Influence of competition on performance factors in under-19 soccer players at national league level.

The aim of this study was to analyse and quantify the acute effects of competition on several performance factors in under-19 male soccer players. To this end, 198 national league players (17.56 ± 0.78 years) performed various tests to measure jump capacity, kicking velocity and sprint times immediately pre-match (T1), at half-time (T2) and post-match (T3). Tests included kicking the ball to measure ball velocity (KICK), sprinting for 40 meters, timing the first 30 meters (30mACCEL), the last 10 meters (10mACCEL) and the total distance (40mACCEL), and performing countermovement jumps (CMJ). For subsequent analysis, the sample was divided into 5 playing positions: goalkeepers (n = 24), defenders (n = 51), midfielders (n = 36), wingers (n = 54) and forwards (n = 33). For all positions, we found a significant decline in performance (p<0.05) for kicking velocity (2.91% - 6.51%) and sprinting (0.44%-5.85%). For the CMJ, all positions except defenders presented a significant decline in performance that ranged from 1.5% to 4.56%. These findings highlight the need to individualise fitness training, taking into account the match needs and demands of the different playing positions in order to minimise the effects of match fatigue and accelerate post-match recovery.

Introduction

As a discontinuous or acyclic team sport, soccer requires dynamic, random and intermittent actions [1] involving acceleration, jumping and changes of direction and speed, all of which exerts great pressure on neuromuscular and metabolic parameters [2]. Sprint times and vertical jump capacity have been used to assess soccer players’ anaerobic profile [3] and to study changes with respect to the beginning of the match [4].

It has been demonstrated that soccer matches cause physical changes in players that lead to reduced physical performance in aspects such the capacity to maintain high speed during sprints [5], and acceleration/deceleration distances during and after the match [6]. Moreover, male players’ jump and knee extension-flexion performance declines immediately after a soccer match [7], and player performance is associated with various factors such as body structure, strength, power and speed [8]. However, the changes these factors undergo do not affect all playing positions equally [1].

Soccer players execute multiple sprints during a match [9], and although these represent less than 10% of the total distance covered, they are nonetheless decisive in the match outcome [10]. Frequent acceleration and deceleration during a soccer match involves high energy consumption and induces momentary fatigue in players [11]. As a result, maximum sprint speed declines over the course of the match [4], hence the capacity to minimise fatigue is related to an increase in performance [12]. The need to execute sprints varies during a match, thus soccer players must be capable of sprinting, recovering and sprinting again at the highest intensity possible [9].

Furthermore, other high intensity acyclic actions are necessary during a match, such as kicking, jumping, braking and explosive starts, and these vary according to playing position. Malina et al. [13] have reported significant differences in vertical jump, speed over 30 meters and intermittent aerobic endurance between defenders, midfielders and forwards in a group of young elite players with 4.5 years of experience. It has also been demonstrated [14] that forwards obtain better results for the 30 meter sprint and vertical jump than other playing positions. In that sense, the acute effect of fatigue on jumps post-match has also been demonstrated in other sports such as handball [15] and rugby [16], but not in basketball [17].

Many researchers consider kicking a fundamental aspect of a soccer player’s performance because it is the most frequently employed action during a match [18], for example when attempting to score a goal or passing the ball to other players. When kicking, players aim to achieve different ball velocities and trajectories with a high degree of accuracy [19]. Thus, some studies have analysed this variable to assess kicking accuracy, using a radar to measure kicking velocity [20].

Acceleration, the vertical jump and kicking velocity could be considered among the most easily measured acyclic actions due to the simplicity of their execution and ease of reproduction. Although these actions exert a decisive influence on performance in soccer, their variations have not been extensively studied in a competitive context. Consequently, the aim of the present study was to analyse and quantify the acute effects of a soccer match on the performance of under-19 players, testing jump capacity, sprint times and kicking velocity pre-match, at half-time and post-match, contrasting results according to playing position. In view of the above, we hypothesized that match fatigue would cause a clear reduction in the values obtained for the variables analysed in all players expecting changes in results regarding the position of the players.

Materials and methods

We used a repeated measures design to assess the effects of regular soccer matches on various performance factors. In order to determine the influence of accumulated match fatigue, the study was conducted during the last third of the playing season, analysing official federation matches. This study was conducted in accordance with the guidelines found in the Helsinki Declaration which establishes ethical principles for investigations using human beings. and was approved by the Ethics Committee of the University of Leon.

Participants

Participants comprised one hundred ninety-eight healthy male soccer players with a mean age of 17.56 ± 0.78 years old and a mean of 9.50 ± 3.11 years’ experience of playing in federation soccer teams. Participants belonged to four different Spanish youth clubs of the same geographic area (Castilla y León, Spain), with same chronological ages and competing in the same youth Spanish national category with regular training. All subjects were in good health and were not taking medication or nutritional supplements that could influence the experimental protocol. Participants were divided into five groups based on playing position and player characteristics: goalkeepers (n = 24), defenders (n = 51), midfielders (n = 36), wingers (n = 54) and forwards (n = 33). Demographic and anthropometric data are shown in Table 1. Study participation was voluntary. All players, parents and coaches were notified of the research procedures, requirements, benefits and risks before giving written informed consent.

Table 1

Descriptive statistics (mean ± standard deviation).

	All players (n = 198)	Goalkeepers (n = 24)	Defenders (n = 51)	Midfielders (n = 36)	Wingers (n = 54)	Forwards (n = 33)
Age (years)	17.56±0.78	17.21±0.84	17.58±0.77	17.55±0.87	17.71±0.72	17.57±0.72
Height (cm)	178.09±5.63	178.00±4.81	180.76±5.88	178.50±6.96	174.72±3.88	179,19±3.80
Weight (kg)	69.03±7.84	71.13±5.43	71.71±6.72	70.75±10.13	65.22±6.73	67.76±7.42
Body Mass Index (kg/m2)	21.74±2.10	22.47±1.81	21.96±1.96	22.18±2.91	21.34±1.86	21.05±1.57
Experience (years)	9.51±3.12	8.38±3.37	10.18±2.02	9.67±2.20	8.83±3.83	10.24±3.54

Procedure

Testing procedures were performed during competition seasons 2016–2017 and 2017–2018. Prior to data collection at matches, players were familiarised with the tests by performing them during training. The tests were then conducted at thirty-three different matches, played in the second part of each analysed season. We selected players who would play for the maximum number of minutes, identified in a prior conversation with the coach. To ensure efficiency and interfere as little as possible in match preparation, the tests were conducted with six players for each match, including at least one player for each of the five positions analysed. To standardise procedure, all tests were performed using the same protocol and in the same order before, during and after the match. As can be seen in Fig 1, the tests were performed after pre-match warm-up (T1), at half-time (T2) and post-match (T3). If a player was substituted, he performed the tests at that time provided that he had completed at least 50% of the second half. All matches were played on natural grass surface and no were register data of matches in rainy days. The average temperature was between 8°C and 20 oC (data obtained from the Spanish Meteorological Agency www.aemet.es)

Fig 1

Timing and order of tests.

Kicking velocity

Kicking velocity was measured as the maximum ball velocity when aiming at the goal, according to the protocol described by Sedano et al. [21]. Velocity was expressed in kilometers per hour (km/h), measured using a Stalker´s type hyperfrequency radar (Stalker Professional Radar, Radar Salts, Plymouth, MA, USA). The players made 3 attempts with 60 seconds’ rest between kicks, and the best result in km/h was used to analyse the KICK variable.

Acceleration capacity

Acceleration capacity was assessed by means of a 40 m sprint test conducted in an adjoining field with an identical surface to that where the match was played, and timed using a single beam photocell system (DSD Laser System, León, Spain). Two photocells were sited at the start, another two at 30 m and two at 40 m. Players started the sprint test half a meter behind the first two photocells [22]. A visual stimulus was used to indicate to players when the cells were ready to start, and then players could start, on their own, a 40-m sprint at maximum intensity. They performed this test once, and times for the first 30 m (30mACCEL), the last 10 m (10mACCEL) and the total 40 m (40ACCEL) were analysed.

Explosive force-jump capacity

Players’ jump capacity was measured using a laser platform (SportJUMP System PRO, DSD Inc., Spain), placed in a small area adjacent to the team dressing room with a surface similar to that of the match surface. Keeping their hands at the waist, players made 3 attempts at the CMJ [23], with 60 seconds’ rest between jumps. The best result in centimeters was analysed for the CMJ variable.

Statistical analysis

Data analysis was carried out using the Statistical Package for Social Sciences (SPSS 24.0). The Kolmogorov-Smirnov test (K-S) was used to determine normal distribution of the variables analysed. Standard statistical methods were used to calculate means and standard deviations (SD). The Student’s t-test was used to determine differences between initial values for the 5 groups and the variables analysed. Effects related to the match were assessed using one-way ANOVA (time) with repeated measures, post Levene's test for equality of variances (p>0.05). When Wilks’ Lambda indicated a significant F-value, Bonferroni post-hoc tests were performed to determine pairwise differences. Statistical significance was set at p<0.05. The effect size (ES) was estimated using Cohen’s d [24], classifying ES as “small” (0.2–0.3), “medium” (0.4–0.7) and “large” (>0.8).

Results

The Kolmogorv-Smirnov test confirmed that the variables presented a normal distribution (p>0.05). The Student’s t-test indicated that there were no significant differences between groups at baseline (p>0.05). Table 2 shows the data for the variables analysed at the three times (T1, T2 and T3) and Table 3 gives the same variables for each playing position.

Table 2

Descriptive data for the variables kicking velocity (KICK), acceleration (ACCEL) and jump capacity (CMJ).

	T1	T2	T3	Time
Variables	Mean ± SD	Mean ± SD	Mean ± SD	F (ES)
KICK (km/h)	97.83±6.16	96.17±5.44	93.31±5.74	71.003* (0.448)
30mACCEL (s)	4.68±0.24	4.75±0.33	4.82±0.25	51.092* (0.349)
40mACCEL (s)	5.95±0.31	6.02±0.28	6.16±0.31	63.156* (0.396)
10mACCEL (s)	1.26±0.10	1.28±0.16	1.33±0.11	41.323* (0,306)
CMJ (cm)	35.72±4.68	34.65±4.79	34.14±4.78	41.281* (0.301)

* p< .05

Table 3

Descriptive data and percentages of variation for the variables kicking velocity (KICK), acceleration (ACCEL) and jump capacity (CMJ), by playing position.

		T1	T2	T3	Percentage of variation
Variables	Position	Mean ± SD	Mean ± SD	Mean ± SD	T1-T2	T1-T3	T2-T3
KICK (km/h)	GOALKEEPER	93.43a±7.28	92.29b±6.14	90.14c±5.88	-1.22%	-3.52%	-2.32%
	DEFENDER	97.87a±6.01	95.60b±5,52	92.93c±5.80	-2.32%	-5.04%	-2.79%
	MIDFIELDER	99.18a±6.42	96,55b±5,27	92.72c±5.86	-2,66%	-6.51%	-3.95%
	WINGER	97.00a±4.66	96.12a±4.45	94.18b±5.14	-0.91%	-2.91%	-2.02%
	FORWARD	100.71a±5.86	99.29a±4.90	95.22b±5.62	-1.41%	-5.44%	-4.09%
30mACCEL (s)	GOALKEEPER	4.85±0.21	4.87±0.19	4.91±0.21	-0.49%	-1.32%	-0.82%
	DEFENDER	4.68a±0.22	4.88b±0.50	4.88b±0.23	-4.16%	-4.14%	0.03%
	MIDFIELDER	4.76a±0.24	4.74a±0.25	4.88b±0.23	0.44%	-2.40%	-2.85%
	WINGER	4.59a±0.22	4.65b±0.25	4.79c±0.31	-1.47%	-4.42%	-2.90%
	FORWARD	4.61a±0.24	4.63 a±0.15	4.71b±0.16	-0.64%	-2.11%	-1.46%
40mACCEL (s)	GOALKEEPER	6.15±0.27	6.17a±0.29	6.22b±0.31	-0.24%	-1.12%	-0.87%
	DEFENDER	6.00a±0.31	6.08a±0.27	6.23 b±0.29	-1.28%	-3.77%	-2.46%
	MIDFIELDER	6.04a±0.34	6.06a±0.31	6.22b±0.31	-0.33%	-3.08%	-2.74%
	WINGER	5.82a±0.27	5.95b±0.28	6.10c±0.38	-2.33%	-4.80%	-2.42%
	FORWARD	5.83a±0.24	5.88a±0.16	6.03b±0.15	-0.92%	-3.37%	-2.42%
10mACCEL (s)	GOALKEEPER	1.3±0.12	1.29±0.12	1.31±0.14	0.64%	-0.38%	-1.03%
	DEFENDER	1.29a±0.09	1.31a±0.11	1.34b±0.12	-1.26%	-4.12%	-2.83%
	MIDFIELDER	1.27a±0.12	1.32b±0.08	1.35b±0.09	-3.20%	-5.60%	-2.32%
	WINGER	1.23a±0.07	1.3b±0.08	1.33b±0.12	-5.85%	-7.75%	-1.79%
	FORWARD	1.25a±0.07	1.26a±0.05	1.32b±0.07	-1.21%	-6.13%	-4.86%
CMJ (cm)	GOALKEEPER	35.26a±7.26	34.39±7.30	33.42b±8.01	-2.48%	-5.22%	-2.80%
	DEFENDER	35.32±3.56	34.79±4.41	34.88±3.17	-1.50%	-1.23%	0.27%
	MIDFIELDER	33.77±5.74	32.44±5.03	31.81±4.13	-3.92%	-5.79%	-1.94%
	WINGER	35.87a ±3.62	34.23b ±3.68	33.35b ±4.56	-4.56%	-7.02%	-2.58%
	FORWARD	38.59±2.46	37.69±2.60	37.36±2.59	-2.33%	-3.17%	-0.86%

Means on the same line with the same subscript do not present significant differences at p<0.05.

Kicking velocity

For the variable KICK, we found significant differences between times (p = 0.00; ES = 0.396). The Bonferroni post-hoc test identified differences between T1 and T2 (p = 0.00; ES = 0.285) and T3 (p = 0.00; ES = 0.758) and between T2 and T3 (p = 0.00; ES = 0.510). As regards differences by playing position (Fig 2), the Bonferroni post-hoc test identified the following significant differences: in goalkeepers between T1 and T2 (p = 0.033; ES = 0.167) and T3 (p = 0.006; ES = 0.488) and between T2 and T3 (p = 0.033; ES = 0.351); in defenders between T1 and T2 (p = 0.00; ES = 0.390) and T3 (p = 0.00; ES = 0.829) and between T2 and T3 (p = 0.00; ES = 0.468); in midfielders between T1 and T2 (p = 0.00; ES = 0.444) and T3 (p = 0.00; ES = 1.038) and between T2 and T3 (p = 0.00; ES = 0.678); in wingers between T1 and T3 (p = 0.00; ES = 0.571) and between T2 and T3 (p = 0.001; ES = 0.401); and in forwards between T1 and T3 (p = 0.00; ES = 0.983) and between T2 and T3 (p = 0.00; ES = 0.771).

Fig 2

Changes in kicking velocity (KICK) by playing position.

Acceleration capacity

As regards acceleration capacity, Table 2 shows that the variable 30mACCEL presented significant differences between times (p = 0.00; ES = 0.301). The Bonferroni post-hoc test identified differences between T1 and T2 (p = 0.00; ES = 0.305) and T3 (p = 0.00; ES = 0.570). As regards differences in this variable by playing position (Fig 3), the Bonferroni post-hoc test identified the following significant differences: in defenders between T1 and T2 (p = 0.026; ES = 0.502) and T3 (p = 0.00; ES = 0.859); in midfielders between T1 and T3 (p = 0.00; ES = 0.483) and between T2 and T3 (p = 0.00; ES = 0.484); in wingers between T1 and T2 (p = 0.003; ES = 0.248) and T3 (p = 0.00; ES = 0.753) and between T2 and T3 (p = 0.00; ES = 0.474); and in forwards between T1 and T3 (p = 0.017; ES = 0.481) and between T2 and T3 (p = 0.015; ES = 0.384).

Fig 3

Changes in acceleration capacity (30mACCEL) by playing position.

For the variable 40mACCEL(Table 2), we found significant differences between times (p = 0.00; ES = 0.396). The Bonferroni post-hoc test identified differences between T1 and T2 (p = 0.00; ES = 0.236) and T3 (p = 0.00; ES = 0.676) and between T2 and T3 (p = 0.00; ES = 0.451). As regards differences by playing position (Fig 4), the Bonferroni post-hoc test identified the following significant differences: in goalkeepers between T2 and T3 (p = 0.014; ES = 0.177); in defenders between T1 and T2 (p = 0.017; ES = 0.263) and T3 (p = 0.00; ES = 0.748) and between T2 and T3 (p = 0.00; ES = 0.535); in midfielders between T1 and T3 (p = 0.00; ES = 0.569) and between T2 and T3 (p = 0.00; ES = 0.474); in wingers between T1 and T2 (p = 0.00; ES = 0.493) and T3 (p = 0.00; ES = 0.838) and between T2 and T3 (p = 0.00; ES = 0.220); and in forwards between T1 and T3 (p = 0.00; ES = 0.968) and between T2 and T3 (p = 0.00; ES = 0.914).

Fig 4

Changes in acceleration capacity (40mACCEL) by playing position.

As shown in Table 2, the variable 10mACCEL presented significant differences between times (p = 0.00; ES = 0.306). The Bonferroni post-hoc test identified differences between T1 and T3 (p = 0.00; ES = 0.664) and between T2 and T3 (p = 0.00; ES = 0.363). As regards differences by playing position (Fig 5), the Bonferroni post-hoc test identified the following significant differences: in defenders between T3 and T1 (p = 0.001; ES = 0.489) and T2 (p = 0.002; ES = 0.317); in midfielders between T1 and T2 (p = 0.016; ES = 0.395) and T3 (p = 0.00; ES = 0.664); in wingers between T1 and T2 (p = 0.00; ES = 0.908) and T3 (p = 0.00; ES = 0.915); and in forwards between T1 and T3 (p = 0.00; ES = 1.061) and between T2 and T3 (p = 0.00; ES = 0.152).

Fig 5

Changes in acceleration capacity (10mACCEL) by playing position.

Explosive force- jump capacity

As shown in Table 2, the results revealed significant differences between times (p = 0.00; ES = 0.301) for the variable CMJ. The Bonferroni post-hoc test identified differences between T1 and T2 (p = 0.00; ES = 0.093) and T3 (p = 0.00; ES = 0.657).

As regards differences between times by playing position (Fig 6), the Bonferroni post-hoc test identified the following significant differences: in goalkeepers between T1 and T2 (p = 0.007; ES = 0.104) and T3 (p = 0.00; ES = 0.209) and between T2 and T3 (p = 0.009; ES = 0.109); in midfielders between T1 and T3 (p = 0.001; ES = 0.339); in wingers between T1 and T2 (p = 0.00; ES = 0.388) and T3 (p = 0.00; ES = 0.531) and between T2 and T3 (p = 0.00; ES = 0.185); and in forwards between T1 and T2 (p = 0.029; ES = 0.309) and T3 (p = 0.003; ES = 0.421).

Fig 6

Changes in explosive force (CMJ) by playing position.

Discussion

Due to the paucity of studies conducted in competitive contexts, the aim of the present study was to analyse the acute effects of a soccer match on jump capacity, sprint times and kicking velocity in under-19 players. We expected to find a general decline in performance due primarily to muscle fatigue occasioned by the demands of the match [25]. We also expected to find that the effect of this fatigue would vary between players according to playing position, since each position involves specific features and functions [1]. Both hypotheses were confirmed by our study. We found that performance declined over time for all the variables analysed, and also that this was associated with playing position, in under-19 soccer players.

In a literature review, Rodríguez Lorenzo et al. [26] reported values for the kicking velocity test according to age, sex, dominant leg, experience, competition level and playing position, and found differences between defenders and midfielders and forwards. In contrast to these findings, our results indicated that kicking velocity was similar for all playing positions except goalkeepers, with forwards obtaining slightly higher values.

With respect to the reduction in kicking velocity caused by match fatigue, it has been reported that accuracy, kick success rate and maximum ball velocity decline significantly in under-19 players in the second half [27]. This finding is consistent with our results, since we observed a statistically significant decline in performance between the start and end of the match and between half-time and the end of the match for all playing positions. It therefore seems clear that fatigue exerts a negative effect on kicking velocity by reducing players’ capacity to transfer velocity from leg to ball [28], possibly due to an increase in lactate production and a reduction in glycogen levels, which also affects coordination [10]. However, some studies have reported that kicking velocity is affected by body posture, kicking technique, boots or a reduced ability to transfer energy to the ball due to loss of muscle strength [29]. Studies of young players [30] have also found that good hamstring flexibility exerts an important influence not only on kicking velocity, but also on jump capacity and sprint times.

Sander et al. [31] related vertical strength training programs to improved sprint times in young soccer players. Along the same lines, a marked decline in CMJ and sprint performance has been reported, indicating that several aspects of the capacity to generate strength are compromised in male and female soccer matches alike [32]. In this study, it was found a significant decline in CMJ and sprint performance, although the above studies assessed the 20-m sprint rather than the 30-m sprint analysed here, and did not differentiate by playing position. Thus, it could be inferred that in the case of the CMJ, changes related to muscle fatigue become evident immediately after the match and are long-lasting [33], while in the case of the sprint, the different rates of recovery between the sprint and the jump may be related to changes in biomechanical behaviour (the duration of concentric and eccentric phases), resulting in neuromuscular fatigue that affects jump performance to a greater extent [33].

Carlin et al. [34] have argued that speed tests may be more appropriate than jump tests to assess performance and fatigue. Recently, it has been demonstrated that performance of a short sprint (10–20 m) in a straight line lacks sensitivity as an indicator of physical fatigue immediately post-match [35]. Such a short distance does not alter times but does affect hip biomechanics, with changes in flexion and extension angles [36]. Nagahara et al. [37] have suggested using running distances in excess of 30 m in order to improve assessment of the effect of post-match fatigue on soccer players’ maximum speed capacity, and obtained differences in this capacity (p = 0.038) when they used a distance of 35 m. However, this effect of fatigue has also been demonstrated using distances of up to 60 m [35]. Our results for all playing positions except goalkeepers and all distances (10, 30 and 40 m) indicate a statistically significant decline in performance between the start and end of the match and between half-time and the end. The reason for the lack of such a difference could be considered to result from the nonexistence of the repeated sprints because of the position of goalkeepers during the match [10].However, even though the distance of 10 m has been studied in other sports, such as basketball [17], it is important to emphasise that these results cannot be compared with our findings since our distance of 10 m was measured at the end of the 40-m test (between 30 m and 40 m) and therefore represented a full speed.

Assessing jump capacity in soccer players by means of the CMJ test it has also proved a reliable indicator of fatigue in soccer players [33]. The fatigue produced over the course of a match was evident in all players analysed in our study except for the defenders. In this sense, sprint times and vertical jump capacity are related as aspects to assess soccer players’ anaerobic profile [3], and defenders spend a significantly less amount of time sprinting and running than the other positions [1]. In a study by Romagnoli et al. [38], players of a similar age to those in our study obtained a better result (49.6±5.1 cm pre-match) than our participants (35.72 cm), but the decline in their performance was similar to the mean obtained in our study. In addition, there is an acute effect on the isometric strength of the associated hamstrings during knee flexion [39], a basic movement in jump technique. This post-match fatigue remains significant 24 hours post- match for this test [7], and in most cases there is no functional recovery until the 72 hours post-match [35, 38]. However, Stone et al. [35] found no significant differences in this decline in their study. It should be noted that age could also be a factor in this test, because although the effect of fatigue has been elucidated in young players [40]. In other sports such as handball [15] and rugby [16], post-match fatigue has been shown to affect jump capacity, no significant decline has been observed.

Although it is acknowledged that there are a number of limiting factors which may account for the present findings, we believe that the present design has high levels of validity and demonstrates the need for further research to continue identifying and quantifying the physical requirements—and their inter-relationships—of different playing positions. In that sense, must be taken into consideration that our sample not investigate how nature of the game (importance of the game or superiority of the opponent) or weather conditions could influence the measurement results. Furthermore, it would be beneficial to explore other player profiles, levels and categories, such as professional or female soccer players, and to study these effects at different times of the playing season, because in the final months of the competitive mesocycle (a decisive phase in the season), the cumulative load of several months of training and competition may exert a different effect to that in other periods.

Conclusions and practical applications

In this study, we have quantified the decline in performance of tests of jump capacity, sprint times and kicking velocity produced as a result of the demands of a soccer match in under-19 players. This decline in the performance of the tests did not affect all playing positions equally, demonstrating the need for individualised fitness training.

Our results provide useful information for coaches and trainers with respect to the organisation of training. In order to attain good physical performance, it is necessary to minimise the effects of match fatigue and accelerate post-match recovery. This can be achieved by tailoring training strategies and programs to each of the players’ needs.

Supporting information.

(XLSX)

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2 Jan 2020

PONE-D-19-31381

Influence of competition on performance factors in under-19 soccer players at national league level

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Reviewers' comments:

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Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

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Reviewer #1: Partly

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: No

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This study aims to examine the influence of competition on performance factors in under-19 soccer players at national league level. The study is quite intriguing, however there are some serious methodological flaws, that can not be neglected. The manuscript is rather well written, although some improvement on that field is also needed. Please find general and specific comments below:

General comments:

1. Introduction needs some thorough revision, regarding writing style. From the perspective of potential readers, sentences seem to be hard to read and connect. It appears to me they are just tossed one after another. Some fluency is needed. Authors should “tell us a story” about this topic (based on the previous research, of course).

2. Since the players were observed by position in team, descriptive data (age, height, weight, experience) should be presented by their position as well (with appropriate statistics to show potential differences). Moreover, BMI data would also be beneficial for this manuscript.

3. One of the major methodological flaws in this manuscript is the fact that the authors tested players in different occasions during the extensive period of time. Great number of factors can influence this methodological design, such as time of a day, different weather conditions (temperature, humidity, wind...), nature of the game (important game of not, better or inferior opponent...), type of the surface (some football fields have better quality of grass, thus allowing greater speeds and better results)... Overall, great number of factors was not controlled by the researchers. Therefore, results could not be accepted such as.

4. This manuscript is lack of study limitations paragraph within the discussion chapter.

Specific comments:

Page 4, Lines 74-75: Hypothesis should be more informative, regarding players’ position. Do you expect changes in results regarding the position of the players or not?

Page 5, Lines 87-89: Can participants give consent by themselves, since majority of them are underage?

Page 6, Line 117: Did they use visual stimulus as a signal to start immediately, or that the cells are ready, and they can start on their own?

Page 6, Line 123: Please specify, how can surface in dressing room be similar to the one on the football field?

Page 5, procedure: I did not noticed explanation for test order in procedure. Authors should specify that.

Page 7, statistical analysis: Leven test should be performed prior to the all ANOVAs, especially since there are unequal numbers of participants within the groups.

Page 7, Lines 134-135: Can authors be more precise? Is Cohen’s coefficient, Cohen's D or something else?

Page 7, Line 140: Please present p level.

Page 11, Line 219: In introduction, only one hypothesis is presented. Please, make sure that aims and hypothesis are the same in abstract, introduction, discussion and conclusion.

Page 11, Line 224: Please change “forward.”

Page 11-12, Lines 227-238: Authors should specifically discuss why there are differences between goalkeepers and other players.

Page 12, Line 242: Please use “in this study” and “it was found”.

Page 12-13, Discussion: Authors did not provide sufficient explanation regarding differences between positions. Discussion was primarily focused on overall decline in performance.

Page 14, Line 280: Please avoid terms such as “interesting”. For example, say: “it would be beneficial (for coaches, scientists) to explore”…

Reviewer #2: This is very interesting work. Easy for reading and good in all methodical procedures. It is a specially interesting for sport sciences and of course for football coaches. Of course it will be interesting that in work we have more date about number of sprinting, jumps and acceleration and deceleration and changing of direction of every player in games.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

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16 Jan 2020

By the academic editor

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming.

- It was revised.

2. In your Methods section, please provide additional information about the participant recruitment method and the demographic details of your participants. Please ensure you have provided sufficient details to replicate the analyses such as:

a) the recruitment date range (month and year).

- In page 5, this paragraph was modified (lines 100-103):

Testing procedures were performed during competition seasons 2016-2017 and 2017-2018. Prior to data collection at matches, players were familiarised with the tests by performing them during training. The tests were then conducted at thirty-three different matches, played in the second part of each analysed season.

b) a description of any inclusion/exclusion criteria that were applied to participant recruitment

- In page 5, this paragraph was added (lines 88-91):

Participants belonged to four different Spanish youth clubs of the same geographic area (Castilla y León, Spain), with same chronological ages and competing in the same youth Spanish national category with regular training. All subjects were in good health and were not taking medication or nutritional supplements that could influence the experimental protocol.

c) a table of relevant demographic details

- Table 1 was updated including data by positions and IMC

d) a statement as to whether your sample can be considered representative of a larger population,

- Considering that the category involves 14 teams in competition and that a total of 182 matches are played, we can regard our sample (33 matches) as representative a priori. Which is confirmed by the normality of the sample and the significance of the differences found in the parametric tests. All this, reinforced by the size of the calculated effect.

e) a description of how participants were recruited

It is the same that answered in section b: Participants belonged to four different Spanish youth clubs of the same geographic area (Castilla y León, Spain)

f) descriptions of where participants were recruited and where the research took place.

- In page 5, this paragraph was added (lines 88-91):

Participants belonged to four different Spanish youth clubs of the same geographic area (Castilla y León, Spain), with same chronological ages and competing in the same youth Spanish national category with regular training.

3. Please provide additional details regarding participant consent.

In page 5, lines 95-96 were added:

- All players, parents and coaches were notified of the research procedures, requirements, benefits and risks before giving written informed consent.

In the ethics statement in the Methods and online submission information, please ensure that you have specified (1) whether consent was informed and (2) what type you obtained (for instance, written or verbal).

- In page 4, this paragraph (lines 82-84) was added:

This study was conducted in accordance with the guidelines found in the Helsinki Declaration which establishes ethical principles for investigations using human beings. and was approved by the Ethics Committee of the University of Leon.

4. We note that you have indicated that data from this study are available upon request. PLOS only allows data to be available upon request if there are legal or ethical restrictions on sharing data publicly. For more information on unacceptable data access restrictions, please see http://journals.plos.org/plosone/s/data-availability#loc-unacceptable-data-access-restrictions.

It was an error. There are no restrictions and we upload our data set necessary to replicate our study findings.

5. Thank you for stating the following financial disclosure:

- Included in the cover letter:

The authors received no specific funding for this work.

By reviewer #1:

1. Introduction needs some thorough revision, regarding writing style. From the perspective of potential readers, sentences seem to be hard to read and connect. It appears to me they are just tossed one after another. Some fluency is needed. Authors should “tell us a story” about this topic (based on the previous research, of course).

We have thoroughly reviewed the introduction with the reader in mind proceeding from the general to the particular. We have tried to link ideas, so it is easier for the reader to understand the structure adding connectors such as furthermore, hence, however or thus.

2. Since the players were observed by position in team, descriptive data (age, height, weight, experience) should be presented by their position as well (with appropriate statistics to show potential differences). Moreover, BMI data would also be beneficial for this manuscript.

In page 5, table 1 was updated including data by positions and BMI

3. One of the major methodological flaws in this manuscript is the fact that the authors tested players in different occasions during the extensive period of time. Great number of factors can influence this methodological design, such as time of a day, different weather conditions (temperature, humidity, wind...), nature of the game (important game of not, better or inferior opponent...), type of the surface (some football fields have better quality of grass, thus allowing greater speeds and better results)... Overall, great number of factors was not controlled by the researchers. Therefore, results could not be accepted such as.

We believe this consideration is very important. First, we made it clear that no match data was recorded on rainy days, although it was not a condition taken into account a priori, since the photocells and radar are not prepared for rain. On the other hand, the AEMET (Spanish Meteorological Agency) database was consulted and the temperature range of the days and time of the matches was established.

So, this paragraph was included in lines 110-113 (page 6):

All matches were played on natural grass surface and no were register data of matches in rainy days. The average temperature was between 8oC and 20oC (data obtained from the Spanish Meteorological Agency www.aemet.es)

Regarding the “nature of the game”, since it has not been included in the study variables, a comment is attached to its limitations.

Added in lines 294-304 (page 14), changing the existing paragraph:

Although it is acknowledged that there are a number of limiting factors which may account for the present findings, we believe that the present design has high levels of validity and demonstrates the need for further research to continue identifying and quantifying the physical requirements —and their inter-relationships— of different playing positions. In that sense, must be taken into consideration that our sample not investigate how nature of the game (importance of the game or superiority of the opponent) or weather conditions could influence the measurement results. Furthermore, it would be beneficial to explore other player profiles, levels and categories, such as professional or female soccer players, and to study these effects at different times of the playing season, because in the final months of the competitive mesocycle (a decisive phase in the season), the cumulative load of several months of training and competition may exert a different effect to that in other periods.

4. This manuscript is lack of study limitations paragraph within the discussion chapter.

Specific comments:

Page 4, Lines 74-75: Hypothesis should be more informative, regarding players’ position. Do you expect changes in results regarding the position of the players or not?

In page 4, lines 76-77 was added:

…expecting changes in results regarding the position of the players.

Page 5, Lines 87-89: Can participants give consent by themselves, since majority of them are underage?

In page 5, lines 95-96 were added:

All players, parents and coaches were notified of the research procedures, requirements, benefits and risks before giving written informed consent.

Page 6, Line 117: Did they use visual stimulus as a signal to start immediately, or that the cells are ready, and they can start on their own?

In pages 6 and 7, lines 128-129 were modified:

A visual stimulus was used to indicate to players when the cells were ready to start, and then players could start, on their own, a 40-m sprint at maximum intensity.

Page 6, Line 123: Please specify, how can surface in dressing room be similar to the one on the football field?

Page 7, line 134, this paragraph was modified:

“placed in a small area adjacent to the team dressing room”

Page 5, procedure: I did not noticed explanation for test order in procedure. Authors should specify that.

This point it exposed in Fig 1, cited in page 6:

Page 7, statistical analysis: Leven test should be performed prior to the all ANOVAs, especially since there are unequal numbers of participants within the groups.

Data analysis was carried out using the Statistical Package for Social Sciences (SPSS 24.0) and Leven test is included the protocol of ANOVA calculations.

So we have included in line 139. Data analysis was carried out using the Statistical Package for Social Sciences (SPSS 24.0).

Added in line 144: post Levene's test for equality of variances (p >0.05).

Page 7, Lines 134-135: Can authors be more precise? Is Cohen’s coefficient, Cohen's D or something else?

It is analized the Cohen’s d effect sizes related to Cohen J. Statistical power analysis for the behaviors science.(2nd). New Jersey: Laurence Erlbaum Associates, Publishers, Hillsdale. 1988.

So we have rewritten the sentence in lines 146-148:

The effect size (ES) was estimated using Cohen’s d [24], classifying ES as “small” (0.2-0.3), “medium” (0.4-0.7) and “large” (>0.8).

Page 7, Line 140: Please present p level.

It was added (p>0.05) in line 144

Page 11, Line 219: In introduction, only one hypothesis is presented. Please, make sure that aims and hypothesis are the same in abstract, introduction, discussion and conclusion.

In page 4, lines 76-77 was added so this aspect has been corrected:

…expecting changes in results regarding the position of the players

Page 11, Line 224: Please change “forward.”

We don’t understand what to do. The word forwards in included in a cite of Rodríguez Lorenzo et al.

Page 11-12, Lines 227-238: Authors should specifically discuss why there are differences between goalkeepers and other players.

When we have review our results (page 9, lines 161-170) we realized that there are no differences between goalkeepers and other players so we have modified this aspect (line 242) and we believe it is no necessary to discuss about this.

Page 12, Line 242: Please use “in this study” and “it was found”.

It was changed (Line 253).

Page 12-13, Discussion: Authors did not provide sufficient explanation regarding differences between positions. Discussion was primarily focused on overall decline in performance.

At this regard it has been added the next paragraphs:

Page 13, Lines 271-273:

The reason for the lack of such a difference could be considered to result from the nonexistence of the repeated sprints because of the position of goalkeepers during the match [10].

Pages 13-14, Lines 280-283:

In this sense, sprint times and vertical jump capacity are related as aspects to assess soccer players’ anaerobic profile [3], and defenders spend a significantly less amount of time sprinting and running than the other positions [1].

Page 14, Line 280: Please avoid terms such as “interesting”. For example, say: “it would be beneficial (for coaches, scientists) to explore”…

It was changed (Line 300)

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

21 Feb 2020

Influence of competition on performance factors in under-19 soccer players at national league level

PONE-D-19-31381R1

Dear Dr. Redondo Castán,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

Within one week, you will receive an e-mail containing information on the amendments required prior to publication. When all required modifications have been addressed, you will receive a formal acceptance letter and your manuscript will proceed to our production department and be scheduled for publication.

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With kind regards,

Dragan Mirkov, Ph.D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: All raised issues, comments and questions have been thoroughly addressed by the authors of this paper.

Reviewer #2: This study aims to examine the influence of competition on performance factors in under-19 soccer players at national league level. I still think that this is very interesting work. Easy for reading and good in all methodical procedures. It is a specially interesting for sport sciences and of course for football coaches.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

6 Mar 2020

PONE-D-19-31381R1

Influence of competition on performance factors in under-19 soccer players at national league level

Dear Dr. Redondo:

I am pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please notify them about your upcoming paper at this point, to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

For any other questions or concerns, please email plosone@plos.org.

Thank you for submitting your work to PLOS ONE.

With kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Dragan Mirkov

Academic Editor

PLOS ONE