ISOKINETIC STRENGTH OF KNEE FLEXORS AND EXTENSORS OF ADOLESCENT SOCCER PLAYERS AND ITS CHANGES BASED ON MOVEMENT SPEED AND AGE

It is generally accepted that during childhood and adolescence muscle strength changes take place. This is, however, a complicated problem, since these changes can progress diff erently in terms of inter-individual differences (maturation, body size, etc.), muscle groups, types of muscle action, sex specifi city, etc. Body height and the lever armʼs role in muscle strength play a key role in strength development; however, some changes during development are independent of body size and are probably caused by the development of the nervous system. Understanding this fi eld is also infl uenced by nuances in the applied testing procedures (Bar-Or, 1996; De Ste Croix, 2007; Faigenbaum, 2008; Forbes, Bullers et al., 2009). Isokinetic dynamometry is considered an objective and reliable diagnostic tool. Its foremost advantages are high reliability, standardization, the possibility to identify the strength level in the entire extent of the mo vement and determine in this way positions, at which the best and the worst value of PT is reached (Baltzopoulos & Brodie, 1989; Dvir, 2004). Isokinetic strength measurements are numerous in the literature; however, fewer pieces of work have been focused on the eva luated changes of the isokinetic strength of the lower extremities in children and youth (Bellew & Malone, 2000; Weir, 2000). The questions of developmental changes in isokinetic strength were summarized in the overview study by De Ste Croix, Deighan and Armstrong (2003), who, among other things, note that although some information on the strength relationships between the knee extensors and fl exors for children exist, data on the age trends which aff ect these muscles are limited. The results of these isokinetic measurements also confi rm that the PT values are infl uenced by the speeds used during testing and that, in accordance with the Hill Curve, the PT decreases with an increasing speed of movement (Chan & Maff ulli, 1996). ISOKINETIC STRENGTH OF KNEE FLEXORS AND EXTENSORS OF ADOLESCENT SOCCER PLAYERS AND ITS CHANGES BASED ON MOVEMENT SPEED AND AGE


INTRODUCTION
It is generally accepted that during childhood and adolescence muscle strength changes take place.This is, however, a complicated problem, since these changes can progress diff erently in terms of inter-individual differences (maturation, body size, etc.), muscle groups, types of muscle action, sex specifi city, etc. Body height and the lever armʼs role in muscle strength play a key role in strength development; however, some changes during development are independent of body size and are probably caused by the development of the nervous system.Understanding this fi eld is also infl uenced by nuances in the applied testing procedures (Bar-Or, 1996;De Ste Croix, 2007;Faigenbaum, 2008;Forbes, Bullers et al., 2009).
Isokinetic dynamometry is considered an objective and reliable diagnostic tool.Its foremost advantages are high reliability, standardization, the possibility to identify the strength level in the entire extent of the mo vement and determine in this way positions, at which the best and the worst value of PT is reached (Baltzopoulos & Brodie, 1989;Dvir, 2004).Isokinetic strength measurements are numerous in the literature; however, fewer pieces of work have been focused on the eva luated changes of the isokinetic strength of the lower extremities in children and youth (Bellew & Malone, 2000;Weir, 2000).The questions of developmental changes in isokinetic strength were summarized in the overview study by De Ste Croix, Deighan and Armstrong (2003), who, among other things, note that although some information on the strength relationships between the knee extensors and fl exors for children exist, data on the age trends which aff ect these muscles are limited.The results of these isokinetic measurements also confi rm that the PT values are infl uenced by the speeds used during testing and that, in accordance with the Hill Curve, the PT decreases with an increasing speed of movement (Chan & Maff ulli, 1996).
Isokinetic testing of muscle strength is used for evaluating the eff ects of training programs, especially for identifying muscle weakening and its compensation and for the identifi cation of imbalance and prior injuries (Baltzopoulos & Brodie, 1989;Brown, 2000;Houweling, Head, & Hamzeh, 2009;Perrine, 1993).The num ber of injuries to young players is lower than the number of mature highly trained soccer players, but the treatment time is longer for young players (Hawkins et al., 2001).The number of injuries increases according to increasing age and in the age category from 17 to 20 years injury occurrence is almost the same as in the adult category (Bahr et al., 2008).
The results of case studies concentrating on the strength of young soccer players' lower limbs have confi rmed the rising trend in strength related to chronolo gical age.Results are not definite regarding the de velopmental period, fl exors or respectively, extensors, the contraction type and speed measurement.The study of young soccer players aged from 12 to 18 years (Forbes, Sutcliff e et al., 2009) has confi rmed that the absolute PT increases with the chronological age of the players.The rise of PT was almost linear in the age group from 11 to 14 years, then the major PT growth was in the age group from 15 to 18 years and then, in the age group from 18 to 20 years, the growth was almost linear again.Authors point out that a similar trend has not been proven in tennis players nor in youth without a sport specialization.That is the reason why the authors suppose that soccer training specifi cally has a strong impact on muscle strength development and particularly on lower limb progress.Kellis S., Gerodimos, Kellis E. and Manou (2001) note that age aff ects the strength of the dominant leg (DL) and non-dominant leg (NL) and that in the age growth period young soccer players gain better trained lower limbs and the bilateral diff erence decreases.Gür, Akova, Pündük and Küçükoğlu (1999) monitored a group of players younger than 21 years old and a group of players older than 21 and they found that PT grows with age only in the dominant leg.Authors supposed that the diff erences were present mostly due to training elements rather than age factors.Forbes, Bullers et al. (2009) pointed out that the infl uence of the actual pubertal state compared to the maximal amount of strength is higher than that of age itself.Lehance, Binet, Bury and Croisier (2009) compared the fl exors' and extensors' knee strength of the player groups U17, U21 and adult players at the speed of 60° × s -1 and 240° × s -1 .They found absolute growth of the concentric PT in all cases.Statistically signifi cant diff erences for both speeds were identifi ed only for the concentric extensors' strength.No signifi cant diff erence was confi rmed by expressing PT with regard to body weight.Higher growth in extensors compared to the fl exors could be, according to the authors, caused by a lack of attention to hamstring strengthening.The results of some studies do not correspond with the trend of increasing strength related to chronological age.For example, De Ste Croix, Deighan and Armstrong (2003) have pointed out that, although age has a strong eff ect on strength development, the rate of anatomical growth and maturation vary independently and their eff ects do not correlate to chronological age.
It is clear that the questions about isokinetic knee fl exors' and the extensors' strength of the DL and NL within the diff erent age periods, in diff erent specifi c groups and according to the speed measured are not yet well answered.The goal of this study is to determine the isokinetic knee strength of fl exors and extensors by highly trained adolescent soccer players and to evaluate whether the chronological age and the speed of the movement signifi cantly aff ect the strength of the knee joint fl exors and extensors.

METHODOLOGY
The group of tested soccer players was formed by 45 mem bers of the SK Sigma soccer team (age 17 ± 1.2 years; height 178.4 ± 5.3 cm; weight 68.5 ± 7.6 kg).44 players had DL on their right, 1 player had DL on their left (a dominant lower limb was determined as the one which is preferred by players for kicking the ball).The players were divided into three subgroups according to their age category U16 (n = 16; height 175.5 ± 4.9 cm; weight 63.4 ± 6.9), U17 (n = 14; height 181.9 ± 4.1 cm; weight 73.4 ± 7.2), U18 (n = 15; height 181.0 ± 5.0 cm; weight 69.2 ± 5.7).All tested players, as well as their parents, were fully informed about the goal and methodology of the measurements.They agreed with the testing process and with the use of the data for further research.Players with acute medical problems were excluded from the research.The day before testing, the players were not exposed to any high training pressure.
Just before testing the players went through nonspe cifi c warm up exercises, which included cycling on a bicycle ergometer for 6 minutes with a rising load and 5 minutes of auto-stretching exercises controlled by a physiotherapist.Unilateral strength of the concentric contraction of the fl exors and extensors of the knee was measured with an isokinetic dynamometer ISOMED 2000 (D. & R. Ferstl GmbH, Hemau, Germany).Players were tested in the sitting position and they held handgrips alongside the seat, the seat back was reclined by 15 grades.For fi xation in the area of the pelvis and thigh fi xed belts were used, shoulders were fi xed by a shoulder rest in the ventral-dorsal and cranialcaudal direction.The axis of the dynamometer rotation was the same as the axis of the knee rotation (lateral femoral condyle).The arm of the dynamometer lever was fi xed to the distal part of the lower leg and the lower edge of the training pad was placed 2.5 cm over the medial apex malleolus.The seat settings were stored in PC memory before measuring the right leg and were automatically activated in the process of measuring the left leg.The angular speed parameters 60° × s -1 , 180° × s -1 and 360° × s -1 were used for measurement and the gravitational correction was activated as well.The testing pro tocol was made up of two contraction sets in each of the measured speed values.In the fi rst -the warm up set -players made fi ve reciprocal contractions (concentric contraction to fl exion, followed by concentric contraction to the extension) with a progressive raise in the contraction strength of the player until the maximal contraction strength was achieved.After a 30 seconds rest period, a set of six repetitions with the maximal con traction strength followed.The rest time during the measurements at particular speeds was 1 minute and the rest time between the measurement of the right and left leg was 3 minutes.During the testing process, the players were motivated to reach the best results and they were provided with concurrent visual feed back in the form of an isokinetic strength curve displayed on the dynamometer monitor.The result of the measurement was the absolute PT (Nm) in the process of concentric fl exion, respectively extension in the knee joint.
The two way ANOVA was used to determine the signifi cance of the PT diff erences between groups (p < 0.05).The statistical signifi cance of the diff erences between age categories was verifi ed by the post hoc -LSD test.Eff ect size was assessed by the coeffi cient "Eta square" (0.01-0.05 low eff ect; 0.06-0.13middle eff ect; > 0.14 large eff ect); (Cohen, 1988).Statistical analysis was per-formed using the data analysis software system STATIS-TICA, version 10.

RESULTS
Isokinetic muscle strength DL and NL values at va rious speeds by the entire tested group of soccer players for given age categories are noted in TABLE 1  and TABLE 2. As expected, the tested players reached higher values for extensors than fl exors at all speeds.The highest values were measured at a speed of 60° × s -1 for all players and for all age categories.Evaluation of iso kinetic muscle strength at three angular speeds for the entire group confi rmed that PT was signifi cantly lo wer for DL and NL with an increasing speed for both flexion and extension.The value of the effect for these diff erences was high [DL: F(2.84) = 99.80;p < 0.001; η 2 = 0.703, resp.F(2.84) = 327.35;p < 0.001; η 2 = 0.886]; [NL: F(2.84) = 91.26;p < 0.001; η 2 = 0.684; resp.F(2.84) = 267.27;p < 0.001; η 2 = 0.864].Detailed comparison of the average values reached at particular speeds with the help of the LSD post -hoc test confi rmed statistically signifi cant diff erences between all speeds used (not mentioned in the enclosed tables).
ANOVA results (TABLES 3-5) show statistically sig nifi cant diff erences in muscle strength (without consideration for lateral preferences) among age categories for fl exors as well as extensors at the speed of 60° × s -1 and 180° × s -1 .The size of these diff erences was high.At the speed of 360° × s -1 important diff erences were not confi rmed.The LSD post -hoc test, on the other hand, confi rm ed signifi cant diff erences only in some cases between categories.For the speed of 60° × s -1 by DL between the categories of U16 and U 18, in fl exion (p = 0.005), between the categories of U16 and U17, U16 and U18 in extension (p < 0.001, resp.p < 0.001).Between catego ries U17 and U18 no signifi cant diff erences were confi rmed.For NL a signifi cant diff erence was confi rmed for the categories U16 and U18, in the case of fl exion and also extension (p = 0.005, resp.p = 0.019) and between U16 and U17 in extension (p = 0.004).Diff erences between the categories U17 and U18 were not statistically signifi cant.At the speed of 180 × s -1 1 for DL, statistically signifi cant diff erences were confi rmed for the categories of U16 and U17 in extension (p = 0.045) and U16 and U18 in fl exion and extension (p = 0.025, resp.p = 0.048).In NL, statistically signifi cant diff erences were confi rmed only for the U16 and U18 categories (p = 0.044) in fl exion.

DISCUSSION
The knowledge of the muscle strength of the knee joint fl exors and extensors in soccer players is necessary from the point of view of productivity as well as injury prevention (Lehance et al., 2009).The average PT for the players from our group was at the speed of 60° × s -1 for DL for fl exion 123.3 Nm, for extension 212.9 Nm, for NL 120.6 Nm, resp.209.7 Nm.When we compare the average PT values of our group of players with those values noted by Lehance et al. (2009) during fl exion, resp.DL extension, with Belgian players in the catego ry U17 (128 Nm, resp.194 Nm) we can say that both groups of players reached similar results.
In comparison with values measured by Forbes et al. (2009) with 18 year-old British soccer players at the speed of 60° × s -1 , the players from our group were substantially stronger (DL 99.7 Nm,resp. 181.8 Nm,NL 94,5 Nm,resp. 182.7 Nm).We can also compare results from our tested group with the results of the group from the same club, but in the category U19 (Botek et al., 2010).The older players were substantially stronger (DL 165.36 Nm versus 233.64 Nm;NL 169.86 Nm versus 243.93 Nm) and even if we consider the higher average weight of the older group (M = 73.4± 4.9) we assume that these diff erences could not have been caused only by age, but also by the goals and content of the training process.
A further fi nding of our study was that PT values in fl exion and extension for DL and also NL were signifi cantly lower with increasing speed.This trend was also confi rmed by Dvir and David (1996); Lehance et al. (2009); Malý, Zahálka and Malá (2010); Kellis et al. (2001).These authors explain this decrease by the fact, that, at high speed levels, muscle contraction is accomplished only by a fraction of the muscle fi ber, mostly that muscle fi ber, which ismorphologically adapted to high speed work.Another reason mentioned is the fact, that for making such a movement at such a high speed, passive components, i.e., the non-contractile muscle elements, participate to a greater extent.Iga, George, Lees and Reilly (2009).With the use of isokinetic dynamometry 15 year old soccer players compared favorably with their contemporaries who were not regularly involv ed in sport activities.For this purpose they were measured at speeds of 60° × s -1 and 250° × s -1 .While the diff erence in PT at the speed of 60° × s -1 was only from 15 to 18%, at the speed of 250° × s -1 soccer players were signifi cantly stronger by 25 to 37%.The results show that training preparation and play performance itself in soccer require the development of muscle strength, particularly at the higher speed levels, which is not yet well developed in adolescents from the common population.
We confi rmed by observing the group of players in our study that the age factor signifi cantly infl uenced the isokinetic strength level.A detailed comparison of the PT with DL as well as NL between the categories nearest to each other, with the help of the LSD post-hoc test, confi rmed statistically important diff erences in PT at the speed of 60° × s -1 only between categories U16 and U17 in the case of extension.Diff erences in the case of fl exion were "on the borderline of statistical signifi cance" (p = 0.062).The diff erence between the categories U17 and U18 were statistically insignifi cant in fl exion as well as extension.In the case of extension the diff erences were on the borderline of statistical significance (p = 0.054).A more detailed analysis shows that statistically signifi cant diff erences in muscle strength between age categoriesʼ fl exors and extensors at the speed of 60° × s -1 and 180° × s -1 were influenced first and fo remost by a more pronounced trend of the strength increase between the U16 and U17 categories.On the other hand for the U17 and U18 categories at the speed of 60° × s -1 for the DL extension we noted PT stagnation and for the NL even a decrease (Fig. 1).This fi nding suggests that within the oldest category monitored, suffi cient attention is not given to strength training.With regard to the importance of strength for accomplishing specifi c movements in current soccer (Verheijen, 1998) we consider this fi nding important.This detailed retrospective data analysis of the training process showed us that the stagnation of strength in the U18 category was not determined by the limit of muscle strength development, but by mistakes in strength training.
Inter-individual data analysis related to his or her me dical history furthermore helped us to determine that the imbalance of the fl exor and extensor knee joint strength in the case of some of the observed players can be the key factor for the development of repeated micro traumas to the knee fl exors.This misbalance in the isokinetic dynamometry can be best expressed by the examination of the fl exor to the extensor strength ratio -the so called H/Q ratio (Houweling et al., 2009;Lehance et al., 2009;Sangnier & Tourney-Chollet, 2007).Diff erences in the strength between the U16 and U18 groups are in accordance with the results of other studies (Forbes et al., 2009).At the highest speed of 360°•s -1 a statistically signifi cant diff erence was not confi rmed in any case.During the result interpretation in this speed it is also necessary to take into consi deration problems related to the validity and reliability of the isokinetic test ing at high speeds (Dvir, 2004).
The growth of the PT connected to the young soccer player's chronological age growth is also mentioned in the results of the Forbes, Bullers et al. (2009) study.In the case of the PTʼs expression relative to the player's weight authors found out that the U18 group of players wasn't signifi cantly stronger than the U16 and U15 group of players.Kellis et al. (2001) measured isokinetic strength at speeds of 60° × s -1 , 120° × s -1 and 180° × s -1 with young Greek players and they determined a significant increase of muscle strength corresponding to the player's age from 10 to 18 years.When we compare aver age PT values during fl exion, respectively, the

CONCLUSIONS
This study of the soccer players in the age group from 16 to 18 years old evaluated the muscle strength preparation by the young highly trained soccer players in the Teen Sport Center SK Sigma Olomouc, CR.The study confi rmed that an increase in the speed of the move ment leads to a decrease in isokinetic strength.It also confi rmed that muscle strength increases with age.In the group of 17 and 18 years old soccer players the diff erences in the PT for fl exors, resp.extensors of DL and NL were not important.In the case of NL extensors we recorded a small decrease in PT, which can indicate a lack in the strength training for the oldest playersʼ group.Results of the study partly deepen the knowledge of the changes in muscle strength, with connection to age, including its production at higher, more specifi c, speeds.They showed the importance of diagnostics during systematic long term sport preparation, specifi cally the importance of strength diagnostics and training in contemporary soccer.

Fig. 1
Fig. 1Muscle strength in connection with age during extension at the speed of 60°•s -1

TABLE 1
Peak torque (Nm) by the fl exion of the knee joint for the dominant and non-dominant leg -basic statistical characteristics (n = 45) Legend: M -average, Med -median, SD -standard deviation, DL -dominant leg, NL -non-dominant leg

TABLE 2
Peak torque (Nm) by the extension of the knee joint for the dominant and non-dominant leg -basic statistics characteristics (n = 45) Legend: M -average, Med -median, SD -standard deviation, DL -dominant leg, NL -non-dominant leg

TABLE 3
Diff erences in the isokinetic strength by fl exion and extension of the knee joint among the age categories at the speed 60°× s -1 (n = 45) Legend: Lat -laterality factor, F -testing criteria level, p -level of statistical signifi cance, η 2 -Eta squareStatistically signifi cant values are in bold characters

TABLE 4
Diff erences in the isokinetic strength by fl exion and extension of the knee joint among the age categories at the speed 180° × s -1 (n = 45)

TABLE 5
Diff erences in the isokinetic strength by fl exion and extension of the knee joint among the age categories for the speed 360° × s -1 (n = 45) Gür et al. (1999)009)the monitored player's group in the U17 category at the speed of 60° × s -1 (125 Nm, resp.226Nm) with the values of the Greek players of the same age (140 Nm, resp.213Nm),we can say that our players lag behind the Greek players in hamstring strength.It can be assumed that this condition can be undesirable, particularly from the point of view of injury prevention.SimilarlyLehance et al. (2009)noted a statistically signifi cant diff erence between Belgian players in the U17 and U21 groups at the speed of 60° × s -1 for the dominant DL.Average results in the U17 playersʼ group were 128 Nm during fl exion and 194 Nm during extension, values not greatly diff erent from the values in the U17 group of our group of players.These authors further state that in the expression of the isokinetic testing results of muscle strength compared to body weight, 17 year old highly trained soccer players have already reached a muscle strength of the fl exor and extensor of the knee very close to their maximal level.From this point of view there are the very interesting results of the De Ste Croix et al. (2001) study.These authors came to the conclusion that age is a non-signifi cantly explanatory variable of isokinetic knee torque once stature and mass are accounted for.Gür et al. (1999)evaluated the diff erence between players up to 21 years of age and the adult category of the Turkish soccer league at the speeds of 30° × s -1 , 180° × s -1 , 240° × s -1 and 300° × s -1 and except for the lowest speed they found a signifi cant diff erence in the strength of the quadriceps and hamstrings only for DL.In comparison, the muscle strength level for Turkish soccer players up to 21 years old for DL was on average 86 Nm for fl exion and 144 Nm for extension.