Acute effect of stretching modalities on global coordination and kicking accuracy in 12–13 year-old soccer players

Human Movement Science 54 (2017) 63–72

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Full Length Article

Acute effect of stretching modalities on global coordination and kicking accuracy in 12–13 year-old soccer players

MARK

⁎

Mohamed Frikhaa, , Mohammad S. Derbelb, Nesrine Chaâric, Adnene Gharbib, Karim Chamarid a

Department of Physical Education, College of Education, King Faisal University, Al-Hufūf, Saudi Arabia High Institute of Sport and Physical Education, Sfax University, Sfax, Tunisia c Faculty of Sciences, Carthage University, Bizerte, Tunisia d Athlete Health and Performance Research Center, Aspetar, Qatar Orthopaedic and Sports Medicine Hospital, Doha, Qatar b

AR TI CLE I NF O

AB S T R A CT

Keywords: Psychomotor task Speed-accuracy trade off Precision Lower limb Range of motion Football

The aim of the study was to compare the effect of stretching procedures on global coordination and accuracy in instep soccer kicks achieved in different stress conditions. Twenty male young soccer players completed the global coordination test (GC), the instep kicking accuracy test in free (FKA) and in time-pressure (TPKA) conditions, either after static (SS), dynamic (DS), ballistic (BS) or no-stretching (CTR) protocols, on nonconsecutive days and in a randomized order. After performing a 5 min standardized intensity jogging (70% of MAV), followed by stretching exercises for 10 min, each participant completed, successively, the GC, FKA and TPKA tests. Accuracy data, heart-rate, rating of perceived exertion and task difficulty perception were recorded and analyzed using a two-way ANOVA. GC scores were analyzed using one way ANOVA with repeated measures. The results showed higher GC and TPKA performances after DS and BS procedures. However, there was no effect of the stretching procedures on FKA. The GC scores increased by 10.8% and 7.2% after DS and BS, respectively, but were not affected by SS. Compared to FKA, the TPKA accuracy significantly decreased by 20.2% after CTR (p < 0.01) and 30.7% SS (p < 0.001) with no significant difference after DS (10.1%; p > 0.05) and BS (11.0%; p > 0.05). The use of dynamic and ballistic stretching yielded to better GC scores and helped reducing the adverse effect of time-pressure on instep kicking accuracy. Consequently, dynamic and ballistic exercises can be recommended before practicing activities requiring coordination and lower limbs speed and accuracy.

1. Introduction The soccer instep kick is an open-chain movement consisting of a series of stretch-shortening-cycle muscle actions about the hip and the knee joint (Andersen & Dörge, 2011). It constitutes a basic element of both soccer and futsal games (Castagna, D’Ottavio, Granda Vera, & Barbero Alvarez, 2009; Naser & Ajmol, 2016) in which both speed and accuracy are required (Van den Tillaar & Ulvik, 2014). The improvement of soccer instep technique is one of the most important aims of training programs in young players (Kellis & Katis, 2007). As a multijoint motion, it depends on the maximum strength and power of the muscles activated during the kick but also, its performance highly relies on the coordination between the agonist and antagonist muscles (Bjelica, 2008; Dorge et al., 1999; Lees & Nolan, 1998; Manolopoulos, Papadopoulos, & Kellis, 2006). It was previously demonstrated that ball speed of the

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Corresponding author at: Department of Physical Education, Faculty of Education, King Faisal University, 31982 Al-Hufūf, Saudi Arabia. E-mail addresses: [email protected], [email protected] (M. Frikha).

http://dx.doi.org/10.1016/j.humov.2017.03.008 Received 8 December 2016; Received in revised form 11 March 2017; Accepted 29 March 2017 0167-9457/ © 2017 Elsevier B.V. All rights reserved.

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soccer kick may be enhanced by quicker leg movement and transfer of the energy between segments (Tsaousidis & Zatsiorsky, 1996). However, an elevated execution-speed induces decreases in the accuracy of motion (Missenard & Fernandez, 2011). Such phenomenon in goal-directed aiming tasks is related to the Fitts principle (1954), showing that when aiming as quickly as possible at a target of width “W” located at a distance “D”, the execution time “ET” increases as a function of an index of difficulty “ID”, defined as log2(2D/W). Therefore, kicking as quickly as possible at a target (time-pressure condition), responding to soccercompetition demands, theoretically leads to an increase of ET and a decrease of accuracy. Researchers focusing on instep kicks demonstrated that by prioritizing accuracy, the kicking ball velocity decreased to 75–85% of maximal values (Andersen & Dörge, 2011; Lees & Nolan, 1998; Van den Tillaar & Ulvik, 2014). An accurate kick requires high level of kinesthesic differentiation ability and regulation of ball trajectory. It was demonstrated that the kinesthesic differentiation ability, develops rapidly from young age and reaches a maximum at around 10 years of age (Derri, Mertzanidou, & Tzetzis, 2000). Moreover, the regulation of ball trajectory can be reached by modifying: (i) the support-leg placement characteristics, (ii) the kicking-leg swing motions and (iii) kicking foot-to-ball contact characteristics (Scurr, Abbott, & Ball, 2011). Such regulation requires activation of several lower limbs muscular groups: kicking accurately to a top target requires an activation of tibialis anterior and biceps femoris muscles, while reducing the gastrocnemius muscle activation of the swinging leg (Katis et al., 2013). In contrast, to perform an accurate kick to a bottom target, player must first be able to keep the ball at ground level by: (i) limiting the activation of the tibialis anterior, and (ii) direct in the ball to the desired target by limiting the activation of the rectus femoris (Katis et al., 2013). Stretching exercises, as part of soccer warm-up procedures, aims amongst others, to prepare players to an optimal state from the first minutes of competition, by increasing the range of motion (ROM; Apostolopoulos, Metsios, Flouris, Koutedakis, & Wyon, 2015), the length of musculo-tendinous unit (MTU) and inhibiting the mechanoreceptor-mediated reflexes (Behm, Blazevich, & McHugh, 2016; Haag, Wright, Gillette, & Greany, 2010). In the last decade, the impact of the stretching procedures on subsequent performance has been widely investigated (see Behm & Chaouachi, 2011; Behm et al., 2016; Kay & Blazevich, 2012 for reviews). It has been demonstrated that dynamic and ballistic stretching have better effect on hip dynamic ROM during instep kicking in professional soccer players (Amiri-Khorasani, Abu Osman, & Yusof, 2011; Curry, Chengkalath, Crouch, Romance, & Manns, 2009), on sprint performance (Fletcher & Jones, 2004; Little & Williams, 2006), on power and velocity in vertical jumping (Fattahi-Bafghi & Amiri-Khorasani, 2012; Kirmizigil, Ozcaldiran, & Colakoglu, 2014; Miranda, Maia, Paz, & Costa, 2015) and on soccer agility (Amiri-Khorasani et al., 2011; Little & Williams, 2006). Nevertheless, studies related to higher limbs demonstrated that static stretching did not affect baseball pitching velocity and accuracy (Haag et al., 2010) or speed and accuracy in the tennis serve performance (Knudson, Noffal, Bahamonde, Bauer, & Blackwell, 2004). More recently, Mascarin, Vancini, Lira, and Andrade (2015) demonstrated that static stretching decreased medicine ball throwing performance when compared to dynamic warm-up exercises, but did not yield to any drop in ball speed during the handball throwing test. Moreover, Frikha et al. (2016) showed no effect of stretching modalities (ie. static, dynamic and ballistic) on free darts throw, among young boys. However, the authors recommended 10 min static stretching exercises before practicing activities requiring both upper limbs speed and accuracy. To the author’s knowledge, no studies investigated the acute effect of stretching on coordination and accuracy in soccer instep kicks. Likewise, the combined effect of stretching exercises and time-pressure on accuracy of lower limbs remains not studied too. Thus, the aim of the present study was to examine the acute effect of static, dynamic and ballistic exercises on global coordination, accuracy and consistency of lower limbs in soccer kicking task in different psychological stress conditions. 2. Methods 2.1. Participants Twenty male, right-footed young soccer players (age: 13.4 (0.7) yrs, body height: 164 (0.1) cm and body mass: 57.3 (8.1) kg; mean (SD)), volunteered to participate in this study. All the participants, belonged to a soccer team, had a playing experience of 2–4 years in regional tournaments and training with an average of 3–5 training sessions per week in addition to one competitive game. After an explanation of the procedures as well as the benefits and risks involved, a written informed consent was received from the subjects and their parents. Participants reported no sleep disorder, do not consume caffeine and none of them was taking any medication. During the experiment, all participants were instructed to maintain normal daily food and water intake, to avoid any kind of strenuous activity for 24 h before each test, to wear the same sportswear and soccer shoes (without studs/cleats). Two subjects were removed from the study for missed testing sessions. Hence, the data from 18 participants were used for analysis. The study was conducted according to the declaration of Helsinki and the local Ethics Committee approved the protocol. 2.2. Procedures Once included, participants were invited to perform the Yo-Yo intermittent recovery test (Level_1; Bangsbo, Iaia, & Krustrup, 2008) between 10:00 and 11:30 h a.m. This test was performed to estimate their maximal “aerobic” velocity (MAV). During one week before the start of the experiment, participants were familiarized with the testing procedures and the stretching techniques for at least four sessions, ie., the global coordination test (GC) (Starosta, 2006; Garbolewski & Starosta, 2013) and the Kicking Accuracy test (Finnoff, Newcomer, & Laskowski, 2002). These familiarizations, performed at the same time-of-day, ensured that participants were fully knowledgeable of the experimental conditions, measurements required, and scoring system. In the last familiarization session, 64

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Counter-balanced and randomized 10 min CTR

GC-test

10 min SS 1min rest

5min jogging (70% MAV)

FKA Time

10 min DS TPKA 10 min BS

Fig. 1. Schematic representation of the experimental design. Arrow: HR, RPE; Dotted arrow: DP15; MAV: maximal aerobic velocity; CTR: control warm-up with no stretches; SS: static stretching; DS: dynamic stretching; BS: ballistic stretching; GC: global coordination; FKA: free kicking accuracy; TPKA: timepressure kicking accuracy.

body mass was measured, using a digital scale (Tanita, Tokyo, Japan; precision: 100 g). The experimental design (Fig. 1) consisted of four test sessions, carried out in a randomized and partially counterbalanced order (because of the large number of rank possibilities), by the same experimenters at the same time-of-day. All sessions were completed during the course of the subsequent 12 days, so that approximately 48 h separated each test session. During three of the testing sessions, participants completed a 5 min jogging at 70% of MAV followed by 10 min of stretching of both legs: static (SS), dynamic (DS) or ballistic (BS). Then they performed the GC test and the kicking accuracy test in free (FKA) and in time pressure (TPKA) conditions, at the same order (GC-FKA-TPKA). In the remaining session (control warm-up: CTR), the procedures consisted only on 5 min of jogging at 70% of MAV, followed by 10 min rest with no stretch exercises performed. The rating of perceived exertion (RPE) and heart rate (HR) were recorded at rest, at the end of the jogging and after the stretching procedures, with respectively the OMNI scale of Robertson et al. (2000) and a cardio-frequency-meter (POLAR S410). Moreover, the DP-15 scale (Delignières, 1998) was used to evaluate the task difficulty perception after the kicking and the global coordination tests performances. Tests were conducted indoors in a playing team sport gymnasium, where the mean ambient temperature and relative humidity were of 24.1 °C ± 0.6 °C and 51.9% ± 0.3%, respectively. 2.3. Stretching protocols The stretching modalities used in this study were static (SS), dynamic (DS) and ballistic (BS) stretches. The SS involved lower limb muscle stretches, conducted at a submaximal intensity (∼90% POD) (Behm & Chaouachi, 2011; Knudson et al., 2004) and reached by subjectively decreasing the ROM of articulation by ∼10% from the angle achieved when participants were stretched at the POD (Young, Elias, & Power, 2006). Each stretch was held for 30 s and repeated two times with 30 s recovery in-between. The SS routine consisted of the major muscle groups involved in football kick: the gastrocnemius, hamstrings, quadriceps and hip flexors, gluteals, and the adductors (Little & Williams, 2006). The SS protocol was based on the protocol used by Chatzopoulos, Galazoulas, Patikas, and Kotzamanidis (2014) and Faigenbaum, Bellucci, Bernieri, Bakker, and Hoorens (2005). The DS involved active and slow movements, without bouncing of antagonist muscles that cause, by reciprocal inhibition, relaxation and stretching of the agonist muscles. The BS consisted of bouncing and rhythmic motions of the legs to an extreme position (Gelen, Dede, Bingul, Bulgan, & Aydin, 2012; Jaggers, Swank, Frost, & Lee, 2008). The SS, DS and BS protocols included exercises that stretched the same muscle groups. Participants were instructed to perform the DS and the BS stretches at a rate of, approximately, 1 stretch cycle every 2 s and 1 stretch cycle every 1 s, respectively, with 15 s of recovery in-between. 2.4. Global coordination test (GC) From semi squat position, hands on the hips, participant performed a vertical jump with maximal revolution around the longitudinal body axis with both feet joined (Starosta, 2006). They were allowed to use their upper limbs to maintain balance at the end of the jump. The result of the test is expressed in degrees (the greater the number of degrees, the higher the level of the coordination). Three repetitions were performed with revolutions to the right side and 3 repetitions with revolutions to the left side. The best jump results in the right and in the left were kept for analysis. This measure was treated as a measure of the global motion coordination (GC) level (Starosta, 2006; Garbolewski & Starosta, 2013). 2.5. Kicking accuracy test Participants performed 10 kicks of a stationary ball to a vertical target using the inner side of their preferred kicking foot and with a self-selected approach angle and distance (Scurr & Hall, 2009). A plywood target measuring system of 243.5 cm wide × 244 cm high was used (Finnoff et al., 2002). Carbon paper was applied to the surface of the target. The distance between the target and balls 65

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positions was of 6.10 m. The distance between bull’s-eye and ground was of 120 cm. Ten size 5 FIFA-approved balls (mass ∼430 g, pressure ∼0.8 bar), were used, and verified before each test-session. Balls were placed on an arc of a circle at the same distance from the target (i.e. 6.10 m) with an inter-ball distance of 10 cm in-between. Participants were allowed to perform two blocks of ten kicks, scored according to the ball position on the target. The first block, performed at a self-selected rhythm (FKA), with no time limitation. The second one, conducted on time-pressure condition (TPKA) within 15 s. The time constraint aimed at increasing the task difficulty by inciting participants to kick the balls as fast as possible in a situation of attentional control disruption (Wilson, 2008). Moreover, in real-life kicking actions, players are often confronted with speed as well as accuracy demands (Andersen & Dörge, 2011). The distance between ball mark and bulls-eye after each kick was measured. The mean distance between the bull’s-eye and balls positions, the number of times the target was missed (ball out of target or not kicked in time pressure condition) and the variability of the scores (VAR=[SD scores]/[mean score]) of the 10 kicks was retained for analysis, so that the smaller the score, the better the accuracy (Finnoff et al., 2002). 2.6. Statistical analyses All statistical tests were processed using STATISTICA Software (StatSoft, France). Data were reported as mean ± SD. The Shapiro-Wilk W-test of normality revealed that the data were normally distributed. Once the assumption of normality was confirmed, parametric tests were performed. Mean distances between ball positions and bull’s-eye, missed kicks, VAR and task difficulty perception were analyzed using a two-way ANOVA (4 [stretching procedures] × 2 [stress conditions]) with repeated measures on all factors. The RPE was analyzed using a two-way ANOVA (4 [stretching procedures] × 3 [moment of measurement]). Comparison between the GC variable means was performed using a one way-ANOVA with repeated measure. When appropriate, significant differences between means were assessed using the Least Significant Different (LSD) post hoc test. The reliability of the free kicking accuracy and GC tests were assessed using repeated-measures analysis of variance and intraclass correlation. Furthermore, the effect size “partial η2” was calculated. The thresholds for small, moderate, and large effects were defined as 0.20, 0.50 and 0.80, respectively. Statistical significance was set at p < 0.05. 3. Results The intraclass correlation showed high reliability coefficients of the free kicking accuracy and the global coordination tests: ICCa = 0.93 and ICCa = 0.87, respectively. 3.1. Kicking accuracy Statistical analysis indicated a significant effect of stress condition (F = 84.54; p < 0.001; η2 = 0.832). The post-hoc revealed, that differences between stress conditions (i.e: FKA vs. TPKA) were higher in CTR (p = 0.002, Cohen’s d = 1.22: large) and SS (p = 2.1E-05, Cohen’s d = 1.43: large) compared to DS (p = 0.122, Cohen’s d = 0.4: small) and BS (p = 0.087, Cohen’s d = 0.51: moderate). In TPKA better accuracy performances were recorded after DS in comparison with CTR (p = 0.049, Cohen’s d = 0.56: moderate) and with SS (p = 0.007, Cohen’s d = 0.69: moderate). The BS led to better accuracy performances in comparison with SS (p = 0.024, Cohen’s d = 0.68: moderate) (Fig. 2). Concerning the number of missed kicks, there was a significant effect of the variable stress condition (F = 31.65; p < 0.001; η2 = 0.651). The post hoc revealed: (i) missed kicks were lower in FKA than in TPKA (p = 4.75E-07, Cohen’s d = 1.54: large in CTR; p = 1.701, Cohen’s d = 1.23: large in SS; p = 0.009, Cohen’s d = 0.76: moderate in DS; and p = 0.000, Cohen’s d = 0.97: large in BS). (ii) for TPKA there were differences between DS vs. CTR (p = 0.002, Cohen’s d = 0.71: moderate) and BS vs. CTR (p = 0.021, Cohen’s d = 0.50: moderate),with lower values in both DS and BS (Fig. 3). 80

Mean distances (cm)

75

†

††

70

##

#

65 60

**

***

FKA TPKA

55 50 45 40 CTR

SS

DS

BS

Fig. 2. Accuracy changes in FKA and TPKA after the four warm-up procedures (ie.: CTR, SS, DS and BS). FKA: free kicking accuracy; TPKA: time pressure kicking accuracy; CTR: control warm-up with no stretches; SS: warm-up with static stretching; DS: warm-up with dynamic stretching; BS: warm-up with ballistic stretching. * Significant difference FKA vs. TPKA. **p < 0.01; ***p < 0.001. #Significantly different from SS at TPKA condition. #p < 0.05; ##p < 0.01. †Significantly different from DS at TPKA condition. †p < 0.05; ††p < 0.01.

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Fequency of missed kicks

3.50 3.00

***

*

** ***

***

***

2.50 2.00

FKA

1.50

TPKA

1.00 0.50 0.00 CTR

SS

DS

BS

Fig. 3. Missed kicks in FKA and TPKA after the four warm-up procedures (ie.: CTR, SS, DS and BS) FKA: free kicking accuracy; TPKA: time pressure kicking accuracy; CTR: control warm-up with no stretches; SS: warm-up with static stretching; DS: warm-up with dynamic stretching; BS: warm-up with ballistic stretching. * Significantly different at: *p < 0.05; **p < 0.01 and ***p < 0.001.

No main effect of both stress condition and stretching procedures on VAR variables. 3.2. Global coordination Statistical analysis showed significant main effects of the stretching procedures (F = 4.84; p < 0.01; η2 = 0.221), with post hoc indicating higher values after DS in comparison with SS (p = 0.008, Cohen’s d = 0.90: large) and CTR (p = 0.002, Cohen’s d = 0.82: large). Likewise, better GC performances were recorded after BS in comparison with SS (p = 0.041,Cohen’s d = 0.69: moderate) and CTR (p = 0.014, Cohen’s d = 0.66: moderate) (Fig. 4). 3.3. Rating of perceived exertion (RPE), heart rate (HR) and task difficulty perception For RPE (Table-1), analysis showed significant main effects of the stretching procedures and the moment of measures variables (F = 11.22; p < 0.001; η2 = 0.397 and F = 286.77; p < 0.001; η2 = 0.944, respectively) with post hoc indicating higher RPE recorded after the DS and BS vs. CTR (p = 6.49E08, Cohen’s d = 1.98: large; p = 4.07E09, Cohen’s d = 2.53: large, respectively) and vs. SS (p = 0.001, Cohen’s d = 0.95: large; p = 0.000, Cohen’s d = 1,23: large, respectively). For HR (Table 1), analysis showed a significant main effect of the moment of measure (F = 603.83; p < 0.001; η2 = 0.973). The post-hoc revealed: (i) HR recorded at the end of CTR and SS were lower than after DS (p = 0.020, Cohen’s d = 0.63: moderate; p = 0.003, Cohen’s d = 0.74: moderate, respectively) and after BS (p = 0.007, Cohen’s d = 1.29: large; p = 0.001, Cohen’s d = 0.99: large, respectively). (ii) no significant differences were observed between HR recorded after both DS and BS (p = 0.704, Cohen’s d = 0.09: small). For task difficulty perception (Table 2), analysis showed significant main effects of the stretching procedures and the stress condition variables (F = 4.79; p < 0.01; η2 = 0.470 and F = 58.81; p < 0.001; η2 = 0.771, respectively). The post-hoc indicated: (i) higher difficulty perception recorded in TPKA compared to FKA after the DS (p = 0.011, Cohen’s d = 0.89: large) and the BS (p = 0.002, Cohen’s d = 1.24: large). (ii) in TPKA, higher difficulty perception values were recorded after CTR in comparison with DS (p = 0.000, Cohen’s d = 0.98: large) and BS (p = 0.001, Cohen’s d = 0.93: large) and after SS in comparison with DS (p = 0.001, Cohen’s d = 1.05: large) and BS (p = 0.002, Cohen’s d = 0.98: large). 4. Discussion The purpose of this study was to compare the effects of different stretching procedures (ie: SS, DS, BS and CTR) on selected *

Global coordination (degrees)

* **

640

**

620 600 580 560 540 520 500 480 CTR

SS

DS

BS

Fig. 4. Comparison of GC after the four warm-up procedures (ie.: CTR, SS, DS and BS) GC: global coordination; CTR: control warm-up with no stretches; SS: warm-up with static stretching; DS: warm-up with dynamic stretching; BS: warm-up with ballistic stretching. *Significantly different at: *p < 0.05; **p < 0.01.

67

68

1.22 ± 0.57 69.7 ± 9*

2.17 ± 0.79¥ 105.1 ± 8†,‡

4.61 ± 1.38# 141.7 ± 16# 1.28 ± 0.57 70.2 ± 9*

4.5 ± 1.79# 135.3 ± 19#

After jog

* Significantly different than after stretching at the same stretching modality at: *p < 0.001. # Significantly higher than after stretching at the same stretching modality at: #p < 0.001. † significantly different than DS at the same moment of measure at: †p < 0.05; ††p < 0.01. ‡ significantly different than BS at the same moment of measure at: ‡p < 0.01; ‡‡p < 0.001 ¥ significantly different than DS and BS at the same moment of measure at: ¥p < 0.001

RPE HR (bpm)

Rest

After stretch

Rest

After jog

SS

CTR

2.83 ± 1.15¥ 102.9 ± 11††‡‡

After stretch 1.39 ± 0.61 73 ± 9*

Rest

DS

5.61 ± 1.09# 142.7 ± 19#

After jog

3.78 ± 0.88 113.7 ± 18

After stretch

1.28 ± 0.57 69.6 ± 9*

Rest

BS

Table 1 Average and standard deviations (n = 18; mean ± SD) of RPE and HR, according to stretching procedures and moment of measures (ie. rest, after jogging and after stretching).

5.94 ± 0.87# 134.2 ± 17#

After jog

3.94 ± 0.64 115.1 ± 14

After stretch

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Table 2 Average and standard deviations (n = 18; mean ± SD) of task difficulty perception, according to stretching procedures and stress condition (ie. FKA and TPKA). CTR Task difficulty perception

FKA TPKA

SS

5.22 ± 1.00 5.72 ± 0.67

5.28 ± 0.83 5.78 ± 0.65

DS

BS *

5.50 ± 0.92 6.89 ± 1.28†,##

5.22 ± 0.73* 6.78 ± 1.06†,#

* Significantly different than TPKA at the same stretching modality at: *p < 0.001 † Significantly different than CTR in the same mode of execution at:†p < 0.001 # Significantly different than SS in the same mode of execution at:#p < 0.01; ##p < 0.001

measures of kicking accuracy and global coordination. The main finding was that there was no effect of the stretching procedures on free kicking accuracy in young football players. However, better accuracy performances were obtained after DS and BS, compared to SS and CTR, when a moderate psychological stress condition (time-pressure) was introduced to the exercise performance. In addition, the GC was affected by stretching procedures with better coordination performances for DS and BS conditions compared to SS and CTR.

4.1. Kicking accuracy performance The kicking accuracy-test consisted on kicking a stationary ball. This test was chosen for 2 reasons: first, the participants of the present study were young players and imposing a rolling ball condition may have caused additional difficulties. Second, as demonstrated by Barbieri, Gobbi, Santiago, and Cunha (2010), there is no significant difference for accuracy between kicking a stationary and a rolling ball in elite soccer players. Moreover, the participants were allowed to use a self-selected approach angle for their instep kicking, because it was demonstrated that there was no significant difference in instep soccer kicks accuracy conducted at different approach angles (Scurr & Hall, 2009). Therefore, it is assumed that the participants of the present study were in optimal conditions to achieve their best kicking accuracy performance. As there is limited research regarding the effect of stretching modalities on kicking accuracy in young soccer players, limited comparisons can be made. The present study’ results show no effect of the stretching procedures on FKA. These findings seem to support those of Young, Clothier, Otago, Bruce, and Liddell (2004), showing that the inclusion of static stretching in a warm-up had no influence on the ROM of the swinging leg during kicking or the foot speed at impact with the ball. Nevertheless, this latter study did not measure accuracy. Similar results were also found by Knudson et al. (2004), Haag et al. (2010) and Mascarin et al. (2015) showing no effect of static stretching on speed and accuracy in tennis serve, baseball pitching performances, and handball throwing speed, respectively. More recently, Frikha et al. (2016) showed no effect of stretching modalities (static, dynamic, and ballistic) on free darts throw among young boys, which is in accordance with the present finings. While direct comparison with the aforementioned studies is difficult due to the differences in tasks characteristics (upper vs. lower limbs), it seems that accuracy demands related to fine motor coordination tasks (Messier, Adamovich, & Berkinblit, 2003), cannot be affected by warm-up stretching exercises. The lack of data relatively to the effect of stretching on kicking accuracy performances suggests making some speculations about the explanation of these results and especially in relation with the lack of effect of dynamic and ballistic stretches in the free kicking accuracy condition. These findings could be explained by: (i) the relatively short duration of muscle stretches used (2 × 30 s for each muscle group). Relatively recently it was proved that only dynamic and high volume upper extremity plyometric warm-up exercises are beneficial to serve speed in elite junior tennis players (Gelen et al., 2012). In contrast, the accuracy of kicks depends on how fast the player approaches the ball (Godik, Fales, & Blashak, 1993). It seems therefore, that there would been optimal approach speed in order to achieve an accurate kick. (ii) the short distance between target and ball position (6.1 m) may represent a low level of task difficulty (iii) the level of children skills recruited for this study. It was suggested that greater endurance and playing experience may improve kicking accuracy under moderate physiological stress (Young et al., 2010). The present finding reveals lower accuracy performances (higher distances between ball mark and bulls-eye and higher missed kicks), in time-pressure condition after the SS and CTR in comparison with DS and BS. Stretch-induced impairment on accuracy in a situation of reduced “ET”, may be related to disturbance mechanisms. Indeed, changes in the MTU length and stiffness caused by the SS exercises, could alter the ability to detect and respond promptly to changes in an unstable environment (Behm, Bambury, Cahill, & Power, 2004) and then to increase electromechanical delay and time from muscle excitation to the initiation of the movement (Cramer et al., 2005). Similar studies reported impairment in movement time of upper limbs (Chatzopoulos et al., 2014) and lower limbs (Behm et al., 2004) following SS bout exercises. Moreover, it was recently concluded, that dynamic stretching is more effective for enhancing powerful soccer instep kick performance (Amiri-Khorasani & Kellis, 2013) and that muscle activation of the kicking leg represents a significant mechanism which largely contributes to soccer kick accuracy (Katis et al., 2013). Although neuromuscular function was not measured in this study, it could be speculated that both DS and BS would contribute to a better muscle activation of the kicking leg in time-pressure condition. These results are in accordance with the findings showing that dynamic stretching leads to a decrease in the inhibition of antagonist muscles (Yamaguchi & Ishii, 2005), to an increase in motor unit excitability and an improvement of proprioception and kinesthetic awareness (Behm & Chaouachi, 2011; Fattahi-Bafghi & AmiriKhorasani, 2012); all the latter characteristics being necessary for such kicking task. Fixing the “ET” to15 s with the same task difficulty index (ID = log22D/W), leads to a decrease of the information transmission 69

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coefficients (Fitts, 1954) responsible to the information processing capacity and then to difficulties in feedback treatments. While those results are in accordance with the “speed-accuracy trade-off” phenomena (see Elliott et al., 2010 for review), showing that movement accuracy depends on the time available for current control, the interpretation about the nature of the motor control operated during this type of movement is still under discussion. Some models consider that the control is operated in an “iterative” manner during the movement execution and based on the sensory feedback (Mackenzie, Marteniuck, Dugas, Liske, & Eickmeier, 1987). Such theory is known by the “iterative-correction model of aiming”; whereas, some others (Meyer, Smith, Kornblum, Abrams, & Wright, 1990) advanced the “optimized-sub-movement correction model”, which considers that the control is operated in a feed forward manner and pre-determined before movement execution. Therefore, further researches are needed to elucidate the nature of the motor control operated during this type fine motor coordination task. Thus, in accordance with Chatzopoulos et al. (2014), Amiri-Khorasani et al. (2011) and Kellis and Katis (2007), it can be concluded that either dynamic or ballistic stretching exercises helped to reduce the adverse effects of time-pressure and resulted more useful than static stretching for the effectiveness of soccer instep kicks. 4.2. Global coordination The GC-test is based on jumping ability, in which lower limbs explosive power, balance, speed reaction and spatial orientation abilities are highly solicited (Garbolewski & Starosta, 2013). The present finding shows better GC performances after DS and BS in comparison with SS and CTR. Dynamic or ballistic stretching has been shown to enhance tasks-performance requiring agility and balance ability (Chatzopoulos et al., 2014; Little & Williams, 2006; Mcmillan, Moore, Hatler, & Taylor, 2006; Van Gelder & Bartz, 2011). However, SS exercises impair it (Behm et al., 2004; Chatzopoulos et al., 2014; Alvoniti et al., 2016). It was reported that this impairment is related to mechanical changes in MTU after SS exercises, that may alter the sensory input from spindles and Golgi tendon organs and which play an important role for postural maintenance (Behm et al., 2004; Chatzopoulos et al., 2014). Others, showed an improvement in lower limbs power outcomes performances after DS or BS bout of stretching exercises (Curry, Chengkalath, Crouch, Romance, & Manns, 2009; Kruse, Barr, Gilders, Kushnick, & Rana, 2015; Turki et al., 2011). These improvements are principally related to the increased electromyographic amplitude and the enhancement of muscle activation after DS and BS (Herda, Cramer, Ryan, McHugh, & Stout, 2008). Nonetheless, the present study findings are at odd with those of Jaggers et al. (2008), reporting that neither dynamic nor ballistic stretching resulted in an increase in vertical jump height or force; those of Chaouachi et al. (2010) proving that DS did not affect agility, sprinting, and jumping performances whether conducted separately or in conjunction with static stretching; and those of Paradisis et al. (2014) showing that DS deteriorated explosive power. Those discrepancies may be attributed to several factors such as age, training experiences, stretch durations and/or intensities used in the different studies (Behm & Chaouachi, 2011), and specifically to the use of post-activation potentiation (PAP). PAP is defined as the increase in the efficiency of the muscle to produce force after a submaximal or maximal contraction and is considered as a contributing factor for better performance of the DS and BS protocols (Mcmillan et al., 2006). Thus the pre-test submaximal intensity contractions performed in the DS and BS protocols may have elicited a PAP response and, consequently, significant differences with SS and CTR protocols. This warrants further investigations. 4.3. RPE and task difficulty perception The main findings of the present study showed higher HR values and RPE after the DS and BS compared to CTR and SS, which suggests that the differences in GC and KA performances could be caused by higher warm-up loads in DS and BS (HR ∼115 bpm; RPE ∼4). However, the difficulty of perception was higher in time-pressure condition in comparison with the free one, which is in agreement with other findings showing that the difficulty perception estimation increases when increasing difficulty in aiming task performances by increasing the distance from the target (Elghoul, Frikha, Chtourou, Abedelmalek, & Souissi, 2014) or creating a condition of moderate psychological stress (Frikha et al., 2016). Indeed, the task difficulty perception concept (Delignières, 1998) is considered as a major determinant of anxiety, and is commonly introduced into psychological models with motivation and emotion (Frömer, Hafner, & Sommer, 2012). Therefore, the higher difficulty estimations in the perception of kicking accuracy performances, carried out in TPKA after DS and BS, demonstrate that there is an optimal difficulty level in order to achieve an accurate kick (Wood, Jordet, & Wilson, 2015). 5. Conclusion The results of the present study confirm that static, dynamic or ballistic stretching did not affect accuracy in free soccer instep kicks. Based on the present investigation, both dynamic and ballistic stretching have better effect than static stretching on global coordination and on kicking accuracy under time-pressure among young soccer players. Therefore, for a positive outcome, dynamic or ballistic exercises should be included in the warm-up routines, before practicing activities requiring coordination and lower limbs speed and accuracy. Acknowledgments The authors wish to express their sincere gratitude to all the participants and parents for their dedication to the study. 70

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