Journal of Electromyography and Kinesiology 23 (2013) 125–131
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Mechanisms that inﬂuence accuracy of the soccer kick Athanasios Katis a,⇑, Emmanouil Giannadakis a, Theodoros Kannas a, Ioannis Amiridis a, Eleftherios Kellis a, Adrian Lees b a b
Aristotle University of Thessaloniki, Laboratory of Neuromechanics, Department of Physical Education and Sport Sciences of Serres, Serres, Greece Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, Liverpool, UK
a r t i c l e
i n f o
Article history: Received 25 May 2012 Received in revised form 28 August 2012 Accepted 29 August 2012
Keywords: Soccer Kicking accuracy EMG GRFs
a b s t r a c t Goal scoring represents the ultimate purpose of soccer and this is achieved when players perform accurate kicks. The purpose of the present study was to compare accurate and inaccurate soccer kicks aiming to top and bottom targets. Twenty-one soccer players performed consecutive kicks against top and bottom targets (0.5 m2) placed in the center of the goal. The kicking trials were categorized as accurate or inaccurate. The activation of tibialis anterior (TA), rectus femoris (RF), biceps femoris (BF) and gastrocnemius muscle (GAS) of the swinging leg and the ground reaction forces (GRFs) of the support leg were analyzed. The GRFs did not differ between kicking conditions (P > 0.05). There was signiﬁcantly higher TA and BF and lower GAS EMG activity during accurate kicks to the top target (P < 0.05) compared with inaccurate kicks. Furthermore, there was a signiﬁcantly lower TA and RF activation during accurate kicks against the bottom target (P < 0.05) compared with inaccurate kicks. Enhancing muscle activation of the TA and BF and reducing GAS activation may assist players to kick accurately against top targets. In contrast, players who display higher TA and RF activation may be less accurate against a bottom target. It was concluded that muscle activation of the kicking leg represents a signiﬁcant mechanism which largely contributes to soccer kick accuracy. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The instep soccer kick is considered the most powerful of the kicking techniques (Kellis and Katis, 2007a; Lees and Nolan, 1998), but a powerful kick is not always a successful one because accuracy has a bearing on a kick’s success, such as goal scoring. Many factors inﬂuence kicking accuracy and inaccuracy ranging from errors from the players’ approach, support leg placement characteristics, kicking leg swing motions and kicking foot-to-ball contact characteristics. These factors have been extensively examined biomechanically, mainly under laboratory conditions (Asai et al., 2002; Kellis et al., 2006; Katis and Kellis, 2010; Scurr et al., 2011), while only a few were performed under ﬁeld conditions (Giagazoglou et al., 2011). Further, most studies examined biomechanics of powerful soccer kicks (De Proft et al., 1988; Kellis et al., 2006), but not the biomechanics of accurate kicks. The placement of the support leg is of great importance for the performance of a kick, since the support leg is considered responsible for stabilizing the body while the kicking leg swings (Lees et al., 2010). During the kick, the support leg lands next to the ball ⇑ Corresponding author. Address: Monastiriou 114, 53100 Florina, Greece. Tel.: +30 2385046146; fax: +30 2385044655. E-mail address: [email protected]
(A. Katis). 1050-6411/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jelekin.2012.08.020
with the knee ﬂexed to absorb the impact of landing. In this way the speed of the kicking movement is reduced, stabilizing body segments and is thought to have a beneﬁcial effect on kicking performance (Lees et al., 2010). This means that the body may assume different postures depending on the direction of the ball to the target. If this is the case, then differences in ground reaction forces made by the support leg should be expected between kicks which hit the target and those which do not. Previous studies have examined muscle activation patterns during powerful soccer kicking using electromyography (EMG) (Dorge et al., 1999; Kellis et al., 2004; Brophy et al., 2007; Scurr et al., 2011). Only recently, Scurr et al. (2011) examined the EMG activity of the quadriceps muscles when kicking towards different targets. Particularly, they found differences in the EMG activity of kicking limb muscles when kicks for accuracy aimed at different corners of the goal post. Kicks aimed to the top right corner demonstrated a higher level of quadriceps activation compared to those aiming to the other corners. This study focused on quadriceps muscles activation only, and while quadriceps activation is crucial for kicking power, the activation patterns of other muscles is also of great importance as kicking involves simultaneous movement of several segments around many joints. Despite these limitations, it seems that kicking accuracy largely depends on differential activation of the muscles during the kick in combination with the position of the target. This
A. Katis et al. / Journal of Electromyography and Kinesiology 23 (2013) 125–131
might also be related to game conditions, i.e. when a player performs two kicks using essentially the same technique, but one hits the target and the other does not. Players have several options when kicking the ball to goal such as to kick it to the top or bottom of the goal. Surprisingly, the biomechanical adjustments taking place when players perform kicks to the top or bottom of the goal have not been investigated in detail. One study, though, has shown that players lean the body away from the ball (backward body lean) and use a lower contact point on the ball when a player kicks the ball to the top of the goal to enable the ball to follow a higher trajectory after release (Prassas et al., 1990). This suggests that in order to position the kicking foot further under the ball there should be a different support leg placement and different activation of lower limb muscles for kicks to the top of the goal compared to the bottom. Goal scoring represents the ultimate purpose of soccer. This is achieved when players perform accurate kicks. In this respect, identifying the mechanisms, such as support leg-ground interaction and details for the nature of activation of muscle groups around a joint, which lead to an accurate kick, may provide insight into the role support leg and muscles play in successful and unsuccessful kicks. Therefore, the purpose of the present study was to compare accurate and inaccurate soccer kicks when aiming to top and bottom targets, focusing on the ground reaction forces made by support leg and on the muscle activation patterns of selected lower extremity muscles of the kicking leg.
Fig. 1. A schematic illustration of the kicking trials on different targets.
the target ‘‘as accurate as possible, as fast as possible’’. A kick was deﬁned accurate every time the ball hit the target or passed through the target area. A 30 s rest interval between consecutive kicks was provided. The average of all the kicking trials was used for further analysis.
2.3. Ground reaction forces
The vertical, anteroposterior, and mediolateral components of the GRF’s during the plant of the support leg were measured using a Kistler piezoelectric force platform (Kistler Type 9281C, Kistler Instruments, Winterthur, Switzerland). The force platform was located beside the ball, in the middle of a 5-m-long pathway and was concealed with plastic turf to avoid disorientation of the player by focusing on stepping inside the force platform when kicking. The force platform was interfaced through Kistler amplifying units (Type 233A) to an Ariel Performance system (Ariel Dynamics Inc., San Diego, CA). The force platform signals were A/D converted at a sampling rate of 1000 Hz and recorded using the analogue converter of the Ariel system. Subsequently, they were analyzed simultaneously with the electromyographic data.
Twenty-one male amateur soccer players (age: 23.7 ± 2.3 yrs, mass: 75.2 ± 6.3 kg, height: 180 ± 2.1 cm) volunteered to participate in the present study. The players were members of two teams participating in the fourth division of the Hellenic Amateur Association League, for the last 4 years. Ten players were strikers, seven were midﬁelders and four were defenders. Participants had a minimum of 8 years of experience and trained at least three times plus one game per week. All participants passed medical examination within 15 days before the tests and had no injury of their lower limbs, while as was stated they refrain from injury the last 6 months before the measurements. Fifteen players preferred to kick with the right foot and six with the left foot. Participant informed written consent was received prior to the testing procedures. The University Ethics Committee approved the protocol. 2.2. Testing procedure A 15-min warm-up consisting of jogging, stretching exercises and several familiarization trials was performed. Following the familiarization session each participant performed three maximum instep kicks in order to generate reference EMG data for normalization purposes. All the kicks of the study were performed against a full length goal (7.32 2.44 m) using a standard size and inﬂated ball (FIFA approved) from a distance of 11 m, thus corresponding to the penalty shoot-out. In the main session, each participant performed 20 consecutive kicking trials. Ten kicks were performed to a top target and ten kicks to a bottom target. All the kicks were performed in a random order. Targets (0.50 m2) were positioned in the center of the goal, o...