Latest research in football - week 29 - 2019

As previous literature updates, I have performed a PubCrawler search looking for football articles in NCBI Medline (PubMed) and GenBank databases.

Following studies were retrieved for this week:


#1 A new approach to study the relative age effect with the use of additive logistic regression models: A case of study of FIFA football tournaments (1908-2012)
Reference: PLoS One. 2019 Jul 16;14(7):e0219757. doi: 10.1371/journal.pone.0219757. eCollection 2019.
Authors: Saavedra-García M, Matabuena M, Montero-Seoane A, Fernández-Romero JJ
Download link:
Summary: The relative age effect plays an important role in the pursuit of excellence, providing advantage to athletes born at the beginning of the year or near the cut-off date. This phenomenon has been observed in areas such as sports, education or business. Traditionally, the chi-square test has been used to analyze whether there are statistically significant differences in the distribution of births in each of the four quarters of the year. However, this approach is limited, focusing only on the analysis of the response variable, without taking into account the effect of a set of predictive variables. In this paper a new approach is proposed to study the relative age effect with the use of a logistic regression additive model. The new method has been evaluated with a sample of 21,639 players involved in football tournaments organized by the Fédération Internationale de Football Association (FIFA) between 1908 and 2012. New conclusions have been established that the relative age effect exists regarding player age and the year of the competition in male FIFA competitions and its effect is dynamic and complex.

#2 Effect of a New Rule Limiting Full Contact Practice on the Incidence of Sport-Related Concussion in High School Football Players
Reference: Am J Sports Med. 2019 Jul 15:363546519860120. doi: 10.1177/0363546519860120. [Epub ahead of print]
Authors: Pfaller AY, Brooks MA, Hetzel S, McGuine TA
Summary: Sport-related concussion (SRC) has been associated with cognitive impairment, depression, and chronic traumatic encephalopathy. American football is the most popular sport among males in the United States and has one of the highest concussion rates among high school sports. Measured head impacts and concussions are approximately 4 times more common in contact practices compared with noncontact practices. The Wisconsin Interscholastic Athletic Association passed new rules defining and limiting contact during practice before the 2014 football season. The purpose was to determine if the SRC rate is lower after a rule change that limited the amount and duration of full-contact activities during high school football practice sessions. A total of 2081 high school football athletes enrolled and participated in the study in 2012-2013 (before the rule change), and 945 players participated in the study in 2014 (after the rule change). Players self-reported previous concussion and demographic information. Athletic trainers recorded athlete exposures (AEs), concussion incidence, and days lost for each SRC. Chi-square tests were used to compare the incidence of SRC in prerule 2012-2013 seasons with the incidence in the postrule 2014 season. Wilcoxon rank sum tests were used to determine differences in days lost because of SRC. A total of 67 players (7.1%) sustained 70 SRCs in 2014. The overall rate of SRC per 1000 AEs was 1.28 in 2014 as compared with 1.58 in 2012-2013 (P = .139). The rate of SRC sustained overall in practice was significantly lower (P = .003) after the rule change in 2014 (15 SRCs, 0.33 per 1000 AEs) as compared with prerule 2012-2013 (86 SRCs, 0.76 per 1000 AEs). There was no difference (P = .999) in the rate of SRC sustained in games before (5.81 per 1000 AEs) and after (5.74 per 1000 AEs) the rule change. There was no difference (P = .967) in days lost from SRC before (13 days lost [interquartile range, 10-18]) and after (14 days lost [interquartile range, 10-16]) the rule change. The rate of SRC sustained in high school football practice decreased by 57% after a rule change limiting the amount and duration of full-contact activities, with no change in competition concussion rate. Limitations on contact during high school football practice may be one effective measure to reduce the incidence of SRC.

#3 Influence of pitch size and age category on the physical and physiological responses of young football players during small-sided games using GPS devices
Reference: Res Sports Med. 2019 Jul 15:1-11. doi: 10.1080/15438627.2019.1643349. [Epub ahead of print]
Authors: Lemes JC, Luchesi M, Diniz LBF, Bredt SDGT, Chagas MH, Praça GM
Summary: This study aimed to compare the physical and physiological responses of young football players of different categories during small-sided games (SSGs) played on different pitch sizes. Forty-eight (24 U-13 and 24 U-14) athletes played a 3 vs. 3 + 1 SSG in two experimental conditions: regular (36 × 27 m) and large pitch sizes (40 × 29 m). The total distance covered, the distances covered at different speed zones (0 to 6.9 km/h, 6.9 to 14.3, and 14.3 to 21.4), maximum heart rate, and mean heart rate were recorded. The results showed that older athletes covered larger distances during SSGs (p = 0.001; d = 0.937; large effect) and lower distances at the lowest (0-6.9 km/h) speed zone (p = 0.001; d = 0.657; moderate-to-large effect). Neither the physical nor physiological variables (except for distance covered between 14.3 and 21.4 km/h) differed between pitch sizes. This result indicates that pitch size may not impact the physical or physiological responses of U-13 and U-14 players during SSGs, but differences between categories were found. In conclusion, the development of tactical skills may be desirable to better explore the available space in the same age categories.

#4 Comparative study on skill and health related physical fitness characteristics between national basketball and football players in Sri Lanka
Reference: BMC Res Notes. 2019 Jul 12;12(1):397. doi: 10.1186/s13104-019-4434-6.
Authors: Kariyawasam A, Ariyasinghe A, Rajaratnam A, Subasinghe P
Download link:
Summary: The purpose was to compare health and skill related physical fitness profiles between healthy, male, basketball and football players of Sri Lankan national teams. Thirty basketball players (mean age 24 ± 4.5 years) and 30 football players (mean age 23 ± 4.3 years) were evaluated for health related fitness characteristics (body fat percentage, cardio-respiratory fitness, isometric hand grip strength, lower body and upper body muscular strength, abdominal and upper body muscular endurance, and flexibility) and skill related fitness characteristics (agility, speed, explosive throwing power, jumping power, reaction time, coordination, static balance). Fat percentage, upper body endurance, grip strength, running speed, explosive power, jumping power, balance and coordination were significantly higher in basketball players than in footballers. Football players had better upper body strength, flexibility, reaction time and agility than those of basketball players. The latter two were statistically significant. Basketball players had better mean lower body strength, although not significant. Fitness characteristics were different between basketball and football players. The results have implications in tailoring training activities to improve relevant fitness characteristics.

#5 Effects of a six-week period of congested match play on plasma volume variations, hematological parameters, training workload and physical fitness in elite soccer players
Reference: PLoS One. 2019 Jul 25;14(7):e0219692. doi: 10.1371/journal.pone.0219692. eCollection 2019.
Authors: Saidi K, Zouhal H, Rhibi F, Tijani JM, Boullosa D, Chebbi A, Hackney AC, Granacher U, Bideau B, Ben Abderrahman A
Download link:
Summary: The aims of this study were to investigate the effects of a six-week in-season period of soccer training and games (congested period) on plasma volume variations (PV), hematological parameters, and physical fitness in elite players. In addition, we analyzed relationships between training load, hematological parameters and players' physical fitness. Eighteen elite players were evaluated before (T1) and after (T2) a six-week in-season period interspersed with 10 soccer matches. At T1 and T2, players performed the Yo-Yo intermittent recovery test level 1 (YYIR1), the repeated shuttle sprint ability test (RSSA), the countermovement jump test (CMJ), and the squat jump test (SJ). In addition, PV and hematological parameters (erythrocytes [M/mm3], hematocrit [%], hemoglobin [g/dl], mean corpuscular volume [fl], mean corpuscular hemoglobin content [pg], and mean hemoglobin concentration [%]) were assessed. Daily ratings of perceived exertion (RPE) were monitored in order to quantify the internal training load. From T1 to T2, significant performance declines were found for the YYIR1 (p<0.001, effect size [ES] = 0.5), RSSA (p<0.01, ES = 0.6) and SJ tests (p< 0.046, ES = 0.7). However, no significant changes were found for the CMJ (p = 0.86, ES = 0.1). Post-exercise, RSSA blood lactate (p<0.012, ES = 0.2) and PV (p<0.01, ES = 0.7) increased significantly from T1 to T2. A significant decrease was found from T1 to T2 for the erythrocyte value (p<0.002, ES = 0.5) and the hemoglobin concentration (p<0.018, ES = 0.8). The hematocrit percentage rate was also significantly lower (p<0.001, ES = 0.6) at T2. The mean corpuscular volume, mean corpuscular hemoglobin content and the mean hemoglobin content values were not statistically different from T1 to T2. No significant relationships were detected between training load parameters and percentage changes of hematological parameters. However, a significant relationship was observed between training load and changes in RSSA performance (r = -0.60; p<0.003). An intensive period of "congested match play" over 6 weeks significantly compromised players' physical fitness. These changes were not related to hematological parameters, even though significant alterations were detected for selected measures.

#6 Effects of an 8-Week Pre-seasonal Training on the Aerobic Fitness of Professional Soccer Players
Reference: J Strength Cond Res. 2019 Jul 19. doi: 10.1519/JSC.0000000000003209. [Epub ahead of print]
Authors: Michaelides MA, Parpa KM, Zacharia AI.
Summary: Pre-season in soccer training develops the physical requisites for competition and usually consists of a high volume of aerobic and anaerobic conditioning training including friendly games. The purpose of the study was to determine the effects of pre-season training on the aerobic fitness of professional soccer players. Nineteen professional male soccer players (age = 27.37 ± 3.67 years, height = 179.61 ± 5.17 cm, and body fat percentage = 11.3 ± 3.19%) participated in this study performed an incremental cardiopulmonary exercise testing on a treadmill before and after the 8 weeks of pre-season preparation. The results were analyzed using paired t tests, revealing significant differences on several indices. The subjects improved significantly on maximal aerobic capacity (V[Combining Dot Above]O2max) and lasted significantly longer on the treadmill (p < 0.05). The V[Combining Dot Above]O2 at ventilatory threshold (VT) and respiratory compensation point (RCP) increased significantly (p < 0.05). The running velocity at ventilatory thresholds (vVT and vRCP) and at V[Combining Dot Above]O2 max (vVO2max) also increased significantly (p < 0.05). In conclusion, the results of this study, as expected, demonstrated that the proposed 8 weeks of pre-season training program was sufficient to cause significant improvements on the aerobic performance indices of professional soccer players. The study confirms the beneficial changes in the process of adaptations that occur with this type of training and can assist coaches and trainers in planning a successful pre-season training program.

#7 In-season in-field variable resistance training: effects on strength, power, and anthropometry of junior soccer players
Reference: J Sports Med Phys Fitness. 2019 Jul 24. doi: 10.23736/S0022-4707.19.09937-7. [Epub ahead of print]
Authors: Izadi M, Arazi H, Ramirez-Campillo R, Mirzaei M, Saidei P
Summary: Soccer players' leg muscular strength and power have been shown to be significant due to their association with soccer-specific performance including jumps, sprints, tackles and kicks. The aim of this study, therefore, was to examine the effects of an in-season in- field variable resistance training (VRT) program on strength, power, and anthropometry of junior soccer players. A team of male soccer players were randomly divided into Experimental (n=10) and Control groups (n=10). The Control group performed 8 weeks of soccer training alone. The Experimental group performed squat VRT using chains in addition to soccer training. Measures before and after training included one repetition maximum (1RM) of squat, countermovement jump (CMJ), and anthropometric estimation of thigh muscle cross sectional area (CSA). The VRT induced large improvements in absolute (34.45%; p=0.001; Cohen's d=1.78) and relative strength to thigh muscle CSA (21.53%; p=0.002; Cohen's d=1.04). Similarly, there were large (18.07%, p=0.007; Cohen's d=1.5) increases in jump height and medium gains in absolute peak power output (16.13%; p=0.009; Cohen's d=0.34) and relative peak power output to thigh muscle CSA (9.6%; p=0.002; Cohen's d=0.31). Further, there was a medium increase (5.9%, p=0.03; Cohen's d=0.36) in thigh muscle CSA. No significant changes were observed in the Control group. In-season in-field biweekly squat VRT enhanced strength and power measures in junior soccer players.

#8 Selection and promotion processes are not associated by the relative age effect in an elite Spanish soccer academy
Reference: PLoS One. 2019 Jul 24;14(7):e0219945. doi: 10.1371/journal.pone.0219945. eCollection 2019.
Auhors: Castillo D, Pérez-González B, Raya-González J, Fernández-Luna Á, Burillo P, Lago-Rodríguez Á
Download link:
Summary: The aim of this study was to analyze the influence of the relative age effect (RAE) on the selection and promotion processes in an elite soccer academy. One hundred and eleven elite youth players who belonged to an elite soccer club from the Spanish "La Liga" participated in this study. Players were classified into three age-categories: under 14 years (U14), under 16 years (U16) and under 18 years (U18); and they were also classified in quartiles based on their date of birth (i.e., January-March, Q1; April-June, Q2; July-September, Q3; October-December, Q4). In addition, two further classification criteria were established based on the selection (i.e., selected and non-selected players) and promotion (i.e., promoted and non-promoted players) processes. The main results showed that in U14 and U16 age-categories, players born early in the year were over-represented compared to players born late in the year, although birth-distribution was not associated with the likelihood of a player to be selected or promoted. In addition, less fat in sum skinfolds, less percentage of fat, higher percentage of muscle and lower endomorphy and mesomorphy components were showed in U14 selected players, in comparison with non-selected players. Likewise, better sprint performance was found in U16 selected players versus non-selected ones. However, no significant differences on anthropometry, body composition, somatotype and physical performance were found between promoted and non-promoted players. Therefore, our results suggest there is need for coaches to reorient their talent identification programs in order to make sure that players selected to continue playing in the club have the potential to promote to the excellence in soccer.

#9 Changing Rules and Configurations During Soccer Small-Sided and Conditioned Games. How Does It Impact Teams' Tactical Behavior?
Reference: Front Psychol. 2019 Jul 9;10:1554. doi: 10.3389/fpsyg.2019.01554. eCollection 2019.
Authors: Machado JC, Ribeiro J, Palheta CE, Alcântara C, Barreira D, Guilherme J, Garganta J, Scaglia AJ
Download link:
Summary: The present study aimed to investigate how team's tactical behavior varies within and between age categories in different Small-Sided and Conditioned Games' configurations and conditions. Twenty non-elite youth male soccer players (U15, n = 10, mean age = 13.5 ± 1.2 years; U17, n = 10, mean age = 16.3 ± 0.5 years) were selected. Thirty-six Small-Sided and Conditioned Games (SSCG) were played in both categories, namely three Representative SSCG (R-SSCG), three Maintaining Ball Possession Games (MBPG) and three Progression to Target Games (PTG) performed for each configuration (Gk+3vs3+Gk and Gk+4vs4+Gk). Teams' tactical behavior was analyzed based on simple and composite performance indicators, as well as through Lag Sequential Analysis. Rules manipulation and SSCG configurations influenced teams' tactical behavior on both categories, but in different ways. Teams composed by younger players presented greater difficulties in MBPG played in smaller games configuration, while Gk+4vs4+Gk configuration can be used to enhance teams' tactical performance of younger players in R-SSCG and MBPG conditions. Moreover, increasing rules manipulations appeared to negatively impact on teams' exploratory behavior. Therefore, practitioners should carefully manipulate key constraints to adapt task demands to players' age category and training session's goals in order to enhance tactical performance.

#10 Differences in Player Position Running Velocity at Lactate Thresholds Among Male Professional German Soccer Players
Reference: Front Physiol. 2019 Jul 9;10:886. doi: 10.3389/fphys.2019.00886. eCollection 2019.
Authors: Schwesig R, Schulze S, Reinhardt L, Laudner KG, Delank KS, Hermassi S
Download link:
Summary: This study investigated the differences in running velocities at specific lactate thresholds among male German soccer players. One hundred fifty-two professional (3rd league: n = 78; 4th league: n = 74) male soccer players (mean ± SD; age: 24.7 ± 4.37 years, body mass: 80.8 ± 7.33 kg, body height: 1.83 ± 0.06 m) volunteered for the investigation. Players were categorized as goalkeepers, central defenders, central midfielders, wings and forward. Players completed a treadmill test, at incremental speeds, to determine running velocity at different blood lactate concentrations (v2 = 2 mmol/l; v4 = 4 mmol/l; and v6 = 6 mmol/l). In addition, the largest difference between positions for running velocity was found at the lactate threshold v2 (p = 0.005). The running data revealed that only goalkeepers had significantly lower velocities at the lactate thresholds compared to outfield players. The central midfielders showed the highest average performance level at the lactate thresholds (v2: 12.5 ± 1.20 km/h; v4: 15.2 ± 1.14 km/h; and v6: 16.6 ± 1.14 km/h). In conclusion, this study provides soccer and position-specific reference data for the running performance of male professional German soccer players to evaluate the endurance performance in a standardized way. In this context, future research should extend the database for the first and second leagues. Further research assessing running performance during competition matches over the entire season is required to validate the endurance test performance data.

#11 Salivary Metabolome and Soccer Match: Challenges for Understanding Exercise induced Changes
Reference: Metabolites. 2019 Jul 11;9(7). pii: E141. doi: 10.3390/metabo9070141.
Authors: Pitti E, Petrella G, Di Marino S, Summa V, Perrone M, D'Ottavio S, Bernardini A, Cicero DO
Download link:
Summary: Saliva samples of seventeen soccer players were analyzed by nuclear magnetic resonance before and after an official match. Two different ways of normalizing data are discussed, using total proteins and total metabolite concentrations. Changes in markers related to energy, hydration status, amino acids and other compounds were found. The limits and advantages of using saliva to define the systemic responses to exercise are examined, both in terms of data normalization and interpretation, and the time that the effect lasts in this biofluid, which is shorter to that commonly observed in blood. The heterogeneous nature and different timing of the exercise developed by players also plays an important role in the metabolic changes that can be measured. Our work focuses mainly on three different aspects: The effect that time sampling has on the observed effect, the type of normalization that is necessary to perform in order to cope with changes in water content, and the metabolic response that can be observed using saliva.

#12 The Importance of Fundamental Motor Skills in Identifying Differences in Performance Levels of U10 Soccer Players
Reference: Sports (Basel). 2019 Jul 22;7(7). pii: E178. doi: 10.3390/sports7070178.
Authors: Jukic I, Prnjak K, Zoellner A, Tufano JJ, Sekulic D, Salaj S
Download link:
Summary: This study examined the differences in fundamental motor skills (FMSs) and specific conditioning capacities (SCCs) between a coach's classification of first team (FT) and second team (ST) U10 soccer players and examined the most important qualities based on how the coach differentiates them. The FT (n = 12; Mage = 9.72 ± 0.41) and ST (n = 11; Mage = 9.57 ± 0.41) soccer players were assessed using the Test of Gross Motor Development-2, standing long jump, sit and reach, diverse sprints, and the 20 m multistage fitness test (MSFT). The coach's subjective evaluation of players was obtained using a questionnaire. No significant differences existed between the FT and ST in any variables (p > 0.05). However, large and moderate effect sizes were present in favour of the FT group in locomotor skills (d = 0.82 (0.08, 1.51)), gross motor quotient (d = 0.73 (0.00, 1.41)), height (d = 0.61 (-0.12, 1.29)), MSFT (d = 0.58 (-0.14, 1.25)), and maximum oxygen uptake (VO2max) (d = 0.55 (-0.17, 1.22)). Furthermore, the coach perceived the FT group as having greater technical and tactical qualities relative to ST players. This suggests that it might be more relevant for players of this age to develop good FMS connected to technical skills, before focusing on SCC. Therefore, it might be beneficial for soccer coaches to emphasize the development of FMSs due to their potential to identify talented young soccer players and because they underpin the technical soccer skills that are required for future soccer success.

The Training Manager -