The isokinetic strength profile of elite soccer players according to playing position
The isokinetic strength profile of elite soccer players according to playing position
Robert Śliwowski 1 2
Monika Grygorowicz 0 2
Radosøaw Hojszyk 0 2
èukasz Jadczak 1 2
0 Rehasport Clinic FIFA Medical Centre of Excellence, Department of Physiotherapy , Poznań , Poland
1 Department of Team Sports Games of the University School of Physical Education in Poznań , Poland
2 Editor: Antoine Nordez, Universite de Nantes , FRANCE
The aim of this study was to compare isokinetic strength performance profiles in elite soccer players across different field positions. A total of 111 elite international players of Polish Ekstraklasa (the top division in Poland) were examined during the 2010±2015 seasons. The players were classified into six positional roles: central defenders (CD), external defenders (ED), central midfielders (CM), external midfielders (EM), forwards (F), and goalkeepers (G). The concentric isokinetic strength (peak torque [PT] of quadriceps and hamstrings, H/Q ratios) was calculated for the dominant leg and the non-dominant leg at angular velocity of 1.05 rad s±1, whereas to assess isokinetic muscle endurance, the total work [TW] at angular velocity of 4.19 rad s±1, was taken into consideration. The results showed that isokinetic strength performance varies significantly among players in different playing positions. The analysis of PT for quadriceps (PT-Q) and hamstrings (PT-H) generally showed that the goalkeepers and central midfielders had lower strength levels compared to other playing positions. In the case of PT-H and hamstring/quadricep (H/Q) peak torque ratios, statistically significant differences were also noted for the legs, where mean values noted for the dominant leg were higher than for the non-dominant leg. For TW for quadriceps (TW-Q) and hamstrings (TW-H), statistically significant differences were noted only between playing positions. TW-Q values for goalkeepers were lower than for central defenders and external midfielders. TW-H values for goalkeepers were lower than for central midfielders, central defenders and external midfielders. This study showed that specific functional activity of players in individual positions on the field influences the varied profile of isokinetic strength performance.
Data Availability Statement: All relevant data are
within the paper.
Funding: The authors received no specific funding
for this work.
Competing interests: The authors have declared
that no competing interests exist.
Measurement of muscle strength is an important factor in the evaluation and prediction of
muscular condition in addition to functional capacity [
]. A lot of research supports this
concept [2±4]. Isokinetic peak torque (PT) assessment is one of the most commonly applied
methods of evaluation of muscle strength of the lower extremities in soccer [5±7]. Knee
flexor and extensor muscle strength and the hamstring-to-quadricep ratio have also been
identified as important criteria to analyse the risk of lower extremity injuries [
2, 6, 8
Numerous studies have focused on the effect of various training programmes in order to
prevent injuries to adult and youth soccer players [9±11]. Several researchers have successfully
described the isokinetic muscle profile at various levels of soccer training [
muscular strength has also been compared in various sub-periods of the frame  or in
other sport groups [
In the context of a wide area of exploration of isokinetic studies of soccer players, little
information is available about the strength isokinetic profile according to the position on the
field. Although the existing reports in this area mainly confirm the varied profile of isokinetic
strength between players in various positions [16±23], some studies have found contradictory
results [16, 17, 21±23]. A study by Tourny-Chollet et al. [
] indicates that forward players
essentially showed higher hamstring concentric strengths compared to midfielders and
defenders. OÈ berg et al. [
] noted a significantly higher knee extensor torque in goalkeepers
and defenders than in forwards. A greater PT of knee extensor muscles in defensive players
compared to midfield players was also confirmed by the recent study of Costa Silva et al. [
However, Goulart et al. [
], who used a more advanced division of players, demonstrated that
full backs' knee flexor muscles exhibited lower PT when compared with the other positions,
and goalkeepers' knee extensor muscles exerted lower PT and had a higher fatigue index when
compared to results for other positions. These findings are in accordance with Ruas et al. [
who found that goalkeepers demonstrated different characteristics and concentric PT across
muscles than most players in other field positions. However, Magalhães et al. [
] did not note
different isokinetic strengths between players of different positions.
These differences between the results are due to a large variability in test protocols, namely
speed, type of dynamometer (Biodex, Cybex), large differences in sports level, and the numbers
of studied participants, which make the object of the studies ambiguous. A few studies refer to
the division of players on the pitch, which has recently been commonly used in the literature,
resulting from different functional requirements and motor profile of play in a given position
17, 21, 22
]. Some studies relate to young players [
]. Others are not representative
enough in terms of the size of the sample (<30) [
16, 18, 20
]. A prevailing majority of the
reports refer their analyses to absolute values of the studied isokinetic indicators [16, 18, 21±
23]. In our opinion, significant differences in the build and weight among players in different
playing positions [
18, 20, 21, 22
] may have a significant influence on the level of isokinetic
strength. Apart from the study of Goulart et al. [
], no references to total muscle work were
found. This variable represents the torque generated during the entire range of motion (ROM)
and is related to the muscle energy expenditure during the whole test [
]. Taking this
variable into consideration in isokinetic assessment of muscular strength with the use of study
protocols with a large number of repetitions may provide important information on muscle
endurance . Another limitation of the majority of the studies is the lack of research
covering the elite level of sports championship. Performing this type of analysis on top-level soccer
players may not only be a valuable cognitive contribution but may also have implications for
widely defined team selection and optimising physical and medical preparation strategies for
the training process.
This information could stress the need to perform further studies on a larger sample size in
order to identify an isokinetic muscle profile in soccer players of the professional elite level,
verifying possible adjustments according to game and training specificity. Therefore, the
purpose of the current study was to compare isokinetic strength performance profiles (PT and
TW of quadriceps and hamstrings, and H/Q ratios) in elite soccer players across different field
2 / 13
Participants, procedures, and data collection
The study covered a group of 111 elite international soccer players of Polish Ekstraklasa (the
top division in Poland). The participants (26±5 years; 79±8 kg; 181±7 cm) included players of
many nationalities (81 Polish, 13 South European, 5 Latin American, 4 African, 3 Eastern
European, 3 Western European, and 2 Northern European). Seventy-eight players (70% of the
total) were members of senior and youth national teams of their countries (mainly European
countriesÐ92%). The participants had at least three years of experience playing soccer at a
professional level with regular training, which constituted a part of their professional contract.
The study was performed from 2009 to 2015. All measurements were taken in December, close
to the end of the first half of the Ekstraklasa season, 48 hours after the final game.
The players were divided into six subgroups according to playing positions: goalkeepers
(n = 14), central defenders (n = 18), external defenders (n = 14), central midfielders (n = 30),
external midfielders (n = 14), and forwards (n = 21). The profile of the individual playing
positions was based on the different activities on the pitch field and the main area in which this
activity was carried out [27±29]. Additionally, for the studies cited above, we decided to
enlarge the player groups and include the goalkeeper position as a separate group to be
analysed. The players were asked about their playing positions that they had played most
frequently in the past year. Only players who played in their usual positions were included in the
sample. Moreover, the analysis was conducted on values collected only from players who
played at least 50% of official club matches in the season preceding the study. All players in the
analysis played in the 1-4-2-3-1 formation. Player and team confidentiality was ensured thanks
to anonymity of all performance data. As part of their professional playing contracts, players
were informed about the experimental risks and provided written consent for their data to be
collected and analyzed. For players younger than 18 years, their parents or guardians were
informed of the risks and signed an informed consent before the investigation. The study was
carried out according to the Declaration of Helsinki, and the protocol was fully approved by
the Bioethical Committee at the Poznań University of Medical Sciences. The basic physical
characteristics of the players are presented in Table 1.
Test procedures (muscle strength profiling)
The measurements were performed by the examiner team in the laboratory for isokinetic
testing at the Rehasport Clinic FIFA Medical Centre of Excellence in Poznań, Poland. In this
study, the measurement of isokinetic knee muscle strength (as measured by peak torque [PT])
and muscle endurance (as measured by total work [TW]) was performed using the Biodex
System 3 (Biodex Medical Systems™ Inc., New York, USA) dynamometer. According to many
] both variables (PT and TW) express strength performance well.
The proper positioning and stabilisation of the subject, the alignment between the axis of
the rotation of the machine and the knee joint, and the gravity compensation and the
elimination of acceleration artefacts were performed according to the instruction manual for the
Biodex Medical Systems and were similar to the ones described in the literature [
1, 7, 10
warm-up that each player performed before the isokinetic assessment took 10±15 minutes and
consisted of mild pedalling on a stationary Monark cycle ergometer at a moderate pace (50±
100 W) and dynamic stretches for the major lower-limb muscle groups [
]. The concentric
isokinetic torque of the quadriceps and hamstrings was assessed during continuous
(bidirectional) knee extension±flexion movements at the angular velocity of 1.047 rad s-1 and 4.189
rad s-1 through a knee range of motion of 0Ê (flexed) to 90Ê (full extension). The above testing
3 / 13
speeds have been widely used in other studies assessing muscle strength in soccer players [
]. Participants were given three trials at sub-maximal efforts with a gradually increasing load
(50%, 75%, and approximately 100% of maximum capability) and then performed one set of
three repetitions at the maximal concentric contraction at angular velocities of 1.047 rad s-1
and, subsequently, 30 repetitions at angular velocities of 4.189 rad s-1. Then, the same protocol
was followed with the opposite leg. The participants were requested to resist the powered leg
extension movement as hard as they could. A 30-s rest was given after the third sub-maximal
trial, a one-minute break was given between two angular velocities, and a three-minute break
was given when the machine setting was changed for the opposite leg. It has been reported that
this number of repetitions has a high reliability for isokinetic testing. Standardised verbal
encouragement was given before each maximal effort, and visual feedback of the recorded
torque was provided. The order of testing was randomised for the dominant (D) and
non-dominant (N-D) legs [
]. Limb dominance was defined as the leg that is preferred when kicking
a ball, and this was determined through an interview [
]. In order to limit the analysis to
constant velocity periods, only windowed data has been used. The statistical analysis included
the relative PTs (N m kg-1) for flexors (PT-Q) and extensors (PT-H) in both legs and the
unilateral ratio of muscle torque for both the dominant and non-dominant extremities (HD/QD
and HN/QN, respectively) at angular velocity of 1.05 rad s±1 and, similarly, the relative TWs
(J kg-1) for flexors (TW-Q) and extensors (TW-H) for both legs at angular velocity of 4.19 rad
s±1. Previous studies  indicated that TW measured during a protocol involving 30
reciprocal maximal concentric contractions represented a good compromise between reliability
(intraclass coefficient of correlation = 0.91 and standard error of measurement = 4%) and
bioenergetical interpretability of the data.
All tests took place between 9:00 am and 1:00 pm and were conducted in the same order for
each player. The same member of the research team performed all isokinetic tests. The
evaluated players were not exposed to intensive training for two days before testing. Before the start
of the study, the subjects were asked to fill in a questionnaire to determine whether they had
4 / 13
any musculoskeletal pain, discomfort, or known injury in a lower extremity. Subjects were free
of previous significant knee injuries and had no history of ACL repairs or rehabilitation.
Furthermore, the participants had no history of a lower extremity fracture or surgery during the
year prior to the study. Those players who reported a major or moderate lower extremity
injury or any injury to the knee or thigh were excluded from further analysis.
All statistical analyses were conducted using STATISTICA 10.0 for Windows Version (SPSS
Inc., Chicago, IL, USA). All results are reported as means and standard deviations (mean SD)
calculated by conventional procedures, unless otherwise stated. The normality of the variables
was tested with the Shapiro±Wilk Test, and the coefficients of asymmetry and kurtosis were
found. For all isokinetic variables (PT and TW of the quadriceps and hamstrings per body
mass, and H/Q ratios), differences between legs (dominant, non-dominant) were tested using
the T-test. Differences between positions on the field (goalkeepers, central midfielders,
external defenders, central midfielders, external midfielders, and forwards) were tested using
oneway analysis of variance (ANOVA). Duncan post-hoc procedures were used to identify specific
differences. In the absence of interactions, only main effects were analysed. This analysis was
the basis for creating models of multifactorial analysis of variance (ANOVA) for each
isokinetic variable (ANOVA) when significant interactions were present. A 2x6 (leg x position)
repeated-measures ANOVA was used to compare only PT of the hamstrings. Statistical
significance was set at an alpha of 0.05 for all statistical procedures.
The physical characteristics of the players demonstrated significant differences in their height
and weight according to their playing positions (see Table 1). Goalkeepers are significantly
heavier than players in all other positions (in all cases p < 0.0001, except for central defenders
p = 0.0006). Central defenders are significantly heavier than external midfielders (p < 0.0001),
external defenders (p = 0.0128), and central midfielders (p = 0.0065). On the other hand,
forwards are significantly heavier than external midfielders (p = 0.0004).
Fig 1 presents the mean values of PT-Q and PT-H for legs and playing positions and mean
values of H/Q ratios for legs. For legs, statistically significant differences were noted only for
PT-H and H/Q ratios (p = 0.0109 and p = 0.0320, respectively; Fig 1A and 1B), where mean
values noted for the dominant leg were higher than for the non-dominant leg. For PT-Q and
PT-H, statistically significant differences were also noted between playing positions (in both
cases p < 0.0001; Fig 1C). PT-Q values for goalkeepers were lower than for external defenders,
central defenders, external midfielders, and forwards, whereas for central midfielders they
were lower than for external midfielders, and forwards. PT-H values for goalkeepers were
lower than for central midfielders, external defenders, central defenders, forwards, and
external midfielders, whereas for central midfielders they were lower than for central defenders and
Fig 2 presents the mean values of TW-Q and TW-H for legs and playing positions. For
TW-Q and TW-H, statistically significant differences were noted only between playing
positions (in both cases p < 0.0001). TW-Q values for goalkeepers were lower than for central
defenders and external midfielders. TW-H values for goalkeepers were lower for central
midfielders, central defenders and external midfielders.
For PT-H, there were no interactions. However, there was a main effect for position
(p < 0.0001), in which PT-H values for goalkeepers were lower than for central midfielders,
external defenders, forwards, central defenders, and external midfielders, whereas for central
5 / 13
Fig 1. Mean values and standard deviation for: (A) PT/BM of the quadriceps and hamstrings by legs (dominant, non-dominant);
(B) H/Q ratios by legs (dominant, non-dominant); (C) PT/BM of the quadriceps and hamstrings by playing positions (G, CD, ED,
CM, EM, F).
midfielders they were lower than for central defenders and external midfielders, and for legs
(p = 0.0055), where mean values noted for the dominant leg were higher than for the
nondominant leg (Table 2).
The various demands of elite soccer isokinetic strength have been frequently described in the
scientific literature. However, few studies attempted to address specific profiles of isokinetic
strength in reference to the playing positions. In the context of recent highly advanced playing
position specialisation and, what follows, a varied motor profile in a given playing position,
these aspects are of increased significance. Therefore, the aim of this study was to compare
isokinetic strength performance profiles in elite soccer players across different field positions.
The principal finding of the present study was that concentric isokinetic strength varies
significantly among players in different playing positions. In terms of the PT for quadriceps,
goalkeepers had lower values than central midfielders, external defenders, central defenders,
external midfielders, and forwards, whereas the values were lower for central midfielders than
for external midfielders, and forwards (p < 0.0001; Table 2). The latest study of Costa Silva
6 / 13
Fig 2. Mean values and standard deviation for TW/BM of the quadriceps and hamstrings by: (A) legs (dominant, non-dominant);
(B) playing positions (G, CD, ED, CM, EM, F).
bIndicates a significantly greater PT-H then CD (p = 0.0493), and EM (p = 0.0087)
7 / 13
et al. [
], referenced at the beginning, indicates that statistically significant differences among
players in the three specified playing positions (defenders, midfielders, forwards) related only
to the PT of extensor muscles, where defenders had statistically higher values than midfielders
but showed no difference compared to forwards. According to the authors, these results can be
justified due to the performance of similar specific actions between defenders and forwards,
mainly composed of short and intense movements like sprints and jumps, demanding great
effort of knee extensors. Contrary to these findings, OÈ berg et al. [
], based on a similar
division of players, found that goalkeepers and defenders had greater quadricep strength
compared to forwards. These findings are in accordance with Ruas et al. [
], who found that
goalkeepers' quadricep strength was greater compared with all other positions (side backs,
central defender midfielders, central attacking midfielders, and forwards), and central backs had
greater quadricep strength compared to central defender midfielders. However, Carvalho and
] demonstrated that goalkeepers and forwards have greater quadricep strength in the
dominant leg, while centre backs have greater quadricep non-dominant leg strength compared
with all other positions. Contrary to these findings, there are reports that do not show
statistically significant differences in the level of concentric PT for quadriceps between players in
different positions [16±18, 20, 23].
In soccer practice, it is normally understood that the quadricep muscle plays an important
role in the execution of sprints, jumps, kicks, and passes, while the hamstrings mainly act as
stabilisers of the knee joint during changes of speed and direction and kicking [
18, 21, 22, 33,
]. In sprinting, turns and tackling of the hamstrings are used concentrically but are mainly
used eccentrically to control, decelerate, and stabilise the knee [
2, 7, 9, 22, 33, 34
]. The results
found in this study were similar for quadriceps, where goalkeepers had lower PT of the
hamstrings relative to all other positions, and central midfielders relative to central defenders and
external midfielders (p < 0.0001; Table 2). These findings are similar to the data obtained by
Tourny-Chollet et al. [
], who found that midfielders have lower concentric hamstring
strength in the non-dominant leg compared to forwards and defenders, and for the dominant
leg compared to forwards alone (goalkeepers were not considered in this study). Weber et al.
, based on the same division of players, demonstrated the same relations, where defenders
had higher values for flexor strength muscles compared to midfielders for the dominant leg
(no statistically significant differences were noted for the non-dominant leg). On the other
hand, Goulart et al. [
] showed higher average PT of right knee flexors in central defenders
compared to central midfielders, external defenders, and forwards. Similar relations were
noted by Carvalho and Cabri [
], who noted that players in almost all field positions had
lower hamstring strength for both dominant and non-dominant legs than goalkeepers and
Briefly, we found that the goalkeepers and central midfielders had lower strength levels of
extensors and flexors compared to other field positions. The results of goalkeepers may be
explained by their specific tasks, which may have contributed to these results. Furthermore,
Ruas et al. [
] suggest that the results of the goalkeepers should be interpreted differently than
other positions. Contrary to the studies referred to above [
], our goalkeepers had the
lowest levels of PT of the quadriceps and hamstrings for both legs (Table 2). Such contradictory
findings should be considered in the context of the lack of relativisation of analysed isokinetic
indicators to players' weight. It is generally known in the literature that goalkeepers have
higher weight compared to other players . On the other hand, the differences noted
between field players can be justified by the performance of similar specific actions among
central and external defenders, external midfielders, and forwards, mainly composed of short and
intense movements, like sprints and jumps, demanding great effort of the knee extensors
compared to central midfielders, whose activities display characteristics of a more prolonged action
8 / 13
]. Consequently, the general trend confirmed by many studies [
16, 18, 20, 21, 23
] is that
central midfielders have a lower level of quadricep and hamstring muscle strength compared
to players in other positions on the field.
According to Fousekis at al. [
], certain skills in soccer such as kicking, passing, and cutting
are distinctly unilateral, require asymmetrical motor patterns, and lead to the development of
more force on one side toward the other. This is in agreement with the results of many earlier
], which showed statistically significant differences in muscle strength between
the dominant leg and the non-dominant leg. On the contrary, our study does not confirm
these differences with respect to extensor muscles among players in different positions. The
lack of significant differences in PT values for the quadriceps between the dominant leg vs.
non-dominant leg is similar to the results noted by Rahnama et al. , Costa Silva et al. [
Weber et al.  and Magalhães et al. [
], who also did not record significant differences
between the dominant and non-dominant strength of the players in different field positions.
This result may have been influenced by the methodology used in the training of both limbs to
perform technical movements [
], as well as by the versatility of tasks performed, which has
led us in recent years to eliminate differences in the muscle force profile of the players in
different positions [
]. In contrast to the extensors, for flexors we noted statistically significant
differences between legs, where mean values for the dominant leg were higher than for the
nondominant leg (Table 2). These findings, in turn, are in accordance with the results of studies
using both concentric [
18, 20, 21
] and eccentric contraction strength flexors [
2, 9, 18, 22
Perhaps in both cases there is a similar pattern of adaptive changes.
These data did not show statistically significant differences in the case of H/Q ratios among
different field positions (Table 2). As in our study, Costa Silva et al. [
], Weber et al. , and
Ruas et al. [
] did not report statistically significant differences between groups in relation to
conventional H/Q ratio for the dominant and non-dominant legs. The lack of any variation
between groups in the case of the analysed indicator shows that it should be mainly of
diagnostic significance in the assessment of injury risk in players. It seems, however, that PT ratio
measurements have greater importance for clinical evaluation than performance in soccer [
Although the present study found significant differences in H/Q ratios between legs, where
mean values noted for the dominant leg were higher than for the non-dominant leg, these
results indicated a better stability of the knee joint for the dominant leg. On the other hand,
these findings concerning conventional H/Q ratios were not confirmed in the studies referred
to above [
16, 18, 22
]. We also found the values of conventional H/Q ratio ranging from 55% to
62% at 1.05 rad s±1. The above values are similar to those reported by other authors [
3, 13, 21,
]. Previously, the H/Q ratios of less than 60% for the angular velocity at 1.05 rad s±1
were linked to ACL injury; knee joint stabilization is affected by activation of the quadriceps
and relative weakness of the hamstrings [14, 15, 22, 38±40]. Therefore, particularly important
factors in monitoring the effects of training programmes and in injury prevention are the
evaluation and control of muscle strength .
Although PT is considered as the favoured measure of isokinetic strength performance,
from the practical point of view other measures such as TW may provide significant
information in terms of the muscle ability to generate strength over a longer time by players in specific
positions. It might seem that due to significantly different functional requirements in the
game, players in different positions on the pitch will represent various levels of muscle
endurance. Our study shows that TW-Q values for goalkeepers were lower than for central defenders
and external midfielders, and TW-H values for goalkeepers were lower than for central
midfielders, central defenders and external midfielders (Table 3). Although the highest mean
values of TW-Q and TW-H were clearly noted for external midfielders and central defenders,
these differences between individual outfield players did not show statistical significance (a
9 / 13
tendency to statistical significance between external midfielders and external defenders for
TW-H should only be noted here). These findings are similar to the data obtained by Goulart
et al. [
], who also did not report statistically significant differences in TW values between
players in specific positions on the pitch. According to the authors [
], this trend may be due
to the fact that muscle endurance training in clubs is similar for all players, which may explain
the lack of differences in this respect between players in specific positions on the pitch. In our
opinion in the context of modern training technologies, these loads should be strongly
individualised in terms of individual components of strength performance.
Although the relationships between isokinetic muscular strength in soccer players and
functional demands in play are not clear, it has been suggested that the specific functional
activity of players in individual positions in the field, may influence isokinetic strength profile
18, 20, 21
]. The present results confirmed these relations especially in terms of muscle
strength, where the goalkeepers and central midfielders had lower strength levels of extensors
and flexors compared to other field positions. The results for muscle endurance looked slightly
different, as the goalkeepers had lower results compared to external midfielders and central
defenders. Goalkeepers, however, due to the specificity of play in their position and a
significantly lower level of these elements, had the lowest values of most analysed isokinetic
indicators. Therefore, it can be assumed that that specific functional activity of players in individual
positions in the field influences the varied profile of isokinetic strength performance. This
study may be a key part of the trend of activities aiming to individualise the training process in
soccer and the preparation strategies in terms of strength preparation of players in individual
positions and in the context of profiling players for a given position. However, future reports
should also take into consideration deficiencies and imbalances in strength and endurance for
individual muscle groups between players of different positions in the field.
The authors would like to thank Witold Dudziński, PhD, the head of the Rehasport
rehabilitation clinic, for making this study possible, and Mrs. Magdalena Lewandowska for her
assistance in statistical analysis.
Conceptualization: Robert Śliwowski.
Formal analysis: Robert Śliwowski, èukasz Jadczak.
Investigation: Robert Śliwowski, èukasz Jadczak.
Methodology: Robert Śliwowski, Monika Grygorowicz, Radosøaw Hojszyk.
10 / 13
Project administration: Robert Śliwowski.
Resources: Monika Grygorowicz, Radosøaw Hojszyk.
Supervision: èukasz Jadczak.
Validation: Robert Śliwowski.
Visualization: Robert Śliwowski.
Writing ± original draft: Robert Śliwowski.
Writing ± review & editing: Monika Grygorowicz, èukasz Jadczak.
11 / 13
Weber FS, Da Silva BGC, Radaelli R, Paiva C, Pinto RS (2010) Isokinetic assessment in professional
soccer players and performance comparison according to their different positions in the field. Rev Bras
Med Esporte 16(4): 264±268.
12 / 13
1. Daneshjoo A , Rahnama N , Mokhtar AH , Yusof A ( 2013 ) Effectiveness of injury prevention programs on developing quadriceps and hamstrings strength of young male professional soccer players . J Hum Kinet 39 ( 31 ): 115 ± 125 .
2. Croisier JL , Ganteaume S , Binet J , Genty M , Ferret JM ( 2008 ) Strength imbalances and prevention of hamstring injury in professional soccer players: A prospective study . Am J Sports Med 36 ( 8 ): 1469 ± 1475 . https://doi.org/10.1177/0363546508316764 PMID: 18448578
3. Fousekis K , Tsepis E , Vagenas G ( 2010 ) Lower limb strength in professional soccer players: profile, asymmetry, and training age . J Sports Sci Med 9 ( 3 ): 364 ± 373 . PMID: 24149628
4. Greco CC , Silva WL , Camarda SRA , Denadai BS ( 2013 ) Fatigue and rapid hamstring/quadriceps force capacity in professional soccer players . Clin Physiol Funct Imaging 33 ( 1 ): 18 ± 23 . https://doi.org/10. 1111/j. 1475 - 097X . 2012 . 01160 . x PMID : 23216761
5. Rahnama N , Lees A , Bambaecichi E ( 2005 ) Comparison of muscle strength and flexibility between the preferred and non-preferred leg in English soccer players . Ergonomics 48 ( 11 ±14): 1568 ± 1575 . https:// doi.org/10.1080/00140130500101585 PMID: 16338722
6. Houweling TA , Head A , Hamzeh MA ( 2009 ) Validity of isokinetic testing for previous hamstring injury detection in soccer players . Isokinet Exerc Sci 17 ( 4 ): 213 ± 220 .
7. Jenkins NDM , Hawkey MJ , Costa PB , Fiddler RE , Thompson BJ , Ryan ED , et al. ( 2013 ) Functional hamstrings: quadriceps ratios in elite women's soccer players . J Sports Sci 31 ( 6 ): 612 ± 617 . https://doi. org/10.1080/02640414. 2012 .742958 PMID: 23150930
8. Fousekis K , Tsepis E , Poulmedis P , Athanasopoulos S , Vagenas G ( 2011 ) Intrinsic risk factors of noncontact quadriceps and hamstring strains in soccer: A prospective study of 100 professional players . Br J Sports Med 45 ( 9 ): 709 ± 714 . https://doi.org/10.1136/bjsm. 2010 .077560 PMID: 21119022
9. Arnason A , Andersen TE , Holme I , Engebretsen L , Bahr R ( 2008 ) Prevention of hamstring strains in elite soccer: An intervention study . Scand J Med Sci Sports 18 ( 1 ): 40 ± 48 . https://doi.org/10.1111/j. 1600- 0838 . 2006 . 00634 . x PMID : 17355322
10. Iga J , George K , Lees A , Reilly T ( 2009 ) Cross-sectional investigation of indices of isokinetic leg strength in youth soccer players and untrained individuals . Scand J Med Sci Sports 19 ( 5 ): 714 ± 719 . https://doi.org/10.1111/j.1600- 0838 . 2008 . 00822 . x PMID : 18627555
11. Śliwowski R , Jadczak è , Hejna R , Wieczorek A ( 2015 ) The effects of individualized resistance strength programs on knee muscular imbalances in junior elite soccer players . PloS One 10 ( 12 ): e0144021. https://doi.org/10.1371/journal.pone. 0144021 PMID: 26630271
12. Forbes H , Bullers A , Lovell A , McNaughton LR , Polman RC , Siegler JC ( 2009 ) Relative torque profiles of elite male youth footballers: effects of age and pubertal development . Int J Sports Med 30 ( 8 ): 592 ± 597 . https://doi.org/10.1055/s-0029-1202817 PMID: 19468968
13. Lehance C , Binet J , Bury T , Croisier L ( 2009 ) Muscular strength, functional performances and injury risk in professional and junior elite soccer players . Scand J Med Sci Sports 19 ( 2 ): 243 ± 251 . https://doi.org/ 10.1111/j.1600- 0838 . 2008 . 00780 . x PMID : 18384493
14. Lehnert M , XaverovaÂ Z , De Ste Croix M ( 2014 ) Changes in muscle strength in U19 soccer players during an annual training cycle . J Hum Kinet 42 ( 10 ): 175 ± 185 .
15. Andrade MDS , De Lira CAB , Koffes FDC , Mascarin NC , Benedito-Silva AA , Da Silva AC ( 2012 ) Isokinetic hamstrings-to-quadriceps peak torque ratio: The influence of sport modality, gender, and angular velocity . J Sports Sci 30 ( 6 ): 547 ± 553 . https://doi.org/10.1080/02640414. 2011 .644249 PMID: 22364375
16. Costa Silva JRL , Detanico D , Dal Pupo J , Freitas CR ( 2015 ) Bilateral asymmetry of knee and ankle isokinetic torque in soccer players U 20 category . Rev Bras Cineantropom Desempenho Hum 17 ( 2 ): 195 ± 204 .
17. Goulart LF , Dias RMR , Altimari LR ( 2007 ) Isokinetic force of under-twenties soccer players: Comparison of players in different field positions . Rev Bras Cineantrop Desempenho Hum 9 ( 2 ): 165 ± 169 .
19. OÈ berg B , Ekstrand J , MoÈller M , Gillquist J ( 1984 ) Muscle strength and flexibility in different positions of soccer players . Int J Sports Med 5 ( 4 ): 213 ± 216 . PMID: 6480206
20. Tourny-Chollet C , Leroy D , Leger H , Beuret-Blanquart F ( 2000 ) Isokinetic knee muscle strength of soccer players according to their position . Isokinet Exerc Sci 8 ( 4 ): 187 ± 193 .
21. Carvalho P , Cabri J ( 2007 ) AvaliacËăo isocineÂtica da forcËa dos muÂsculos da coxa dos futebolistas . Rev Port Fisiot Desporto 1 ( 2 ): 4± 13 .
22. Ruas CV , Minozzo F , Pinto MD , Brown LE , Pinto RS ( 2015 ) Lower-extremity strength ratios of professional soccer players according to field position . J Strength Cond Res 29 ( 5 ): 1220 ± 1226 . https://doi. org/10.1519/JSC.0000000000000766 PMID: 25436632
23. Magalhães J , Oliveira J , Ascensão A , Soares JMC ( 2004 ) Concentric quadriceps and hamstrings isokinetic strength in volleyball and soccer players . J Sports Med Phys Fit 44 ( 2 ): 119 ± 125 .
24. English R , Brannock M , Chik WT , Eastwood LS , Uhl T ( 2006 ) The relationship between lower extremity isokinetic work and single-leg functional hop-work test . J Sport Rehabil 15 : 95 ± 104 .
25. Amaral GM , Marinho HVR , Ocarino JM , Silva PLP , Souza TR , Fonseca ST ( 2014 ) Muscular performance characterization in athletes: a new perspective on isokinetic variables . Braz J Phys Ther 18 ( 6 ): 521 ± 529 . https://doi.org/10.1590/bjpt-rbf. 2014 .0047 PMID: 25590444
26. Manou V , Arseniou P , Gerodimos V , Kellis S ( 2002 ) Test-retest reliability of an isokinetic muscle endurance test . Isokinet Exerc Sci 10 ( 4 ): 177 ± 181 .
27. Di Salvo V , Baron R , Tschan H , Calderon Montero FJ , Bachl N , Pigozzi F ( 2007 ) Performance characteristics according to playing position in elite soccer . Int J Sports Med 28 ( 3 ): 222 ± 227 . https://doi.org/ 10.1055/s-2006 -924294 PMID: 17024626
28. Bradley PS , Sheldon W , Wooster B , Olsen P , Boanas P , Krustrup P ( 2009 ) High-intensity running in English FA Premier League soccer matches . J Sports Sci 27 ( 2 ): 159 ± 168 . https://doi.org/10.1080/ 02640410802512775 PMID: 19153866
29. Carling C ( 2010 ) Analysis of physical activity profiles when running with the ball in a professional soccer team . J Sports Sci 28 ( 3 ): 319 ± 326 . https://doi.org/10.1080/02640410903473851 PMID: 20077273
30. Montgomery LC , Douglas LW , Deuster PA ( 1989 ) Reliability of an isokinetic test of muscle strength and endurance . J Orthop Sports Phys Ther 10 ( 8 ): 315 ± 322 . PMID: 18796953
31. Daneshjoo A , Mokhtar AH , Rahnama N , Yusof A ( 2012 ) The effects of injury preventive warm-up programs on knee strength ratio in young male professional soccer players . PloS One 7 ( 12 ): e50979. https://doi.org/10.1371/journal.pone. 0050979 PMID: 23226553
32. Bosquet L , Maquet D , Forthomme B , Nowak N , Lehance C , Croisier JL ( 2010 ) Effect of the lengthening of the protocol on the reliability of muscle fatigue indicators . Int J Sports Med 31 ( 2 ): 82 ± 88 . https://doi. org/10.1055/s-0029-1243168 PMID: 20221999
33. Bogdanis GC , Kalapotharakos VI ( 2016 ) Knee extension strength and hamstrings-to-quadriceps imbalances in elite soccer players . Int J Sports Med 37 ( 2 ): 119 ± 124 . https://doi.org/10.1055/s-0035- 1559686 PMID: 26509377
34. Cometti G , Maffiuletti NA , Pousson M , Chatard JC , Maffulli N ( 2001 ) Isokinetic strength and anaerobic power of elite, subelite and amateur French soccer players . Int J Sports Med 22 ( 1 ): 45 ± 51 . https://doi. org/10.1055/s-2001 -11331 PMID: 11258641
35. Reilly T , Bangsbo J , Franks A ( 2000 ) Anthropometric and physiological predispositions for elite soccer . J Sports Sci 18 ( 9 ): 669 ± 683 . https://doi.org/10.1080/02640410050120050 PMID: 11043893
36. McLean B , Tumilty D ( 1993 ) Left-right asymmetry in two types of soccer kick . Br J Sports Med 27 ( 4 ): 260 ± 262 . PMID: 8130965
37. Mognoni P , Narici M , Sirtori M , Lorenzelli F ( 1994 ) Isokinetic torques and maximal velocity in young soccer players . J Sports Med Phys Fit 34 ( 4 ): 357 ± 361 .
38. Dauty M , Potiron-Josse M , Rochcongar P ( 2003 ) Identification of previous hamstring muscle injury by isokinetic concentric and eccentric torque measurement in elite soccer player . Isokinet Exerc Sci 11 ( 3 ): 139 ± 144 .
39. Camarda SRA , Denadai BS ( 2012 ) Does muscle imbalance affect fatigue after soccer-specific intermittent protocol? J Sci Med Sport 15 ( 4 ): 355 ± 360 . https://doi.org/10.1016/j.jsams. 2011 . 11 .257 PMID: 22197067
40. Aagaard P , Simonsen EB , Magnusson SP , Larsson B , Dyhre-Poulsen P ( 1998 ) A new concept for isokinetic hamstring: quadriceps muscle strength ratio . Am J Sports Med 26 ( 2 ): 231 ± 237 . https://doi.org/10. 1177/03635465980260021201 PMID: 9548116