Variability of Objective and Subjective Intensities During Ball Drills in Youth Soccer Players.

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VARIABILITY OF OBJECTIVE AND SUBJECTIVE INTENSITIES DURING BALL DRILLS IN YOUTH SOCCER PLAYERS ASIER LOS ARCOS,1 RAU´L MARTI´NEZ-SANTOS,2 JAVIER YANCI,2 JUAN MARTI´N,1 CARLO CASTAGNA3

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Club Atle´tico Osasuna, Pamplona, Spain; 2Faculty of Physical Activity and Sport Science, University of the Basque Country, UPV/EHU, Vitoria-Gaste´iz, Spain; and 3Technical Department, Football Training and Biomechanics Laboratory, Italian Football Federation (FIGC), Florence, Italy ABSTRACT

Arcos, AL, Martı´nez-Santos, R, Yanci, J, Martı´n, J, and Castagna, C. Variability of objective and subjective intensities during ball drills in youth soccer players. J Strength Cond Res 28(3): 752–757, 2014—The aim of this study was to examine the intrasubject (intraclass coefficient correlation [ICC]) and intersubject variability (coefficient of variation [CV]) of soccer ball drills (BD) involving or not opposition in male youth soccer. For this purpose, a collective ball dribbling (DB) exercise and a 7-a-side ball game without coach encouragements were considered. Exercise intensity was assessed as heart rate (HR), training load (TL), and perceived exertion scales. Fourteen U-14 male soccer players (age, 14.79 6 0.43 years and experience, 6.5 years) of a Spanish First Division club academy participated in the study. Ball drills were examined for variability over 5 successive training sessions in similar field conditions. Results showed that 7-aside was significantly (p = 0.000) more demanding than DB. Indeed the TL, HRmax, HRmean, overall perceived exertion, and leg muscular perceived exertion (MPE) resulted 141, 8.7, 11, 56, and 72%, higher in 7-a-side than in DB, respectively. In the 7-a-side condition, good intersubject (CV , 10%) and low intrasubject (ICC , 0.7) variability were observed. In the DB condition, CVs were below 10% CV only for HR variables and the ICC values were higher than 0.7 only for MPE. Despite the moderate reproducibility of BD not considering opponents, this condition did not reveal to induce homogeneous physiological responses in young soccer players. Therefore, the use of this kind of drills may be questionable when considered as alternative of moderate intensity generic aerobic training. Despite the higher interaction between players variability in the opposition

Address correspondence to Carlo Castagna, [email protected]. 28(3)/752–757 Journal of Strength and Conditioning Research Ó 2014 National Strength and Conditioning Association

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drills resulted lower, this suggests their use as a specific conditioning exercise.

KEY WORDS football, small-sided games, perceived exertion, collaboration, collective duel INTRODUCTION

B

all drills (BD) are considered a viable way of practicing aerobic and anaerobic fitness and skills in soccer concurrently and successfully (15). However, the intermittent nature of small-sided games (SSG) and their supposed interplayers exercise variability requires an accurate training load (TL) control to warrant targeted results (7). In this sense, some sustainable objective-subjective and subjective methods have been proposed recently to assess intensity in the SSG (3–5). The intensity of SSG has been monitored in field conditions using on-line and post hoc exercise heart rate (HR) (i.e., objective-subjective method). Additionally, the use of perception scales to rate subjective exercise strain during the SSG practice revealed to be a valid method to track individual responses to fitness development–oriented BD (23). Interestingly, the combined use of HR and perceived exertion scales showed to be a valid approach to assess exercise intensity of SSG in soccer (9). Despite the interest of BD in daily soccer practice and their expected uneven individual drill responses, only a few articles addressed interplayers’ intensity variability and repeatability during BD in soccer (15). Furthermore, they were performed with adult elite-level players and provided results might not be applicable to less experienced and skilled young soccer players. The information about interindividual and day-to-day intensity variability of BD is of great impact on coach daily practice when training drills are chosen to match methodological (i.e., fitness development) aims in youth soccer (15). The exercise intensity of SSG may be affected by structural constraints, such as the number of players, the set-up dimensions, the rules, and the coach encouragements (23). The BD intensity may also be affected by the presence of opposition, supposed to be more variable when opponents are involved.

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Journal of Strength and Conditioning Research Although the optional use of opposition is a common routine in young soccer to satisfy methodological aims, no research is currently available about the possible effect of opposition on BDs variability. Therefore, the aim of the present study was to examine the variability and reproducibility (i.e., day-to-day variability) of 2 popular youth soccer BD proposing or not opposition in soccer and its effect on the exercise intensity. This is in the attempt to provide full control of coaches over training BD during daily practice. The occurence of higher inter-players’ variability in BD with opponents was assumed as work hypothesis.

METHODS Experimental Approach to the Problem

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method to assess exercise intensity during SSG and training in soccer (6,7,10,16). Categorization of the HR was performed according to the procedures suggested by Stagno et al. (25). The subjective rating of perceived exercise (RPE)-effort was measured using the Foster’s 0–10 scale. The use of RPE was reported to be a valid measure of the internal load during the SSG in soccer (9,23). Subjects

Fourteen U-14 male soccer players (age, 14.79 6 0.43 years; height, 1.74 6 0.04 m; body mass, 60.77 6 5.14 kg; and body fat, 9.96 6 0.77%) from a Spanish First Division (Liga BBVA) club academy participated in the study. All players had a minimum of 6.5 years’ experience in competitive soccer and competed for the same youth category at regional and national level. The players and their parents were informed (word and written) about the research and its benefits and risks, and all of them gave their informed consent before the commencements of all the procedures considered in this study. The study was conducted according to the Declaration of Helsinki (2008) and the fundamental law of Personal Data Protection. The local Ethics Committee approved the study protocol. In the attempt to avoid team imbalances, players position, players technical-tactical level, physical fitness, and participation in competitive matches were considered for team allocation according to Casamichana and Castellano (3). More specifically, allocation was performed using coach technical-tactical rate, time participation in matches, and endurance level (i.e., time to exhaustion on Leger-Boucher test) (20). Because of injury, lack of compliance with the research design requirements, and technical failure, 9 of the original players were considered for calculations eventually.

To examine the effect of opposition consideration on BD intravariability and intervariability, 2 popular drills representing the 2 exercise categories (i.e., with or without opponents) were considered taking into account their structural traits (22). The opposition-free BD was represented by a passing conditioning game involving ball dribbling (DB) and passing (11). To produce opposition effect on exercise intensity, a modified rules 7-a-side SSG (i.e., collective duel drill, CD) was considered according to Ford et al. (11). The study was conducted at the end of the 2010–11 competitive season (May-June). During this period of the season, players practiced 3 times per week reducing the TL to avoid undue effect of fatigue on BD. No matches were played during the period of the experimentation. Before the study, all players performed familiarization sessions to master SSG and BD. However, the drills used in this study were part of the usual training sessions during the considered competitive season. Data Procedures were collected during 5 consecutive training sessions that always In this study, 2 popular BDs were considered to improve took place at the same time of the day (19:00 hours) with individual and team skills. The individual skill development task a minimum of 48-hour rest in between. Each training session consisted in DB as fast as possible in a relay wise mode between comprised the same training tasks performed in the same order 2 lines 9 m apart before passing the ball to a teammate. Each after a standard 5-minute warm-up. The DB and 7-a-side drill team composed of 8 players who had to complete 6 relays. Two were played in reason of 3 sets of 5 minutes with a between-set balls per team were provided, and no coach encouragement was rest of 2 minutes. The recovery between both drills was 6 miconsidered during the drill. nutes. All the BDs were performed outdoors on a third-generA 7-a-side game without goalkeepers was considered as ation artificial grass soccer pitch that was watered 10 minutes conditioning and team skill development drill. The considered before the training session according to the manufacturer’s guidelines. To promote proper hydration ad libitum, water assumpTABLE 1. Exercise variables during the considered ball drills.* tion was promoted to all players during the training sessions. % HRmean OPE MPE Stagno’s method % HRmax Players were advised to maintain DB 40.6 6 9.1 90.1 6 3.6 82.3 6 4.3 4.8 6 1.1 3.6 6 1.4 their usual diet to promote maxCD 62.9 6 6.3† 97.9 6 1.7† 91.3 6 2.2† 7.5 6 0.9† 6.2 6 1.6† imal performance. *% HRmax = percentage of maximal heart rate; % HRmean = mean of heart rate respect to In this study with the aim to the maximal heart rate; OPE = overall perceived exertion; MPE = muscular perceived exertion evaluate the DB and SSG exer(the legs); DB = dribbling the ball; CD = collective duel. cise intensity, HRs were used. †p , 0.0001. HRs monitoring was reported to be a valid and sensitive VOLUME 28 | NUMBER 3 | MARCH 2014 |

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Opponent Effect on Exercise Intensity in Youth Soccer and 71–65% of HRmax were multiplied by 5.16, 3.61, 2.54, TABLE 2. Coefficients of variation in the dribbling ball drill per day.* 1.71, and 1.25, respectively. One minute after the end of % HRmean OPE MPE Stagno’s method % HRmax soccer, BD players individually Day 1 23.4 3.2 4.9 20.8 40.5 rated their effort exertion using Day 2 24.4 4.5 5.5 19.2 32.8 the Foster’s 0–10 scale (12). Day 3 11.9 2.7 3.2 27.3 34.4 They responded separately Day 4 23.2 4.1 4.9 20.9 39.0 Day 5 25.9 3.8 4.9 24.7 40.0 about the overall perceived Mean 21.8 6 5.6 3.7 6 0.7 4.7 6 0.9 22.6 6 3.3 37.3 6 3.5 exertion (OPE) and the leg muscular perceived exertion *% HRmax = percentage of maximal heart rate; % HRmean = mean of heart rate respect to (MPE) (1,13,14,26). All particthe maximal heart rate; OPE = overall perceived exertion; MPE = muscular perceived exertion (the legs). ipants were familiar with this procedure; they have used it to rate exercise effort perception during SSG, friendly SSG was performed in a 45 3 45-m pitch with 5 nonstandard matches, and maximal aerobic test training drills. All the goals and no ball touch restriction. The pitch was divided in to 4 procedures involved in this study were performed at the zones containing a goal, each having the fifth one placed in the same hours of the day to avoid circadian variation that center of the field. Scores were considered valid only if made at may have an effect on effort outcomes. the first touch, and no goal could be scored in the very same Statistical Analyses quadrant where the team regained the possession of the ball The results are reported as mean 6 SDs. Assumption of noruntil it was played on any of the other 3. The 7-a-side game was mality was verified using the Shapiro-Wilk W-test. Day-to-day played without providing encouragements, with the coaches mean variations were detected with repeated-measures analonly counting scored goals for the players to hear. ysis of variance with post-hoc Bonferroni test. Interplayer Exercise intensity was assessed recording (i.e., 5-second variability and intraplayer variability were calculated using interval) HR with short-range telemetry (Polar Team Sport the CV (17) and the intraclass correlation coefficient (ICC) System; Polar Electro Oy, Finland). Maximal heart rate (2), respectively. Statistical significance was set at p # 0.05. (HRmax) was assessed using the Leger-Boucher test (19). Data analysis was performed using the Statistical Package for The endurance test was performed outdoors on an artificial Social Sciences (version 18.0 for Windows; SPSS, Inc, Chicagrass soccer pitch 3 days before the study. The participants go, IL, USA). Power calculation revealed that to achieve completed the test at the same hours of day as everyday a power of 80%, at least 12 subjects were required at the practice. assumed level of significance. The obtained HRmax was used as the reference for developing individual training intensity zones (3,15). Intensity categories were considered as the time spent in the arbitrary chosen HR zone multiplied by a selected weighting factor (i.e., TL) according to Stagno et al. (25). Specifically, time (in minutes) spent at 100–93%, 92–86%, 85–79%, 78–72%

RESULTS

Players’ mean endurance test performance was 21.51 6 1.34 minutes. The CD was more intense than DB drill (p , 0.001) in all variables (Table 1). Indeed, the TL, HRmax, HRmean, OPE, and MPE resulted 141, 8.7, 11, 56, and 72%, higher in 7-a-side than TABLE 3. Coefficients of variation in 7-a-side drill (collective duel) per day.* in DB, respectively. The CVs for the variables of % HRmean OPE MPE Stagno’s method % HRmax interest are reported in Tables Day 1 4.3 1.1 2.0 9.7 26.8 2 and 3. Coefficient of variaDay 2 11.7 1.6 2.6 12.8 28.9 tion was greater in DB than in Day 3 4.8 1.1 1.0 13.8 29.6 7-a-side for all measures. In Day 4 10.3 1.5 2.2 9.9 28.8 the DB, only HRmean and Day 5 12.4 1.9 2.8 14.1 10.4 HRmax values were below Mean 8.7 6 3.9 1.5 6 0.3 2.1 6 0.7 12.0 6 2.1 24.9 6 8.2 10%, with the remaining var*% HRmax = percentage of maximal heart rate; % HRmean = mean of heart rate respect to iables lying above 20%. In the maximal heart rate; OPE = overall perceived exertion; MPE = muscular perceived exertion the 7-a-side condition, only (the legs). the CV for the MPE exceeded 12%.

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enjoyable alternative to medium-to-low-intensity conTABLE 4. Intraclass correlation coefficient values in the different indicators for tinuous running in young socboth exercises in 5 days.*† cer players. This finding is in Stagno’s method % HRmax % HRmean OPE MPE line with that previously reported by Hoff et al. (16), which DB 0.65 0.61 0.70 0.37 0.72 showed relevant cardiovascular CD 0.3 0.07 0.19 0.41 0.63 effect as consequence of DB. In Days 1–2z Days 1–2z Days 2–3z Days 2–3z light of this, skill development Days 2–5z should be regarded as a component of soccer training that *% HRmax = percentage of maximal heart rate; % HRmean = mean of heart rate respect to must be considered to affect the maximal heart rate; OPE = overall perceived exertion; MPE = muscular perceived exertion (the legs); DB = dribbling the ball; CD = collective duel. TL in young soccer players †No significant difference was found between days in any indicator. and proportionally sized in the zSignificant differences between days at p , 0.05. daily training routine. Consistency of training response is vital for training prescription in team sports. We know that BD produce per se intraplayers’ and interThe ICC values for both exercises are presented in Table players’ variability in physiological responses (7,21). The 4. Significant differences between days were found only for supposed activity-related variability might logically be exacthe 7-a-side drill. In this BD, the ICC value was over 0.5 only erbated considering or not opponent in drill development. In for MPE. The ICC values for DB variables were higher than the collaboration drill (i.e., DB), maximal and mean exercise 0.5 for Stagno’s method, HRmax, HRmean, and MPE. HR showed low interplayers’ variability as per reported CV DISCUSSION values (i.e., 3.7 and 4.7%, respectively). These CV values are lower than the 10% threshold suggested by Atkinson et al. The aim of this study was to examine the variability of the (2) to determine practical acceptance of clinical variations. intensity of 2 structurally different soccer drills considering the Despite this, a remarkable intersubject variation in the values presence or absence of opposition. The main finding of this obtained in the perceived exertions (i.e., general and muscustudy was the evidence of a significant effect of opposition on lar) and in the TL was detected (i.e., CV .10%). Therefore, the exercises’ intensity and variability. This suggests that the taking into account all intensity variables, we can assume 7-a-side drill may be useful to promote conditioning training that there is a considerable intersubject variation in the col(i.e., HR above 90% of HRmax) in academy soccer. laboration drill. The training physiological stimulus varies The study results showed that spontaneous playing in the between players. The constancy of a RPE in guiding training collaboration-opposition drill (i.e., 7-a-side) promoted HR intensity in young soccer players may be questionable but intensities in the range of those reported to be functional in the TL values (CV = 21.8 6 5.6%) also indicated a high enhancing aerobic fitness in soccer players (i.e., 90–95% of variation interplayer in this kind of drills. Because of the HRmax) (15). This enforcing HR peaks (i.e., 98% of HRmax) huge practical interest of this kind of information, further were close to the individual maximal values detected under studies on the applicability of RPE to drill prescription are field condition using usual shuttle running tests for endurance. warranted in soccer. These results are in line with those reported in other studies Differently from initially supposed, the 7-a-side BD that examined SSG with rules, number of opponents, and field showed a low interplayer variability. The corresponding dimension variation (15). Interestingly, the reported effect of CVs for %HRmax and HRmean in the opposition exercise the 7-a-side drill was achieved without any external encourwere only 1.5 and 2.1%, respectively. This applies also for agement (23). As a result, coaches are advised to consider the 7-a-side TL that resulted in a CV lower than 10%. These opposition when in need to develop aerobic fitness in acadfindings are in line with those previously reported in other emy players attending to variety as one of the basic principles studies that used HR variations to profile intersubject and of training planning (23). intrasubject variability during SSG (15,23). The resulting Given that the CD here represented by DB may be small interplayer variability in the 7-a-side may be the result considered as a useful training drill to develop endurance of the highly competitive nature of opposition games that (i.e., 80–85% HRmax) or to provide lower TL, this may result seem to elicit close to maximal response in the players, of greater practical interest during the week for the practice resulting in a reduced variability as far as the individual maxsessions closest to the match. Furthermore, the resulting imal range of HR was achieved (7). In this regard, other response promotes DB, aiming to individual skill developexercise variables, such as blood lactate concentrations, ment, as a potential activity functional to conditioning if may be suggested (9) to avoid the limitation of HR values proper strategies are considered. This may result in a more VOLUME 28 | NUMBER 3 | MARCH 2014 |

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Opponent Effect on Exercise Intensity in Youth Soccer in monitoring very high-intensity activities. Given this, further studies considering more sensible variables to very high-intensity exercise (i.e., higher than speed at HRmax) responses are warranted. Even though, if we consider the CV results in all variables, we can affirm that the interplayer variation in opposition drill is lower than in the collaboration drill. In this study, we also assessed session-to-session drill consistency (i.e., reproducibility) using ICC. The results showed that for the collaboration drill (i.e., DB), the reproducibility of the considered variables was good only in few cases (ICC . 0.70) (8), specifically HRmean and MPE. The HR variables (i.e., ICC = 0.61–0.70) do not match with those obtained in other technical exercises (24) performed by elite junior soccer players that showed poor peak HR reproducibility (ICC = 0.38). In the 7-a-side condition, the resulting ICC was very poor except for MPE (ICC = 0.63), although mean differences were found between training days in the selected variables (TL, %HRmax, and %HRmean). Similarly, these results are consistent with the findings obtained by Rampinini et al. (24) for the considered 4 vs. 4, 4 vs. 2, and 10 vs. 10 SSG. However, differences in BD rules, players’ skill, and exercise set-up or modality (i.e., set or free displacement) make comparison between the studies difficult. Therefore, the opposition BD should be thought of as low session-to-session consistent while the collaboration drill presented a better reproducibility. It could be concluded that despite the moderate reproducibility of BD without opponents (i.e., collaborative drill), this condition did not reveal to induce homogeneous physiological responses in young soccer players. Therefore, the use of this kind of drills may be questionable when considered as an alternative to moderate-intensity generic aerobic training. Interplayers’ variability may be lower during team opposition drills despite the occurrence of frequent duel this suggesting their use as conditioning exercise when specific training (i.e., ball use during drills) is to be prescribed. However, session-tosession low reproducibility suggests caution when implementing BD. In this case, strategies enabling higher players drill commitment as per provided encouragements may result of interest. Therefore, further studies looking for training consistency enhancing strategies for specific training are warranted in youth soccer.

PRACTICAL APPLICATIONS Specific training conducted at high intensity has been reported to significantly improve aerobic fitness and to moderately affect match performance in youth soccer (18). Therefore, SSGs were proposed as aerobic fitness enhancing drills (15). This study findings showed that BD involving opposition may result of interest for high-intensity training (i.e., .90% HRmax) although played spontaneously. This promotes intrasession homogenous demand across players. However, caution should be paid when proposing this kind of drills again during the week as the response of the individual player may be different. In this case, on-line monitoring of

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HR may result of great interest to attain the targeted training intensity. Skill development drills performed as collective exercise may result useful for endurance (i.e., 80–85% HRmax) development but strategies to warrant homogenous exercise intensity must be considered as per drill games session-tosession reproducibility has to be addressed in this kind of activities. In light of the practical interest of BD, studies addressing exercise reproducibility enhancing strategies in youth soccer are necessary.

ACKNOWLEDGMENTS The authors would like to thank C.A. Osasuna and coaches of C.A. Osasuna U-14 team for the opportunity to investigate, and the help of Ibai Guridi and Txaber Hurtado students of University of Basque Country, UPV/EHU.

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