Physical Training Oct 2008

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Savvas Lazaridis
16 Giannakopoulou
56121 Ampelokipi
Thessaloniki
Hellas
Tel: (+30) 2310 720843
E-mail: sav200m@hotmail.com

Introduction

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 non-soccer 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 radar-gun 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 Kolmogorov-Smirnov 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 t-test 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 non-soccer 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	t-value	p-value
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	t-value	p-value
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 t-value 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. (2-tailed)	.	,000
		N	24	24
	extensor_240	Correlation Coefficient	,767**	1,000
		Sig. (2-tailed)	,000	.
		N	24	24
Spearman's rho	extensor_90	Correlation Coefficient	1,000	,904**
		Sig. (2-tailed)	.	,000
		N	24	24
	extensor_240	Correlation Coefficient	,904**	1,000
		Sig. (2-tailed)	,000	.
		N	24	24
**. Correlation is significant at the 0.01 level (2-tailed).

Untrained (12) Kick velocity Correlation coefficient p-value Knee extensor_90 0,746 0,015** Knee extensor_240 0,624 0,030*	Skilled (12) Kick velocity Correlation coefficient p-value Knee extensor_90 0,541 0,069 Knee extensor_240 0,262 0,411
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

The current study revealed that trained soccer players presented higher levels of strength regarding kicking performance compared to untrained individuals. Both groups presented high values of correlation at the slower angular velocity (240ْ) which is reasonable, as kicking movement is a multijoint movement which depends on many joints, not only the knee joint (De Proft et al., 1988a, b; Isokawa & Lees, 1988). But untrained subjects seemed to have a better correlation on kicking performance and knee extensors muscle torque because the high kicking performance of trained players not only depends on strength parameters but also on coordination and technical parameters.

As is already known, professional soccer players should present high levels of strength and electromyography activation of their lower extremity muscles in order to perform a strong and powerful kick. But, this does not translate to the fact that muscle strength or torque is the main factor which affects a soccer kick, as kicking movement is a multijoint movement. For this reason, a trainer or a coach should use in his daily training program, a variety of training methods and specific soccer exercises in order to improve, apart from muscle strength, coordination and many technical kicking  parameters.

References

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