Peak Torque of the Knee Extensor and Flexor Muscles in Elite and
Amateur Male Soccer Players
Zakas Athanasios, Galazoulas Christos1,
Doganis George, Zakas Nikolaos
Aristotle University of Thessaloniki
Department of Physical Education and Sports Sciences
Dr. Galazoulas Christos,
Department of Physical Education and Sports Science,
Aristotle University of Thessaloniki,
Thessaloniki 540 06, Greece.
Tel. +32310 992233
Fax: +32310 992232
Participation in intensive sports leads to muscular specializations
that may generate alterations in involved articular forces. The purpose
of this study was to examine the peak torque and strength balance in
the extensor and flexor muscle groups, as well as the hamstring to
quadriceps (H/Q) ratios on the right and left limbs in soccer players
with different conditioning levels. Fifteen elite soccer players of the
first division and thirteen amateur soccer players participated in this
study. All players had dominance to the right leg and systematic soccer
activities for about 15 years. Maximum voluntary concentric torque of
the hamstring and quadriceps muscles of both legs was assessed using a
Cybex II isokinetic dynamometer at angular velocities 12, 60, 180 and
300o.s-1. No significant differences in peak
torque and strength balance were found between the soccer players
groups in the extensor and flexor muscles in right or left leg.
Furthermore, no differences were recorded in the H/Q ratios between
right and left legs for any of the subject groups. These findings
suggest that participating in soccer training and matches for several
years may produce strength-specific gains that are equal in maximal
strength and muscular balance between both sides of the lower
Keywords: Isokinetic strength, muscular balance, hamstring to
quadriceps ratio, soccer players
Soccer is one of the most popular sports in the world, since it
combines several combinations of movements that require strenuous
effort, such as sudden feints, stops, starts, duels, sprints, jumps and
kicking (Reilly and Thomas 1976). These efforts depend on the maximal
strength of the neuromuscular system, particularly of the lower
extremities (Cometti et al. 2001).
The musculature around the knee is important in the prevention of
injuries, as well as in the enhancement of the knee function. Training
and sports practice may result in the development of a specific
muscular model according to the modality practiced (Scranton et al.
1985). These muscular specializations may initiate an imbalance of the
forces acting statically and dynamically on the joints and may lead to
alterations of articular mechanics (Siqueira et al. 2002).
The quadriceps and hamstring muscle groups are the most frequently
injured muscle groups during a soccer match, often causing prolonged
absence from training (Muckle 1981). There are several intrinsic and
extrinsic factors that contribute to these injuries. Some of these are
related to muscle strength imbalances. Over-emphasis on one-side
activities, such as kicking, may lead to asymmetry and dominance of one
leg, which in turn may cause greater than normal differences in
strength between contra lateral muscle groups (Brady et al. 1993).
Thus, it has been argued that the weaker side is the one most liable to
injury (Reilly 1996). An unfavorable difference between agonist and
antagonist muscle groups is considered to leave the weaker muscle group
at a disadvantage. Hypertrophy of the quadriceps at the expense of the
hamstring may cause hamstring injuries (Reilly 1996). A high degree of
skill in using both feet improves the ability to carry out soccer motor
The muscular strength between dominant and non-dominant legs of
footballers, healthy untrained individuals, as well as athletes of
various other sports, has been the point of research that has
highlighted contradictory results. Several researchers have reported
symmetry between the dominant and non-dominant legs (Brady et al. 1993,
Rosene et al. 2001, & Siqueira 2002), whereas others suggest the
existence of a significant asymmetry (Molnar and Alexander 1974, Goslin
and Charteris 1979, Wyatt and Edwards 1981). These contradictory
results may be due to different definitions of the word “dominance” by
various authors. Some researchers define it, as the leg preferred for
kicking (Wyatt and Edwards 1981), others describe it as the stronger
limb (Goslin and Charteris 1977), and others do not specify how
dominance was determined (Molnar and Alexander 1974).
Hamstring to quadriceps (H/Q) ratios have been used to assess knee
functional ability and muscle balance (Li et al. 1996). Cambell and
Glenn (1982) suggested the flexor/extensor knee ratio as a better
assessment tool for muscle function, than absolute strength. This ratio
has conventionally been expressed as concentric-hamstring-to-quadriceps
strength (Lund-Hanssen et al. 1996), and more recently as
eccentric-hamstring-to-concentric-quadriceps strength (Aagaard et al.
1998). H/Q ratios have been thoroughly assessed on both right and left
legs of untrained females and males (Holmes and Alderink 1984) at
different age groups (Gilliam et al. 1979) among athletes (Brady et al.
1993, Rosene et al. 2001, Sigueira et al. 2002, Zakas et al. 2002) and
in rehabilitation settings (Lund-Hanssen et al. 1996). However, to date
there has been no research regarding the concentric peak torque of
extensors and flexor muscle groups and concentric H/Q ratios in both
legs in athletes of the same sport with different levels of
Given that training and sports practice result in the development of a
specific musculature according to the modality practiced (Scranton et
al. 1985), it would be useful to study whether the participation in
soccer practice and the playing of matches for several years produces
specificity in the muscular strength and the H/Q ratios between
bilateral muscle groups of lower extremities in soccer players with
different level of conditioning, since it has been reported that
different demands in various sports produce differences in muscle
The aim of the current study was therefore to evaluate, through
isokinetic tests, the influence of soccer practice and playing matches
on the peak torque of the knee flexor and extensor muscles as well as
the H/Q ratios on the right and left limbs in elite and amateur level
soccer players respectively.
Fifteen elite male soccer players of a first division and thirteen
amateur soccer players volunteered to participate in the study. As
reported by the respective coaches and participants, only the right leg
was used in the soccer related skills. Substitute players were excluded
from the study. The mean ±SD age, height, body mass and
years of training of the elite players were 25.8±3.5 years,
height 178.8±4.7 cm, body mass 75.1±5.2 kg and
15.0±2.7 years training respectively. Whereas the amateur
players’ values were 25.6±3.5 years, height 177.7±5.5 cm,
body mass 74.9±5.6 kg and 14.8±2.5 years training
The frequency of the training programme was somewhat different between
both groups. Elite soccer players trained 9 times per week, whereas,
the amateur players trained only 3 or 4 times per week. However,
training duration times between the groups were similar (approximately
2 hours per day). During the competition season the participants
refrained from participating in muscle strength programmes using free
weights or other resistance training machines. However, in pre-season,
all participants underwent soccer-specific strength training
programmes. The training and competition season lasted 11 months for
the elite soccer players and 10 months for the amateur soccer players
respectively. Muscle strength measurements were performed one month
before the end of the competition season in both groups.
All participants had no history of knee surgery, hip or ankle lesions,
or any known pathology that interfered with their motor function.
Furthermore, none were taking any medications at the time of the study
with known musculoskeletal side effects. Thus, the participants did not
show any conditions that could be aggravated by the testing protocol or
confound the test results. The participants signed an informed consent
form in accordance to the University guidelines for research involving
human subjects. They were asked to refrain from any unusual activities
or vigorous exercise 24 hours before each testing session.
Isokinetic torque measurement
Concentric peak torque of the quadriceps and the hamstring muscles was
measured for both legs using an isokinetic dynamometer (Cybex II, Lumex
Inc, Ronkonkoma 11779 NY) at angular velocities of 12, 60, 180 and 300o.s-1
respectively. All tests were performed from a seated position. The
participants were sitting on the chair of the dynamometer, with
stabilization straps on the trunk, thigh and tibia in order to prevent
extraneous joint movement. The tested knee was positioned to 90o
of flexion (0o=fully extended knee), to align the axis of
the dynamometer arm with the distal point of the lateral femoral
condyle. Participants were instructed to kick and bend the leg as hard
and as fast as possible through a complete range of motion. Verbal
encouragement was given during every trial. Furthermore, participants
were instructed to work as hard as possible in both directions of the
movement. In addition, participants were instructed to hold their arms
comfortably across their chest to further isolate knee joint flexion
and extension movement.
The testing protocol consisted of three separate maximal concentric
efforts of the knee extensors and flexors at each of the aforementioned
velocities with a 30-second rest period between each contraction. All 3
measurements at each velocity were completed before the velocity was
changed, and a minimum of 1 minute was allowed to elapse before
measurements were recorded at the next velocity. The protocol was
performed first for the extensor and then for the flexor muscles. Each
trial started from the high angular velocity and proceeded to the lower
velocities. Following the testing of one leg, there was a 5 minute
rest, and then the testing of the other leg began with the same
conditions. The order of testing participants’ legs was randomly
In each angular velocity the best peak torque of the three test
contractions was recorded for data analysis for both extensor and
flexor muscle groups. Torque values from the trials were recorded in Nm
and were corrected for effects of gravity according to Zakas et al.
A mixed within-and between-participants 2x2 ANOVA model with repeated
measures over tests was applied for each dependent variable. The
repeated factor was the leg and this had 2 levels (right and left leg).
The between-participants factor was the group, and that also had two
levels (elite and amateur soccer players). Paired t-test was applied to
determine the significance of the means differences in both legs.
T-tests for independent samples were used to compare the peak torque
values of dependent variables between elite and amateur groups in both
legs. The significance level of p<0.05 was set for all analyses.
The analysis of the main effect revealed no significant differences for
the legs or for the groups, except for angular velocities (extensors:
F(7,182)=298.37, p<0.000, flexors: F(7,182)=177.68, p<0.000),
indicating that the peak torque is influenced by the angular velocity
both quadriceps and hamstring muscle groups in both lower extremities.
Torque values were higher in lower angular velocities and lower in
higher angular velocities. In addition, no significant interactions
were recorded for Velocity x Group, for Leg x Group, for Velocity x Leg
or for Velocity x Leg x Group respectively, which indicates that the
peak torque for any of the muscle groups tested is not affected by the
dominance between groups.
The H/Q ratios revealed no significant main effects. Thus, we observed
no significant interactions for Velocity x Group, for Leg x Group, for
Velocity x Leg or for Velocity x Leg x Group respectively.
A further analysis by a paired t-test used in each group, showed no
difference between right and left legs with regards peak torque values
for any of the muscle groups tested in each angular velocity (Table 1).
The recorded peak torque in both soccer groups is presented in Figure 1
for the right leg and in Figure 2 for the left leg. The statistical
analysis showed no significant differences in the peak torque, either
for knee extensor or the knee flexor muscles, in any angular velocity
Hamstring to quadriceps ratios in right muscles group and the left
muscles group in each angular velocity of elite and amateur soccer
players are presented in (Table 2). No significant differences were
recorded for the H/Q ratios between the two limbs in any participant
Quadriceps and hamstring peak torques (Nm) at velocities 12, 60, 180
and 300os-1 and comparisons between right and
left legs in elite and amateur soccer players (means ±SD).
Comparisons of hamstring to quadriceps ratio on both legs for each
angular velocity by Nm data in elite and amateur soccer players (%
Extensors and flexors peak torques (Nm) at velocities 12, 60, 180 and
in right leg in elite and amateur soccer
players (means ±SD).
Extensors and flexors peak torques (Nm) at velocities 12, 60, 180 and
in left leg in elite and amateur soccer
players (means ±SD).
The findings of this study showed similar values of maximal power in
the extensor and flexor muscles of the right or left lower limb in both
elite and amateur players. This may suggest that the strength
associated with the demands in sport, specifically basketball and
soccer, results in specific training adaptations to muscle strength
(Zakas et al. 1995). Thostensson et al. (1977) found differences in
muscular power among alpine skiers, jumpers and sprinters. Zakas et al.
(1995) found that the flexor and extensor muscles were more powerful in
basketball players at angular speeds of 60 and 180os-1
in comparison to soccer players. Nevertheless, in the same sport there
were not many differences in the maximal strength of the flexor and the
extensor muscles of the knee when they were compared separately in
elite and amateur soccer players (I to IV), or in basketball players in
the same division (Zakas et al. 1995).
During soccer training and game scenarios, participants were required
to undertake two dynamic efforts on the dynamometer, which according to
Bangsbo (1994) requires a good level of muscular power and strength. It
therefore seems that soccer training and playing games for many years
can contribute to improvements in the muscular strength of both the
flexor and extensor muscles of the lower limbs. Similar findings were
previously reported by Oberg et al. (1986) and they postulated that
duration of soccer training applied for many years helped to improve
the strength of the extensor muscles. Thus, in this study where
participants had trained and played for 15 years results tended to
agree with this hypothesis.
The findings of this study support the lack of asymmetry between the
right and the left limb for each muscle group examined. The findings
also comply with the previous findings of several other recent studies
(Brady et al. 1993, Holmes and Alderink 1984, Gur et al. 1999, Rosene
et al. 2001, and Siqueira et al. 2002). However, research findings in
this study contradict those earlier studies that supported the presence
of asymmetry (Molnar and Alexander 1974, Goslin and Charteris 1979,
Wyatt et al. 1981). A possible reason for this may be the
misunderstanding of the word “dominance”. For example, Wyatt and
Edwards (1981) defined dominance as the leg preferred for kicking,
whereas Goslin and Charteris (1979) defined dominance as the most
powerful member, while Molnar and Alexander (1974) failed to mention
what dominance consisted of.
The muscular symmetry that has been registered for the flexors and the
extensors of the knee in the present study might be the result of
specific soccer loads placed on the lower extremities. It seems that
players involved in soccer for several years have adapted to the
specific soccer loads and are able to maintain similar strength on both
body sides. During soccer training and matches, players perform various
explosive-type efforts, such as starts, sprint, and jump, duels and
feints. For these types of actions both parts of the body were
active. In addition, when shooting, the non-dominant limb has a
support function, while the dominant leg has a propulsion function.
These efforts depend on the maximal strength of the neuromuscular
system and place more emphasis on the lower extremities (Cometti et al.
2001). Although we did not examine the involved mechanisms, Oda (1997)
suggested that a common drive from the central nervous system towards
the right and left muscles act during bilateral contractions. Thus,
bilateral strength deficit is due to the decreased neural activations
of the pre-central motor cortex of both hemispheres.
The muscular symmetry that was registered in this work both in the
flexor and extensor muscles of the knee might be due to the power
training which took place during the preparation of the players in the
pre-competition period. Bangsbo (1994) suggested that one of the most
important aims of the training programmes during the pre-competitive
period is soccer-specific strength. The latter can be improved with
soccer training (Cabri et al. 1988, Bangsbo 1994) and with programmes
of muscular training. Soccer players that participated in this study
did programmes of muscular power during the pre-competitive period.
Nevertheless, their total duration did not exceed the 2 weeks for both
groups and more emphasis was given to the muscular endurance and not to
the maximum muscular power.
The H/Q ratios present symmetry on both limbs of elite and amateur
soccer players. This is in agreement to a study of high school-aged
students (Holmes and Alderink 1984), ball game athletes (Rosene et al.
2001) as well as jumper and runner athletes (Siqueira et al.
In this work, the H/Q ratio was maintained in the same levels, even if
there was an increase in the angular speed. The scientific bibliography
offers plenty of studies, which offer contradictory results with,
regard the H/Q analogy. The H/Q analogy does not seem to be influenced
by the angular speed and is maintained in similar levels in both the
low and the high angular speeds, when gravity is considered during the
isokinetic tests (Zakas et al. 2002, Fillyaw et al. 1986). On the
contrary, the H/Q analogy is significantly higher in the higher angular
speeds, when gravity is not taken into consideration (Zakas et al.
2002, Fillyaw et al. 1986).
According to Moffroid and his associates (1969), the peak torque of the
quadriceps is approximately twice that of the hamstring muscles, due to
a greater muscular mass and irrespective of the angular velocity.
Similar findings accrue from the present study. Zakas et al. (2002)
reported similar H/Q ratios on the same angular velocities of elite
Greek basketball, volleyball and soccer players.
The findings regarding the relationship between the muscular power and
lesions in the hamstrings are contradictory. The inequality or the
asymmetry in the power of the hamstrings between the right and the left
limb can predispose individuals to lesions in the most weak muscular
group (Brady et al. 1993). Previous research indicates that a lack in
the symmetry of the hamstrings in isometric power of more than 10% was
predictive of hamstring injury (Burkett 1970). On the contrary, Bennell
and his associates (1998) considered imbalances in side-to-side
hamstring strength greater than 10% or H/Q ratios fewer than 60% on
either leg did not place the player at greater risk for subsequent
hamstring injury. Stafford and Grana (1984) reported that bilateral H/Q
ratios need to be compared to each speed in addition to the bilateral
comparison of strength, for when bilateral torques are within the
normal comparison limits of 90%, the ratio of the two muscle groups is
not necessarily within the normal bounderies for the two limbs.
In athletes the H/Q could be a sensitive marker of the functional
capacity and readiness to return to competition. Consequently the H/Q
ratio of the contrary limb can be used as a score to recovery (from
lesions). However, the opposite limb ratio should only be used if it is
not injured, because otherwise it may result in misleading H/Q ratios
(Kannus 1988). When asymmetry exists in the muscular power between the
two limbs or in the H/Q ratios, it is suggested to consider
rehabilitation and muscle invigoration for the balance of muscular
strength between the two limbs. Although in soccer players preseason
assessments of strength and the assessment of the H/Q ratios usually
take place during the preparation season, such data is doubtful since
soccer players usually avoid performing maximal efforts during that
time, in order to avoid possible injury.
According to the results of the present study, elite and amateur soccer
players do not appear to encounter muscle strength asymmetry on the
knee extensor and flexor muscles or differences on muscle strength in
these muscle groups in both legs. Thus, participation in soccer
training and matches for several years may play a critical role that
helps to develop a level of maximum strength and power. The H/Q ratio
of these muscle groups does not imply the presence of an asymmetry
between the two body sides either. However, since all experimental
tests have limitations, coaches are advised to design personalized
strength programmes for players with bilateral asymmetry on the knee
extensor or flexor muscles. In case of asymmetry, proper rehabilitation
should be incorporated to the strength programme.
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