Copenhagen Adduction Exercise to Increase Eccentric Strength: A Systematic Review and Meta-Analysis

Abstract

Background: The purpose of this study was to systematically review the scientific literature about the benefits of using the Copenhagen Adductor (CA) exercise to improve eccentric hip adduction strength (EHAD), as well as to provide directions for training. Methods: A systematic search was conducted in three electronic databases following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The inclusion criteria were: (a) EHAD as outcome variable, with means and standard deviations before and after the intervention, (b) the study was a randomized controlled trial using the CA in the program intervention, (c) the article was written in English. The quality of evidence was evaluated according to the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) guidelines. A mini meta-analysis was performed to determine the overall effect. Results: Three studies were selected and included in this systematic review and mini meta-analysis. All articles reported significant enhancement in EHAD in football (soccer) players. The overall effect for the EHAD test was significant in favor to CA group (mean difference = 0.61; 95% CI from 0.20 to 1.01; p = 0.003) with large heterogeneity. Conclusions: CA exercise performed 2–3 times for 8 weeks is useful for improving EHAD in football players.

Full text

applied
sciences
Review
Copenhagen Adduction Exercise to Increase Eccentric
Strength: A Systematic Review and Meta-Analysis
Jorge Pérez-Gómez 1, Santos Villafaina 2, JoséCarmelo Adsuar 1, Jorge Carlos-Vivas 1, * ,
Miguel Ángel Garcia-Gordillo 3and Daniel Collado-Mateo 4
1Health, Economy, Motricity and Education (HEME) Research Group, Faculty of Sport Sciences,
University of Extremadura, 10003 Cáceres, Spain; jorgepg100@gmail.com (J.P.-G.);
carmelo.adsuar@gmail.com (J.C.A.)
2Physical Activity and Quality of Life Research Group (AFYCAV), Faculty of Sport Sciences,
University of Extremadura, 10003 Cáceres, Spain; svillafaina@unex.es
3Facultad de Administración y Negocios, Universidad Autónoma de Chile, Sede Talca 3467987, Chile;
miguelgarciagordillo@gmail.com
4Centre for Sport Studies, Rey Juan Carlos University, Fuenlabrada, 28943 Madrid, Spain;
danicolladom@gmail.com
*Correspondence: jorge.carlosvivas@gmail.com
Received: 2 April 2020; Accepted: 17 April 2020; Published: 21 April 2020


Abstract:
Background: The purpose of this study was to systematically review the scientific literature
about the benefits of using the Copenhagen Adductor (CA) exercise to improve eccentric hip adduction
strength (EHAD), as well as to provide directions for training. Methods: A systematic search was
conducted in three electronic databases following Preferred Reporting Items for Systematic Reviews
and Meta-Analyses (PRISMA) guidelines. The inclusion criteria were: (a) EHAD as outcome variable,
with means and standard deviations before and after the intervention, (b) the study was a randomized
controlled trial using the CA in the program intervention, (c) the article was written in English.
The quality of evidence was evaluated according to the Grading of Recommendations, Assessment,
Development and Evaluation (GRADE) guidelines. A mini meta-analysis was performed to determine
the overall effect. Results: Three studies were selected and included in this systematic review and mini
meta-analysis. All articles reported significant enhancement in EHAD in football (soccer) players.
The overall effect for the EHAD test was significant in favor to CA group (mean difference =0.61;
95% CI from 0.20 to 1.01;
p
=0.003) with large heterogeneity. Conclusions: CA exercise performed
2–3 times for 8 weeks is useful for improving EHAD in football players.
Keywords: groin injuries; hip adduction strength; injury prevention; football; soccer
1. Introduction
Muscle injuries are common in many sports, such as basketball [
1
], rugby [
2
], football [
3
], tennis [
4
],
track and field [
5
]. Muscle injury can represent more than 40% of total injuries in football [
3
,
6
], and track
and field [
5
]. Ice hockey and football are susceptible to many adductor muscle strains [
7
]. In this
regard, the adductor injury is the second most common muscle injury among football players [
3
] and
is associated with lost competition time [
8
]. Therefore, intervention programs aimed to reduce the
incidence of adductor muscle injuries are recommended.
The low level of adductor muscle strength has been linked to the possibility of suffering an injury
for instance in ice hockey, where the muscle strength ratio between adduction and abduction has been
reported as a risk factor [
9
]. Tyler et al. also found that adduction strength was lower (18%), during the
preseason in players who sustained groin strains compared with uninjured players [9].
Appl. Sci. 2020,10, 2863; doi:10.3390/app10082863 www.mdpi.com/journal/applsci

Appl. Sci. 2020,10, 2863 2 of 8
Furthermore, large adductor eccentric contraction demands in sports training and competition
seem to be one of the reasons to have a high prevalence of adductor injuries [
10
]. Thus, many different
exercises have been included in preventive programs [
11
,
12
] focused on improving adductor strength,
in order to avoid injuries and limit the amount of losing time in competition [
13
] including, among
others, adduction partner, hip adduction with an elastic band, isometric adduction, sliding hip
abduction/adduction exercise, side-lying adduction and the Copenhagen adduction (CA) exercise.
The CA exercise is a dynamic high-intensity exercise that does not require equipment and can be
easily performed on-field and at any training facility [
14
]. It is an eccentric partner exercise performed
to outer range of motion, the player who is exercising is in a side-lying position using the forearm
as support on the floor, the other arm is placed along the body, the upper leg is held by the partner,
approximately at the height of his hip, with one hand on the ankle and the another one on the knee.
Then, the player must raise the body from the floor, around 3 s concentric hip adduction, until the
body is in a straight line, so the feet are touching each other. Then the body is lowered halfway to the
ground, around 3 s eccentric adduction, until touching the floor without using it for support [12].
Although the high prevalence of adductor injuries and the potential benefits of the CA exercise,
to our knowledge there are no meta-analyses aimed to analyze the effect of CA on eccentric hip
adduction strength (EHAD). Therefore, the purpose of this study was to systematically review the
scientific literature about the effect of CA on EHAD and to provide practical recommendations for
training. Besides, a mini meta-analysis was carried out to determine the effect sizes of CA on EHAD.
2. Materials and Methods
The review was conducted following the statements of the Preferred Reporting Items for Systematic
Reviews and Meta-Analyses guidelines (PRISMA) [15].
2.1. Literature Search
To find the studies reported in this review, several electronic databases were screened: PubMed
(Medline), Web of Science (which includes other databases such as Current Contents Connect, Derwent
Innovations Index, Korean Journal Database, Russian Science Citation Index and SCIELO citation
index) and Google Scholar. The searches were limited to studies published up to and including
2 March 2020
. The search terms were: (a) the type of exercise (“Copenhagen adduction”) and (b) the
word (“randomized”). The search was conducted using the type of exercise and the word separated
by the Boolean operator “AND”. Figure 1shows the flow chart for selection of studies according to
PRISMA statements.
2.2. Study Selection
The inclusion criteria were: (a) EHAD as outcome measure, reporting means and standard
deviations (SD) before and after the intervention, (b) the study was a randomized controlled trial (RCT)
using the CA in the program intervention, (c) the article was written in English. Two independent
authors selected the potentially eligible articles from the databases. There were no disagreements.
2.3. Quality of the Evidence
The quality of the evidence was categorized using the Grading of Recommendations, Assessment,
Development and Evaluation (GRADE) approach [
16
]. The evidence of the effects on EHAD was
initially classified as “high quality” due to all the selected articles were RCT, but the evidence was
downgraded once because of the small sample size and once because of the potential risk of bias.
Therefore, the final quality of the evidence was low.

Appl. Sci. 2020,10, 2863 3 of 8
2.4. Data Collection
Two authors independently extracted data from the studies. The information included:
participants, interventions, comparisons, outcomes, and study design (PICOS), following the
recommendations from the PRISMA statement. Table 1shows age, sex, sample size and condition
of the participants. Table 2presents intervention and the comparison groups, including the type of
training intervention, duration of the study, sessions per week, number of sets, repetitions, weekly
volume and when the intervention was performed during the season. Figure 2displays results for the
different outcomes. The study design was not included in any table because all studies were RCT.
2.5. Statistical Analysis
The primary outcome of this mini meta-analysis was the EHAD test. Figure 2displays the results
of each study on this variable. All analyses were performed using the Review Manager Software
(RevMan, 5.3, Cochrane Collaboration, Oxford, UK). The analysis method used was inverse variance
and random effects due to the heterogeneity of articles [
17
]. The mean difference was used for EHAD.
The results obtained where represented with a confidence interval (CI) of 95%. Heterogeneity has
calculating by the I
2
statistic model and for the overall effect, Z-test was used [
18
,
19
]. Although there
is no consensus about the definition of “mild”, “moderate” or “severe” heterogeneity, Higgins and
Thompson suggested that H values exceeding 1.5 might indicate severe heterogeneity, while values
below 1.2 might represent low heterogeneity. Similarly, they proposed I
2
of 56% as cutofffor large
heterogeneity and 31% for low heterogeneity [20].
3. Results
3.1. Study Selection
PRISMA flow diagram is showed in Figure 1. A total of 126 records were identified, eight of
which were removed because they were duplicates. Of the remaining 118 articles, 97 were excluded
because they were not related to the topic, five studies were not written in English and three were
congress abstracts or books. After reading the remaining 13 articles, another 10 studies were excluded
because they did not include EHAD as outcome measure, not report means and SD before and after the
intervention or they were no RCT that applied the CA in the training program. Finally, three studies
were included in the meta-analysis.

Appl. Sci. 2020,10, 2863 4 of 8
Appl. Sci. 2020, 10, x 4 of 8
Figure 1. Flow chart for selection of studies according to PRISMA statements.
3.2. Study Characteristics
Study characteristics are summarized in Table 1. The total sample was 79. Of these, 41 belonged
to the CA group, and 38 were the control group. The average age ranged from 16.7 to 17.5 years old.
The sample was comprised exclusively of football players.
Table 1. Characteristics of the sample.
RCT
Group
Sample Size
and Sex
Age*
Condition
Kohavi et al., 2018 [21]
CA
CG
14 (males)
12 (males)
17.5 (1.1)
Young football athletes
Harøy et al., 2017 [22]
CA
CG
17 (males)
16 (males)
16.7 (0.9)
16.9 (1.0)
Elite football players
Ishøi et al., 2016 [14]
CA
CG
10 (males)
10 (males)
17.3 (SD not reported)
17.4 (SD not reported)
Sub-elite football players
RCT: Randomized Controlled Trial; CA: Copenhagen Adduction; CG: Control group, * Years (SD).
3.3. Interventions
The characteristics of the CA intervention and control group are displayed in Table 2. All studies
lasted 8 weeks, sessions per week were 2 or 3, the number of sets and repetitions varied between 1 to
4 and 3 to 15 respectively, the total number of repetitions ranged from 9 up to 90 and the interventions
were performed during pre- and in-season.
Figure 1. Flow chart for selection of studies according to PRISMA statements.
3.2. Study Characteristics
Study characteristics are summarized in Table 1. The total sample was 79. Of these, 41 belonged
to the CA group, and 38 were the control group. The average age ranged from 16.7 to 17.5 years old.
The sample was comprised exclusively of football players.
Table 1. Characteristics of the sample.
RCT Group Sample Size and Sex Age* Condition
Kohavi et al., 2018 [21]CA
CG
14 (males)
12 (males) 17.5 (1.1) Young football athletes
Harøy et al., 2017 [22]CA
CG
17 (males)
16 (males)
16.7 (0.9)
16.9 (1.0) Elite football players
Ishøi et al., 2016 [14]CA
CG
10 (males)
10 (males)
17.3 (SD not reported)
17.4 (SD not reported)
Sub-elite football players
RCT: Randomized Controlled Trial; CA: Copenhagen Adduction; CG: Control group, * Years (SD).
3.3. Interventions
The characteristics of the CA intervention and control group are displayed in Table 2. All studies
lasted 8 weeks, sessions per week were 2 or 3, the number of sets and repetitions varied between 1 to 4
and 3 to 15 respectively, the total number of repetitions ranged from 9 up to 90 and the interventions
were performed during pre- and in-season.

Appl. Sci. 2020,10, 2863 5 of 8
Table 2. Characteristics of the interventions.
RCT CA Group Control
Group Duration
Sessions
Per
Week
Sets Reps Reps Per
Week
Season
Period
Kohavi et al.,
2018
CA resistance
training +RFTS
Mobility
exercises +
RFTS
8 weeks 2 2–4 6–10 24–72 In-season
3–5
(Beginner) 9–15
Pre-season
Harøy et al.,
2017
FIFA 11 (with
CA) +RFTS
FIFA 11
(with NH) +
RFTS
8 weeks 3 1 7–10
(Intermediate)
21–30
12–15
(Advanced) 36–45
Ishøi et al.,
2016
CA resistance
training +RFTS RFTS 8 weeks 2 2–3 6–15 24–90 In-season
RCT: Randomized Controlled Trial; CA: Copenhagen adduction exercise; RFTS: Regular football training sessions;
NH: Nordic hamstring exercise.
3.4. Outcome Measures
This systematic review included articles focused on EHAD. All included studies [
14
,
21
,
22
]
measured the EHAD using a hand-held dynamometer and according to the protocol and
recommendations by Thorborg et al. [
23
]. The EHAD measure varied from 2.71 to 4.11 Nm/kg.
All three articles reported significant between-group or within-group improvement in EHAD
after the treatment [
14
,
21
,
22
]. In this regard, two studies observed significant between-group
improvement [
14
,
21
], while the other one found significant within-group improvements [
22
] and
almost significant (p=0.06) between-group changes.
Results revealed a significant overall effect for EHAD in favor to CA compared to CG (Z =2.94;
p=0.003
). The overall between-group mean difference was 0.61, with a 95% confidence interval from
0.20 to 1.01 (see Figure 2). The heterogeneity level was large (I
2
=68%). The quality of the evidence
was low according to the GRADE classification.
Appl. Sci. 2020, 10, x 5 of 8
Table 2. Characteristics of the interventions.
RCT
CA group
Control
Group
Duration
Sessions
Per
Week
Sets
Reps
Reps
Per
Week
Season
Period
Kohavi
et al.,
2018
CA
resistance
training +
RFTS
Mobility
exercises +
RFTS
8 weeks
2
2–4
6–10
24–72
In-season
3–5
(Beginner)
9–15
Pre-season
Harøy
et al.,
2017
FIFA 11
(with CA)
+ RFTS
FIFA 11
(with NH)
+ RFTS
8 weeks
3
1
7–10
(Intermediat
e)
21–30
12–15
(Advanced)
36–45
Ishøi et
al.,
2016
CA
resistance
training +
RFTS
RFTS
8 weeks
2
2–3
6–15
24–90
In-season
RCT: Randomized Controlled Trial; CA: Copenhagen adduction exercise; RFTS: Regular football training
sessions; NH: Nordic hamstring exercise.
3.4. Outcome Measures
This systematic review included articles focused on EHAD. All included studies [14,21,22]
measured the EHAD using a hand-held dynamometer and according to the protocol and
recommendations by Thorborg et al. [23]. The EHAD measure varied from 2.71 to 4.11 Nm/kg. All
three articles reported significant between-group or within-group improvement in EHAD after the
treatment [14,21,22]. In this regard, two studies observed significant between-group improvement
[14,21], while the other one found significant within-group improvements [22] and almost significant
(p = 0.06) between-group changes.
Figure 2. Meta-analysis results of CA exercise intervention on eccentric hip adduction strength
(EHAD).
Results revealed a significant overall effect for EHAD in favor to CA compared to CG (Z = 2.94;
p = 0.003). The overall between-group mean difference was 0.61, with a 95% confidence interval from
0.20 to 1.01 (see Figure 2). The heterogeneity level was large (I2 = 68%). The quality of the evidence
was low according to the GRADE classification.
4. Discussion
The purpose of this study was to systematically review the scientific literature about the effect
of CA exercise training on the EHAD test and to carry out a mini meta-analysis to determine the
overall effect as mean difference. Results indicate that eight weeks of training intervention with CA
exercise, 2-3 times per week, is effective to improve EHAD in football players [14,21,22]. Therefore, it
could be advisable the inclusion of CA in preventive training programs to potentially reduce injuries
in athletes and, specifically, in football players.
Figure 2.
Meta-analysis results of CA exercise intervention on eccentric hip adduction strength (EHAD).
4. Discussion
The purpose of this study was to systematically review the scientific literature about the effect of
CA exercise training on the EHAD test and to carry out a mini meta-analysis to determine the overall
effect as mean difference
.
Results indicate that eight weeks of training intervention with CA exercise,
2–3 times per week, is effective to improve EHAD in football players [
14
,
21
,
22
]. Therefore, it could be
advisable the inclusion of CA in preventive training programs to potentially reduce injuries in athletes
and, specifically, in football players.
The CA is a dynamic high-intensity exercise that includes eccentric contractions and showed
high muscle activity of the adductor longus using surface electromyography [
12
], so it could be
recommended in the prevention of adductor and/or groin injuries in athletes. Previous studies
observed that additional eccentric exercise reduced the rate of overall, new and recurrent muscle
injuries in professional and amateur football players [
24
]. The CA exercise seems to produce high peak
electromyography values for abdominal muscles [
12
] which may explain why preventing programs
based on CA exercise are appropriate against groin problems [25].

Appl. Sci. 2020,10, 2863 6 of 8
In the three studies included, the CA exercise was performed in football teams and implemented
during the pre-season [
22
] and in-season [
14
,
21
]. A previous study of 12-week eccentric hamstring
strengthening program observed a similar magnitude of effect, irrespective of its schedule relative
to the football training session. In this regard, they found an increase in strength and surface
electromyography, but the adaptations differed according to the timing of the injury prevention
program [
26
]. Therefore, future studies should explore if there is additional benefit when CA is applied
in different schedules, not only in football but also in other sports, and establish the optimal volume of
sets and repetitions.
As can be seen in Table 2, all training programs lasted eight weeks. This means that more studies
are recommended to see if the benefits can be obtained with interventions shorter than eight weeks due
to the lack of time at training is one of the barriers identified to carry out evidence-based sport injury
prevention programs [
27
], specifically in football [
28
]. In this regard, it is important to highlight the
short time required to carry out the CA intervention, which is usually shorter than 15 min [
21
]. Some
strengths of the CA exercise are that it seems to be well-tolerated and safe since none of the three articles
reported any important problem as consequence of the intervention. There was high compliance with
the training: 91% [
14
], 90% [
22
], and 85% [
21
] what explains the adherence and applicability of the
training. It is known that eccentric training intervention for muscle injury prevention depends on good
compliance to have success [
29
]. Another strength is that no special equipment is needed, so it can be
performed on the pitch or at any training facility.
Some limitations in the present mini meta-analysis can be mentioned. The first one is related to the
search strategy since only articles published in English were included. Another limitation can be that
the dominant leg was not reported in the study by Kohavi et al. [
21
], so the right leg was considered as
dominant. However, there was not much difference between the right and left legs. Given the low
quality of evidence based on GRADE and the large heterogeneity more studies are needed to have
a better understanding of CA exercise to enhance EHAD. Moreover, future studies should compare
CA exercise with another alternative training programs for adductor injuries prevention including
an injury register and assess the effect of CA depending on the athletes’ level, since all participants
included in this study were adolescents and soccer players.
5. Practical Applications
Based on this mini meta-analysis, some recommendations for the training intervention are
highlighted: (a) eight weeks, 2–3 times per week, seems to be effective, (b) weekly volume from 9 to
90 reps is enough to achieve a significant improvement. Nevertheless, further studies with different
duration, number of sets, repetitions and in athletes from other sports, not only football, could report
additional information to these findings.
6. Conclusions
The CA is an easy and useful exercise for improving EHAD in youth football players and,
consequently, has a positive effect on one of the factors that influence the injury risk. Thus, the inclusion
of CA in prevention programs could be an adequate strategy that could help to reduce the injury risk
and consequently suppose a decrease in injury incidence in football.
Author Contributions:
Conceptualization, J.P.-G., J.C.A. and J.C.-V.; methodology, J.P.-G., S.V., J.C.-V. and D.C.-M.;
software, J.C.A., and D.C.-M.; formal analysis, J.P.-G., S.V., J.C.A., J.C.-V., and D.C.-M.; investigation, J.P.-G.,
S.V., J.C.A., J.C.-V., M.
Á
.G.-G. and D.C.-M.; data curation, J.P.-G., J.C.A. and J.C.-V.; writing—original draft
preparation, J.P.-G., S.V. and D.C.-M.; writing—review and editing, J.P.-G., S.V., J.C.A., J.C.-V., M.
Á
.G.-G. and
D.C.-M.; supervision, J.P.-G., S.V., J.C.A., J.C.-V., M.
Á
.G.-G. and D.C.-M. All authors have read and agreed to the
published version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.

Appl. Sci. 2020,10, 2863 7 of 8
References
1.
Rodas, G.; Bove, T.; Caparros, T.; Langohr, K.; Medina, D.; Hamilton, B.; Sugimoto, D.; Casals, M. Ankle
Sprain Versus Muscle Strain Injury in Professional Men’s Basketball: A 9-Year Prospective Follow-up Study.
Orthop. J. Sports Med. 2019,7, 2325967119849035. [CrossRef] [PubMed]
2.
Williams, S.; Trewartha, G.; Kemp, S.; Stokes, K. A meta-analysis of injuries in senior men’s professional
Rugby Union. Sports Med. 2013,43, 1043–1055. [CrossRef] [PubMed]
3.
Ekstrand, J.; Hagglund, M.; Walden, M. Epidemiology of muscle injuries in professional football (soccer).
Am. J. Sports Med. 2011,39, 1226–1232. [CrossRef] [PubMed]
4.
Rogowski, I.; Creveaux, T.; Genevois, C.; Klouche, S.; Rahme, M.; Hardy, P. Upper limb joint muscle/tendon
injury and anthropometric adaptations in French competitive tennis players. Eur. J. Sport Sci.
2016
,16,
483–489. [CrossRef]
5.
Edouard, P.; Branco, P.; Alonso, J.M. Muscle injury is the principal injury type and hamstring muscle injury
is the first injury diagnosis during top-level international athletics championships between 2007 and 2015.
Br. J. Sports Med. 2016,50, 619–630. [CrossRef]
6.
Netto, D.C.; Arliani, G.G.; Thiele, E.S.; Cat, M.N.L.; Cohen, M.; Pagura, J.R. Prospective Evaluation of Injuries
occurred during the Brazilian Soccer Championship in 2016. Rev. Bras. Ortop.
2019
,54, 329–334. [CrossRef]
7. Nicholas, S.J.; Tyler, T.F. Adductor muscle strains in sport. Sports Med. 2002,32, 339–344. [CrossRef]
8.
Emery, C.A.; Meeuwisse, W.H.; Powell, J.W. Groin and abdominal strain injuries in the National Hockey
League. Clin. J. Sport Med. 1999,9, 151–156. [CrossRef]
9.
Tyler, T.F.; Nicholas, S.J.; Campbell, R.J.; McHugh, M.P. The association of hip strength and flexibility with
the incidence of adductor muscle strains in professional ice hockey players. Am. J. Sports Med.
2001
,29,
124–128. [CrossRef]
10.
Tegner, Y.; Lorentzon, R. Ice hockey injuries: Incidence, nature and causes. Br. J. Sports Med.
1991
,25, 87–89.
[CrossRef]
11.
Krommes, K.; Bandholm, T.; Jakobsen, M.D.; Andersen, L.L.; Serner, A.; Holmich, P.; Thorborg, K. Dynamic
Hip Adduction, Abduction and Abdominal Exercises from the Holmich Groin-Injury Prevention Program
Are Intense Enough to Be Considered Strengthening Exercises-a Cross-Sectional Study. Int. J. Sports Phys.
Ther. 2017,12, 371–380. [PubMed]
12.
Serner, A.; Jakobsen, M.D.; Andersen, L.L.; Holmich, P.; Sundstrup, E.; Thorborg, K. EMG evaluation of hip
adduction exercises for soccer players: Implications for exercise selection in prevention and treatment of
groin injuries. Br. J. Sports Med. 2014,48, 1108–1114. [CrossRef] [PubMed]
13.
Hagglund, M.; Walden, M.; Magnusson, H.; Kristenson, K.; Bengtsson, H.; Ekstrand, J. Injuries affect team
performance negatively in professional football: An 11-year follow-up of the UEFA Champions League
injury study. Br. J. Sports Med. 2013,47, 738–742. [CrossRef] [PubMed]
14.
Ishoi, L.; Sorensen, C.N.; Kaae, N.M.; Jorgensen, L.B.; Holmich, P.; Serner, A. Large eccentric strength increase
using the Copenhagen Adduction exercise in football: A randomized controlled trial. Scand. J. Med. Sci.
Sports 2016,26, 1334–1342. [CrossRef]
15.
Moher, D.; Shamseer, L.; Clarke, M.; Ghersi, D.; Liberati, A.; Petticrew, M.; Shekelle, P.; Stewart, L.A. Preferred
reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015 statement. Syst. Rev.
2015,4, 1. [CrossRef]
16.
Gopalakrishna, G.; Mustafa, R.A.; Davenport, C.; Scholten, R.J.; Hyde, C.; Brozek, J.; Schünemann, H.J.;
Bossuyt, P.M.; Leeflang, M.M.; Langendam, M.W. Applying Grading of Recommendations Assessment,
Development and Evaluation (GRADE) to diagnostic tests was challenging but doable. J. Clin. Epidemiol.
2014,67, 760–768. [CrossRef]
17.
Schmidt, F.L.; Oh, I.-S.; Hayes, T.L. Fixed-versus random-effects models in meta-analysis: Model properties
and an empirical comparison of differences in results. Br. J. Math. Stat. Psychol.
2009
,62, 97–128. [CrossRef]
18.
Hsiao, M.-Y.; Hung, C.-Y.; Chang, K.-V.; Chien, K.-L.; Tu, Y.-K.; Wang, T.-G. Comparative effectiveness of
autologous blood-derived products, shock-wave therapy and corticosteroids for treatment of plantar fasciitis:
A network meta-analysis. Rheumatology 2015,54, 1735–1743. [CrossRef]
19.
Chang, K.-V.; Wu, W.-T.; Han, D.-S.; Özçakar, L. Ulnar nerve cross-sectional area for the diagnosis of cubital
tunnel syndrome: A meta-analysis of ultrasonographic measurements. Arch. Phys. Med. Rehabil.
2018
,99,
743–757. [CrossRef] [PubMed]

Appl. Sci. 2020,10, 2863 8 of 8
20.
Higgins, J.P.; Thompson, S.G. Quantifying heterogeneity in a meta-analysis. Stat. Med.
2002
,21, 1539–1558.
[CrossRef]
21.
Kohavi, B.; Beato, M.; Laver, L.; Freitas, T.T.; Chung, L.H.; Dello Iacono, A. Effectiveness of Field-Based
Resistance Training Protocols on Hip Muscle Strength Among Young Elite Football Players. Clin. J. Sport.
Med. 2018. [CrossRef] [PubMed]
22.
Haroy, J.; Thorborg, K.; Serner, A.; Bjorkheim, A.; Rolstad, L.E.; Holmich, P.; Bahr, R.; Andersen, T.E. 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, 3052–3059.
[CrossRef] [PubMed]
23.
Thorborg, K.; Coupp
é
, C.; Petersen, J.; Magnusson, S.; Hölmich, P. Eccentric hip adduction and abduction
strength in elite soccer players and matched controls: A cross-sectional study. Br. J. Sports Med.
2011
,45,
10–13. [CrossRef] [PubMed]
24.
Petersen, J.; Thorborg, K.; Nielsen, M.B.; Budtz-Jorgensen, E.; Holmich, P. Preventive effect of eccentric
training on acute hamstring injuries in men’s soccer: A cluster-randomized controlled trial. Am. J. Sports.
Med. 2011,39, 2296–2303. [CrossRef]
25.
Haroy, J.; Clarsen, B.; Wiger, E.G.; Oyen, M.G.; Serner, A.; Thorborg, K.; Holmich, P.; Andersen, T.E.;
Bahr, R. The Adductor Strengthening Programme prevents groin problems among male football players:
A cluster-randomised controlled trial. Br. J. Sports Med. 2019,53, 150–157. [CrossRef]
26.
Lovell, R.; Knox, M.; Weston, M.; Siegler, J.C.; Brennan, S.; Marshall, P.W.M. Hamstring injury prevention in
soccer: Before or after training? Scand. J. Med. Sci. Sports 2018,28, 658–666. [CrossRef]
27.
Donaldson, A.; Callaghan, A.; Bizzini, M.; Jowett, A.; Keyzer, P.; Nicholson, M. A concept mapping approach
to identifying the barriers to implementing an evidence-based sports injury prevention programme. Inj. Prev.
2019,25, 244–251. [CrossRef]
28.
Joy, E.A.; Taylor, J.R.; Novak, M.A.; Chen, M.; Fink, B.P.; Porucznik, C.A. Factors influencing the
implementation of anterior cruciate ligament injury prevention strategies by girls soccer coaches. J. Strength
Cond. Res. 2013,27, 2263–2269. [CrossRef]
29.
Goode, A.P.; Reiman, M.P.; Harris, L.; DeLisa, L.; Kauffman, A.; Beltramo, D.; Poole, C.; Ledbetter, L.;
Taylor, A.B. Eccentric training for prevention of hamstring injuries may depend on intervention compliance:
A systematic review and meta-analysis. Br. J. Sports Med. 2015,49, 349–356. [CrossRef]
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