ABSTRACT
Aim.The
present study was carried out to
examine the relationships between muscular strength, in terms of knee
extension at two different angular velocities and kicking performance
among soccer players of different skill levels.
Methods. Twenty four amateur soccer players (aged 21.5±0.7 years, body mass: 79.08 kg, height: 179.04cm) constituted the experimental group.They were divided into 2 groups, the superior group (n=12), the average group (n=12), and kicked the ball with maximal effort towards a target 15 m. away. Maximal ball velocity was recorded using a speed gun device and maximal isokinetic and concentric muscular strength was measured in terms of motion of the knee extensors using an isokinetic dynamometer. Results. The results showed that the skilled group presented more powerful kicks than the untrained and demonstrated significantly greater peak torque values at concentric knee extension strength at both angular velocities. It was found that both groups presented positive correlation between peak torque values and kick velocity score. Also, the untrained group revealed greater correlation over the two knee extension conditions with the kick velocity. Conclusions. Soccer trained players have more powerful instep kicks than untrained players and better knee extension concentric strength at slow and fast angular velocities. Possible explanations for the differences between the two groups could be the fact that skilled players are more experienced and adopt a different neuromuscular coordination compared to untrained ones. It is obvious that strength is not the only reason that allows players to perform powerful kicks as the untrained group presented better correlations (kicking score with knee extension peak torque) than the skilled group. On the other hand, trained soccer players have better kicking performance due to the fact that they use specific training methods (such as kicks) in their daily training course and increase this ability but not their isokinetic strength. Key words: soccer, strength, kicking, skilled

Kicking is a
very
important factor in soccer and one of the most fundamentally used
skills. It should be practiced from the early ages (Bloomfield et
al..1979). It is the most widely studied soccer skill as many
researches have examined kinematics, kinetics and EMG variables. It
is a multijoint movement which depends on various factors such as the
maximum strength and power of the muscles of the kicking and
supporting leg that take part during the kicking action ((De Proft et
al., 1988a, b; Isokawa & Lees, 1988; Weineck, 1992; Lees &
Nolan, 1998; Dorge et al., 1999), the angle that the player
approaches the ball and the speed that he has before the impact phase
(Isokawa & Lees, 1988; Opavsky, 1988). It
also, depends on the coordination between
the agonist muscles (vastus lateralis and medialis, rectus femoris,
tibialis anterior and m. iliopsoas) and the antagonists (gluteus
maximus, biceps femoris and semitendinosus) during the kick (De Proft
et al., 1988a, b; Isokawa & Lees, 1988; Lees & Nolan, 1998;
Dorge et al., 1999).
During the game, muscle strength is
required,
because each player performs such dynamic
movements as kicks, headers, tackling, and sprinting. So, power,
endurance
and muscle strength are needed to compete in the game (Cabri et al.,
1988; Bangsbo, 1994). The relation between
muscle strength and performance in the field is a subject of
controversy in many research fields dealing with muscle power. Some
articles have demonstrated positive correlations between force,
measured in the laboratory and performance in the field. Perrine and
Everton (1975) found high correlations between isokinetic strength
(at a speed of: 6.29 rad/sec) and vertical jump and demonstrated
improvements after fast isokinetic leg thrust exercises. Kanehisha
and Myashita (1983) found significant relations between peak force
(3.65 rad/sec) and 50m sprint velocity. Oberg et al. (1986) and Cabri
et al. (1988) found that in general soccer players are stronger than
nonsoccer players and they perform more powerful instep kicks.
Many researchers have mentioned that the coordination between the agonist muscles (vastus lateralis and medialis, rectus femoris, tibialis anterior and m.iliopsoas) and the antagonists (gluteus maximus, biceps femoris and semitendinosus) during the kick and the maximum power of the lower limbs are very important factors for a strong instep kick (De Proft et al., 1988a, b; Isokawa & Lees, 1988; Lees & Nolan, 1998; Dorge et al., 1999). Also, studies with ball velocity and kicking distance (Cabri et al. 1988; Isokawa et al., 1988) have revealed that lower limb and/or toe velocity (Luhtanen et al.1988) and maximal strength of the knee extensor muscle (McLean et al., 1993; Mognoni et al. 1994) are important determinants of kick performance and muscle strength is directly responsible for increasing the speed of the foot.
The purpose of this study was to examine the relation between knee extension strength muscles and kicking performance at two different angular velocities and to investigate if trained soccer players have better relations than untrained subjects. A slow at 90° and a faster (more related to kicking) at 240° angular velocities were chosen according to Masuda et al 2004.
Methods
Participants
n 
24 
age 
21.5±0.7 years 
Body mass 
79.08 kg 
height 
179.04 cm. 
Procedures
Kick performance test. To evaluate kicking performance, subjects kicked the ball with maximal effort. Ball velocity was measured using a speed gun device (PSK Professional, Toa Sports Machine Co., Japan), located 1.22 m above the ground, behind the goal. The participants performed 10 full instep kicks in order to record their best kick. The approach to the stationary ball was free at 15m distance from the goal. The sports radargun was put back from the goal at 1,22m from the ground (Masuda et. al., 2004). All participants were asked to give their maximal effort.
Muscular strength test. Isokinetic concentric peak torque of the dominant leg was measured using an isokinetic dynamometer machine (cybex norm) at two different angular velocities, to find their knee extension concentric strength. The first angular velocity was slow, at 90° (1.57 rad/sec) and the second was faster at 240° (4.19 rad/sec). The number of repetitions was 3 for lower angular velocity movements and 5 for faster angular velocity movements. All the participants practiced isokinetic movements in order to familiarize themselves with the test protocol. Their best values were chosen for further statistical analysis.
Statistics
The KolmogorovSmirnov test of normality revealed that none of the studied variables required logarithmic transformation. Calculations were performed with a SPSS/PC 16 statistical package including mean, standard deviation of the mean and Student's ttest for independent samples. Spearman correlation coefficient was used to show the relationships and to quantify possible relationships between kick velocity release and knee extensors torque at the two angular velocities in the two groups. The level of statistical significance was set at P<0.05.
Results
The skilled group presented more powerful kicks than the untrained group. Those results agree with Cabri et al. (1988), who found a significant difference in kicking performance between soccer and nonsoccer players. Table 2, presents the descriptive statistics for this parameter at which was found a statistically significant difference between the two groups t (22) =7,420, p<0.001.

group 
N 
Mean 
Std. Deviation 
Std. Error Mean 
tvalue 
pvalue 
kick velocity 
untrained 
12 
85,9 
6,7 
1,9402 
7,420 
0,000 *** 
skilled 
12 
106,0 
6,5 
1,8870 
Table 2. Descriptive statistics for the parameter kick velocity in the two groups (untrained, skilled). Statistically significant differences between groups are presented with red font.
Also, the skilled group demonstrated significantly greater peak torque values at concentric knee extension strength at both examined velocities (90 or 240 rad/sec). In the table 3, are presented the descriptive statistics for the knee extensor torque values at both velocities in the two groups with the t and p values.

category 
N 
Mean 
Std. Deviation 
Std. Error Mean 
tvalue 
pvalue 
extensor 90 
untrained 
12 
177,000 
10,3221 
2,9797 
4,134 
0,000*** 
skilled 
12 
211,417 
26,9291 
7,7737 

extensor 240 
untrained 
12 
127,000 
9,6389 
2,7825 
2,687 
0,013 * 
skilled 
12 
145,583 
21,9315 
6,3311 
Table 3. Descriptive statistics and tvalue for the knee extensor peak torque at two conditions (90/240 rad/sec) in the two groups (untrained, skilled). Statistical significant differences between groups are presented with red font.
Regarding the correlation between the kick velocity score and the knee extensors peak torque values at the two velocities (90 and 240 rad/sec) in the two groups it was found that: first of all, there was a positive correlation between the peak torque values at 90rad/sec velocity condition and 240rad/sec overall in the groups as it is presented in the table 4 below. The untrained group revealed greater correlation over the two knee extension conditions with the kick velocity as it is presented in the tables below (5,6) compared to skilled ones.
Correlations 




extensor_90 
extensor_240 
Kendall's tau_b 
extensor_90 
Correlation Coefficient 
1,000 
,767^{**} 
Sig. (2tailed) 
. 
,000 

N 
24 
24 

extensor_240 
Correlation Coefficient 
,767^{**} 
1,000 

Sig. (2tailed) 
,000 
. 

N 
24 
24 

Spearman's rho 
extensor_90 
Correlation Coefficient 
1,000 
,904^{**} 
Sig. (2tailed) 
. 
,000 

N 
24 
24 

extensor_240 
Correlation Coefficient 
,904^{**} 
1,000 

Sig. (2tailed) 
,000 
. 

N 
24 
24 

**. Correlation is significant at the 0.01 level (2tailed). 





Table 5 
Table 6 
Tables 5,6. Correlations between the two condition velocities (90 and 240rad/sec) and kick velocity in the two groups. Statistically significant differences between groups are presented with red font.
Discussion
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