Elite Youth Soccer Players' Physiological Responses,... : The Journal of Strength & Conditioning Research (original) (raw)
Introduction
Nowadays, small-sided games (SSGs) are often used in soccer training to combine physiological, technical, and tactical demands. To maximize adaptations during the demanding schedules of top-level soccer players, coaches should be able to carefully judge the impact of different SSG variables on training intensity to control training load. Hill-Haas et al. (11) reviewed various factors influencing SSGs (e.g., pitch size or number of players). Some of these factors have been investigated to a large extent, for example, the pitch dimensions (13,15,17,18,24) and the number of players (1,10,17,19,23,24). Other factors, however, such as coach encouragement, have not been addressed sufficiently. There are only few studies comparing the physiological response and ratio of perceived exertion (RPE) during SSGs with and without coach encouragement (18,19). In these studies, the heart rate (HR) response showed no or only a moderate effect, but all findings confirmed an increased RPE when consistent coach feedback was provided. Highly demanding SSGs will potentially increase perceived exertion and may induce mental fatigue, thereby limiting the game performance of the players, as shown by Smith et al. (21). Thus, if physiological adaptations and the development of game performance are aimed for, working at a very high intensity could negatively affect the development of technical and tactical performance during SSG.
The majority of studies published to date focused on physiological responses, RPE, and/or time-motion characteristics during SSG (for a brief overview, please refer to Hill-Haas et al. (11)). Few studies investigated technical and tactical performance and indicated a potential interaction between intensity and game performance (7,15–17,22). However, it is hypothesized that most SSG studies were below a critical intensity to negatively impact on game performance. Therefore, in this study commonly used SSGs with 4 vs. 4 players were used, ensuring a high intensity, and a strongly pushed coach feedback was added to increase the intensity even more. To our knowledge this is the first study examining the influence of coach encouragement on physiological response, RPE, time-motion characteristics, and game performance in SSGs with a high intensity (mean HR around 90% maximum HR [90% HRmax]). This approach would allow for showing coaches how to manipulate the physiological response, time-motion characteristics, RPE, and game performance by their type of feedback.
Methods
Experimental Approach to the Problem
In this study, the physiological response, perceived exertion, time-motion characteristics, and game performance were examined in elite youth soccer players during 2 different 4 vs. 4 SSG formats. All investigations were conducted within 2 weeks during late preseason. During the first week, the participants completed a treadmill test until maximum exhaustion (ELG2; Weil am Rhein, Woodway, Germany) to obtain V̇o2max and HRmax. All subjects started with a velocity of 10 km·h−1 that was increased by 1.8 km·h−1 every 3 minutes. A mobile oxygen analyzer including a wireless HR monitor (MetaMax 3b; Cortex Biophysik GmbH, Leipzig, Germany) permanently quantified V̇o2 and HR. During the second week, SSGs were played at the beginning of the regular training session in the afternoon. All participants maintained their standard nutrition as requested by the coaches. The players were familiar with the SSG design from their regular training sessions. This approach allowed identifying changes in the independent variables generated by varying just the type of feedback (strongly pushed vs. mild feedback).
Subjects
Sixteen youth male soccer players (age 17.2 ± 0.7 years [range: 16–18 years], height 182.3 ± 7.3 cm, body mass 73.5 ± 6.7 kg, V̇o2max 62.1 ± 3.8 ml·kg−1·min−1, HRmax 198.7 ± 6.4 b·min−1) participated in the study. They were members of the high performance under-19 team playing in the second highest national division in Germany. The procedures of the study were approved by the local ethics committee. Written informed consent was obtained from the players and additionally from the parents of players younger than 18 years. Training frequency included 4 days per week during the preseason. After consulting with the coach, the players were divided into 4 equally balanced teams with respect to their technical and tactical abilities as well as to the results of the treadmill test. Each team consisted of defenders, midfielders, and forwards.
Small-Sided Games
All subjects performed 2 different SSG formats, 1 with a strongly pushed coach encouragement (SSG-P) and 1 with only mild, unobtrusive feedback (SSG-M). In SSG-P, the coach judged each action of the players, encouraged the attacking team to put pressure on the goal and to score, and drove the defending team to conquer the ball. The feedback was given loudly in a demanding manner and continuously over the entire game duration. In SSG-M, mainly positive, commendatory feedback was provided in an unobtrusive manner. The frequency was much less than in SSG-P, about 1 feedback within 20 seconds of game play and was given equally to the attacking and defending teams.
SSGs were played within the same regular training week during preseason. Only 1 SSG was played per training day. In both SSG formats, there were 4 outfield players per side and 1 goalkeeper per team. Both games were played with unlimited touches of the ball, regular goals (7.32 × 2.44 m) and without offside. For scoring a goal, all teammates had to be in the attacking half of the field to avoid players from staying in the backfield. If a team scored, they were accorded ball possession with the ball being given to their goalkeeper. The SSGs were performed on an artificial pitch sized 40 × 40 m (relative grid area per player: 200 m2) as requested by the coach as a commonly used SSG format. The game duration was 3 × 4 minutes with 2 minutes passive rest during which blood samples were taken. A whistle signaled the end of each third. To avoid longer interruptions due to the ball leaving the field, replacement balls were immediately available inside the goals. Before the SSGs were started, the players completed a 15-minute standardized warm-up program.
Physiological Responses
Heart rate was recorded continuously using a HR monitor (RCX5; Polar Electro Oy, Kempele, Finland) at 5-second intervals. The stored data were downloaded to a personal computer (PC) afterward. Exercise intensity was expressed as %HRmax by relating raw values to the individual HRmax measured during the treadmill test. Four defined intensity zones differentiated exercise intensity: <75, 75–84, 85–90, and >90% HRmax (8). During passive rest, blood samples for blood lactate analysis were immediately taken from the right earlobe. Participants also assessed RPE scores of the previous game on a 15-point scale, ranging from 6 (very, very easy) to 20 (maximal exertion) (3).
Time-Motion Characteristics
Time-motion parameters were measured by small Global Positioning System (GPS) tracking system (Q1000Ex; Qstartz International Co. Ltd., Taipei, Taiwan) recording GPS data with a frequency of 5 Hz. The received data were downloaded later to a PC. The GPS systems were firmly fixed to the HR belt on the player's back. The study's independent variables, for analyzing GPS data, were total distance covered, distance traveled in 5 speed zones (0–6.9, 7.0–12.9, 13.0–17.9, 18.0–21.0, and >21.0, all in km·h−1), number of sprints, and work:rest ratio. The work:rest ratio is defined as the distance traveled at a speed ≥4 km·h−1 (period of activity or work) divided by the distance traveled at a speed <4 km·h−1 (period of recovery or rest) (5,12).
Game Performance
The players' individual game performances were rated using the Team Sports Assessment Procedure described in detail by Gréhaigne et al. (9). In short, 6 tactical actions were measured for each player throughout the game: the number of conquered balls (CBs; e.g., successfully tackling the ball-possessing opponent), the number of received balls (RBs; e.g., receiving and controlling a pass from a teammate), the number of lost balls (LBs; e.g., intercepted pass to teammate), the number of neutral balls (e.g., complete pass to teammate without adding pressure on the goal), the number of passes (P, e.g., complete pass to teammate with adding pressure on the goal), and the number of successful shots on goal (S). The common variables scaled by these tactical actions are volume of play (VP) = CB + RB, the efficacy index (EI) = (P + S)/(10 + LB), and the overall game performance score (PS) = (VP/2) + (EI × 10). Video recordings were later deconstructed to quantify variables and game performance.
Statistical Analyses
Descriptive statistics were calculated as mean ± SD and 95% confidence interval (CI). Statistically significant differences in the dependent variables were computed by _t_-test for paired samples (SPSS 23; IBM Corporation, Armonk, NY, USA). The independent variable was the SSG format. The significance level was set at p ≤ 0.05. Magnitude-based inferences between the SSG formats were calculated according to Batterham and Hopkins (2). The minimal clinically important difference was set to SD × 0.2 for all variables. Furthermore, the probabilities that the effect between SSGs was an increase, trivial, or a decrease of the observed parameter were expressed as percentages and translated to qualitative descriptors according to the following schema: <1%, almost certainly not; 1–5%, very unlikely; 5–25%, unlikely; 25–75%, possibly; 75–95%, likely; 95–99, very likely; and >99%, almost certain (2). Furthermore, Pearson correlation coefficients were calculated between measures of game performance and physiological and time-motion parameters.
Results
Due to illness and device malfunction, 2 players completed only 1 format and were excluded from the analysis, thus the results are based on complete data sets of 14 players. The mean HR did not change significantly between SSG-M (178.9 ± 4.5; 90.5 ± 2.8 %HRmax) and SSG-P (178.5 ± 5.2; 90.2 ± 1.8 %HRmax). Details of the physiological responses are given in Table 1. The RPE possibly increased in SSG-P (0.4 ± 1.2, 95% CI: −0.3 to 1.1; p = 0.27, Table 1). A statistically and qualitatively significant increase in game duration spent with less than 75% of HRmax is seen during SSG-M, whereas the time played with 85–90% of HRmax is possibly decreasing during SSG-P (Figure 1). The 95% CI in each intensity zone indicates a pronounced variability of the HR response to the SSG formats. The overall distance traveled was possibly shorter (−32 ± 104 m, 95% CI: −93 to 28 m, p = 0.26) in SSG-M (1,573 ± 133 m, 95% CI: 1,496 to 1,649 m) than in SSG-P (1,605 ± 122 m, 95% CI: 1,535 to 1,676 m). Minor differences were found for distances walked, jogged, or run with specific velocities (Table 2). The work:rest ratio was possibly increased (0.9 ± 5.5, 95% CI: −2.3 to 4.0; p = 0.57) during SSG-P (18.3 ± 5.8, 95% CI: 14.9 to 21.6) compared with SSG-M (17.4 ± 5.1, 95% CI: 14.5 to 20.4). Players were likely to perform more sprints (1.4 ± 4.0, 95% CI: −0.9 to 3.7; p = 0.23) during SSG-P (13.2 ± 4.5, 95% CI: 10.6 to 15.8) than during SSG-M (11.9 ± 3.8, 95% CI: 9.7 to 14.1).
Physiological parameters.*†‡
Time spent in intensity zones with respect to maximum heart rate (HRmax), expressed in percentage of game duration. Data are provided as mean ± SD (95% confidence interval). SSG-M = small-sided games with only mild, unobtrusive feedback; SSG-P = small-sided games with a strongly pushed coach encouragement.
Differences in distances walked, jogged, run, and sprinted (meters).*†‡
All measures of game performance were possibly or likely to decrease in SSG-P but did not reach the level of statistical significance (e.g., VP: −2.5 ± 4.8, 95% CI: −5.3 to 0.3; p = 0.08; Table 3). The time spent with 85–90% HRmax correlated to VP during both SSG-M (r = 0.5, p = 0.06) and SSG-P (r = 0.5, p = 0.06). The time spent with more than 90% of HRmax correlated negatively for SSG-P (r = −0.6, p = 0.03). The VP and PS correlated negatively with RPE only for SSG-P (r = −0.5, p = 0.04 and r = −0.4, p = 0.09, respectively). Additionally, the time spent above 21.0 km·h−1 correlated significantly with VP (r = 0.6, p = 0.03) in SSG-P.
Game performance measures, including scaled variables (upper part) and tactical actions (lower part).*†‡
Discussion
The aim of our study was to measure the impact of coach encouragement on the physiological response, time-motion characteristics, the RPE, and game performance during SSGs. Overall, only minor differences were observed between both SSG formats in the physiological response. The RPE and movement at higher velocities were pronounced in SSG-P, whereas game performance measures were lower in SSG-P.
The physiological response seen in this study group is more pronounced than that observed in the previous work of Brandes et al. (4) and other research (6,11). As this study group spent more time >90% HRmax, this presumably raised the blood lactate accumulation and RPE as compared with other studies. In part, slightly different game rules, such as requiring all teammates being in the opponents' half before scoring, could explain these differences, although this rule was used in the work by Brandes et al. (4) and in some other previous research (11). However, the load was still acceptable for the players to cover the whole game duration without having to prematurely stop playing.
In general, only a small impact of the coach feedback on the physiological response was found. The mean HR did not change during the 2 formats. Time played with less than 75% of HRmax was slightly increased during SSG-P but did not account for more than 7% of the overall playing time. In consequence, time played with 85–90% HRmax possibly decreased and blood lactate concentration possibly increased in SSG-P. The high variability in the HR response observed in our study compares well with that of previous studies (4,12), reflecting the dependency of the physiological response on the form of the day and the unstructured and stochastic movement characteristics of game formats in contrast to strict training regimes such as interval running (11). Additionally, more sprints and a higher work-rest ratio during SSG-P were observed. Therefore, it is assumed that a pushed coach feedback slightly increases the playing intensity by causing more short-distance sprints, taxing the anaerobic energy supply, and reducing the HR response. This is in accordance with the possible increase in the distances traveled with medium and fast velocities between 13 and 21 km·h−1. Regarding game performance, all parameters were likely to decrease or possibly decreased during SSG-P.
The negative correlations of the possibly increased RPE and game performance seen in SSG-P underline our hypothesis that the strongly pushed feedback by the coach did not increase the physiological response. On the contrary, it appears to add psychological stress, resulting in moving with higher velocities and poorer game performance, as demonstrated by the pronounced correlation between sprinting and VP. Therefore, strongly pushed feedback could provoke the mechanisms between RPE and game performance as described by Smith et al. (21). In our data, a decrease in actions on the ball was found as represented by lower numbers in all subcategories used to calculate the VP, EI, and PS. Thus, it is assumed that the psychological stress raised by the pressure of the coach pushing the players is primarily responsible for the decrease of game performance during SSG-P.
Given the unchanged physiological response but the likely lower game performance, a pushed feedback by the coach is not advisable for elite youth soccer players. This conclusion is in line with previous findings, demonstrating that intensity and RPE increases when SSGs are played under supervision by the coach, in contrast to SSGs without supervision by the coach (18,19). Our findings demonstrate that adding measures of game performance is beneficial in SSGs research because, otherwise, contradictory effects of high intensity during SSGs will not be discovered.
Our study does have some limitations that should be addressed. The use of 5-Hz GPS devices provides only limited accuracy in the measurement of time-motion characteristics, also with respect to our findings. Johnston et al. (14) found acceptable validity and reliability of 5-Hz devices for velocities <20 km·h−1 in a team sport simulation circuit and flying 50-m sprints. A recently published review demonstrates the deficiencies and conflicting results of previous validation studies (20). However, it is acknowledged that 5-Hz devices can accurately measure high velocities, when they occur after moderate velocity motion, for example, during sprinting, which is the case in team sports. However, future studies should determine the reliability of their GPS devices to calculate the error and minimal detectable changes for the entire range of soccer-specific actions, such as standing or change of directions.
As only 1 rater of game performance was included, no data on the reliability of game performance measures are available.
In this study, one of the most common SSG to evaluate the impact of coach encouragement on physiological response, time-motion characteristics, RPE, and game performance was chosen. It is assumed that the findings in this study are transferable to other SSGs; however, the dimension of the impact might vary between SSGs and in comparison to full-court soccer matches.
Practical Applications
The main practical relevance of the study findings is that a strongly pushed feedback provided by the coach does not change the physiological response in terms of an increased HR given that SSG are played with 4 players and at high intensity. On the contrary, a strongly pushed feedback increases the RPE and negatively affects the players' game performance, reducing the success rate of actions on the ball. According to the findings, coaches should be encouraged to provide only mild feedback, at least during soccer training with SSGs. This approach would provide a high stimulus for developing soccer-specific endurance capacities and for successful actions on the ball. Furthermore, the lower RPE when mild feedback is given will improve the tolerance of high training loads and training frequency.
The physiological response could possibly be raised by a strongly pushed feedback in less physiologically demanding SSG, although a deterioration of game performance can still be expected. However, future studies should investigate other SSG formats with different types of coaches' feedback and include measures of game performance.
Acknowledgments
The authors confirm that there is no conflict of interest.
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Keywords:
heart rate; global positioning system; blood lactate; RPE; TSAP
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