Physical Training Aug 2005
 
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Bilateral 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
Thessaloniki, Greece

1Correspondence:
Dr. Galazoulas Christos,
Department of Physical Education and Sports Science,
Aristotle University of Thessaloniki,
Thessaloniki 540 06, Greece.
Tel. +32310 992233
 Fax: +32310 992232
e-mail: galaz@phed.auth.gr

Abstract

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 extremities.

Keywords: Isokinetic strength, muscular balance, hamstring to quadriceps ratio, soccer players 

 INTRODUCTION

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 performance. 

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 conditioning.

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 strength.

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.

METHODS

Participants
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 respectively.

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 selected.

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. (2002).

Statistical analysis
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.

RESULTS

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 tested. 

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 group.  

Table 1 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).
Table 1

Table 2 Comparisons of hamstring to quadriceps ratio on both legs for each angular velocity by Nm data in elite and amateur soccer players (% means ±SD).
table 2

Figure 1 Extensors and flexors peak torques (Nm) at velocities 12, 60, 180 and 300os-1 in right leg in elite and amateur soccer players (means ±SD).
Figure 1 right leg
 
Figure 2 Extensors and flexors peak torques (Nm) at velocities 12, 60, 180 and 300os-1 in left leg in elite and amateur soccer players (means ±SD).
 Figure 2 left leg

 DISCUSSION

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. 2002). 

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.

CONCLUSION

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.


 REFERENCES

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Physical Training Aug 2005