer): a
prospective cohort study. Br J Sports Med 2016;50:1524–35.
27 Harøy J, Thorborg K, Serner A, et al. Including the copenhagen adduction exercise in
the FIFA 11+ Provides missing eccentric hip adduction strength effect in male soccer
players: a randomized controlled trial. Am J Sports Med 2017;45:3052–9.
28 McCall A, Dupont G, Ekstrand J. Injury prevention strategies, coach compliance and
player adherence of 33 of the UEFA Elite Club Injury Study teams: a survey of teams’
head medical officers. Br J Sports Med 2016;50:725–30.
29 Finch C. A new framework for research leading to sports injury prevention. J Sci Med
Sport 2006;9:3–9.
30 McKay CD, Steffen K, Romiti M, et al. The effect of coach and player injury knowledge,
attitudes and beliefs on adherence to the FIFA 11+ programme in female youth
soccer. Br J Sports Med 2014;48:1281–6.
31 Bolling C, van Mechelen W, Pasman HR, et al. Context matters: revisiting
the first step of the ’sequence of prevention’ of sports injuries. Sports Med
2018;48:2227–34.
32 Dijkstra HP, Pollock N, Chakraverty R, et al. Managing the health of the elite athlete:
a new integrated performance health management and coaching model. Br J Sports
Med 2014;48:523–31.
33 Brooks JH, Fuller CW, Kemp SP, et al. Incidence, risk, and prevention of hamstring
muscle injuries in professional rugby union. Am J Sports Med 2006;34:1297–306.
34 Arnason A, Andersen TE, Holme I, et al. Prevention of hamstring strains in elite soccer:
an intervention study. Scand J Med Sci Sports 2008;18:40–8.
35 Soligard T, Myklebust G, Steffen K, et al. Comprehensive warm-up programme to
prevent injuries in young female footballers: cluster randomised controlled trial. BMJ
2008;337:a2469.
36 Owen AL, Wong delP, Dellal A, et al. Effect of an injury prevention program on muscle
injuries in elite professional soccer. J Strength Cond Res 2013;27:3275–85.
37 Grooms DR, Palmer T, Onate JA, et al. Soccer-specific warm-up and lower extremity
injury rates in collegiate male soccer players. J Athl Train 2013;48:782–9.
38 Sebelien C, Stiller C, Maher S, et al. Effects of implementing Nordic hamstring
exercises for semi-professional soccer players in Norway. Orthop Pract 2014;26:90–7.
39 Seagrave RA, Perez L, McQueeney S, et al. Preventive effects of eccentric training
on acute hamstring muscle injury in professional baseball. Orthop J Sports Med
2014;2:232596711453535.
40 Silvers-Granelli H, Mandelbaum B, Adeniji O, et al. Efficacy of the FIFA 11+
injury prevention program in the collegiate male soccer player. Am J Sports Med
2015;43:2628–37.
41 del Ama Espinosa G, Pöyhönen T, Aramendi JF, et al. Effects of an eccentric training
programme on hamstring strain injuries in women football players. Biomed Hum Kinet
2015;7.
What is already known
►Hamstring muscle injury is a common muscle injury across
different sporting codes that affects many athletes. The
Nordic hamstring exercise(NHE) is an effective injury
prevention tool that may reduce the number of hamstring
injuries if implemented successfully.
►Systematic reviews should inform clinical practice where
possible, and clinicians can be confident that the inclusion
of a NHE programme is supported when hamstring injury
reduction is a goal.
►To improve adaptation and implementation of the NHE,
thefocus should be directed towards dose–response
relationships, as well as compliance and adherence with
the prescribed exercise to improve prevention programme
efficacy.
What are the findings
►Hamstring injuries are reduced by 50% when the NHE is
introduced as preventative training, confirming previous
findings isolated to football.
►The overall effectiveness of the NHE remains unchanged
despite a large amount of heterogeneity between the studies
in terms of age, gender and type of sport.
►There is a large amount of variability in the training protocols
used to introduce eccentric training through the NHE.
on 26 February 2019 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2018-100045 on 26 February 2019. Downloaded from

10 van DykN, etal. Br J Sports Med 2019;0:1–10. doi:10.1136/bjsports-2018-100045
Review
42 Nouni-Garcia R, Carratala-Munuera C, Orozco-Beltran D, et al. Clinical benefit of
the FIFA 11 programme for the prevention of hamstring and lateral ankle ligament
injuries among amateur soccer players. Inj Prev 2018;24:149–54.
43 González JR. El entrenamiento de la fuerza para la mejora del rendimiento físico-
deportivo y la prevención de lesiones en el fútbol: Universidad Pablo De Olavide,
2017.
on 26 February 2019 by guest. Protected by copyright.http://bjsm.bmj.com/Br J Sports Med: first published as 10.1136/bjsports-2018-100045 on 26 February 2019. Downloaded from