Latest research in football - week 4 - 2018

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 The Relative Age Effect on Soccer Players in Formative Stages with Different Sport Expertise Levels
Reference: J Hum Kinet. 2017 Dec 28;60:167-173. doi: 10.1515/hukin-2017-0100. eCollection 2017 Dec.
Authors: Praxedes A, Moreno A, Garcia-Gonzalez L, Pizarro D, Del Villar F
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Summary: The Relative Age Effect (RAE) in sport has been targeted by many research studies. The objective of this study was to analyze, in amateur clubs, the RAE of soccer players, according to the sport expertise level of the team (e.g., A, B, C and subsequent) that they belong to within the same game category. 1,098 soccer players in formative stages took part in the study, with ages varying between 6 and 18 years old (U8 to U19 categories). All of them were members of 4 Spanish federated clubs. The birth dates were classified into 4 quartiles (Q1 = Jan-Mar; Q2 = Apr-Jun; Q3 = Jul-Sept; Q4 = Oct-Dec)according to the team they belonged to. The results obtained in the chi-squared test and d value (effect size) revealed the existence of RAE in the teams with the highest expertise level, "A" (X2 = 15.342, p = .002, d = 0.4473) and "B" (X2 = 10.905, p = .012, d = 0.3657). However, in the lower level teams, "C and subsequent", this effect was not observed. Present findings show that players born during the first months of the year tend to be selected to play in teams with a higher sport expertise level of each category, due to their physical maturity. Consequently, this causes differences in terms of the experience they accumulate and the motivation that this creates in these players.

#2 Physical Performance and Anthropometric Characteristics of Male South African University Soccer Players
Reference: J Hum Kinet. 2017 Dec 28;60:153-158. doi: 10.1515/hukin-2017-0098. eCollection 2017 Dec.
Authors: Kubayi A, Paul Y, Mahlangu P, Toriola A
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Summary: Soccer is the most popular sport worldwide. Despite its global acclaim, scientific studies of soccer have tended to focus on tactics and techniques, thereby neglecting the physical and physiological profile of the players. Therefore, the purpose of this study was to examine physical and anthropometric characteristics of male South African university soccer players. Twenty-seven male soccer players aged 19 to 24 (mean age: 22.1 years; s = 1.5 years) volunteered to participate in the study. The results showed that goalkeepers (77.5 ± 9.7 kg) and defenders (68.2 ± 6.5 kg) were the heaviest compared to players in other playing positions. The goalkeepers also had the highest percentage of body fat (11.3 ± 2.3%), in contrast to midfielders who had the lowest body fat content (9.1 ± 0.9%). With regard to flexibility, defenders (45.1 ± 4.9 cm) and midfielders (45.9 ± 5.4 cm) performed better than goalkeepers (37.1 ± 4.3 cm) and strikers (40.1 ± 3.4 cm). Midfielders (57.2 ± 3.1 ml1·kg-1·min1) and defenders (56.1 ± 5.1 ml1·kg-1·min1) had significantly higher values of maximal oxygen uptake (VO2max) than goalkeepers (47.9 ± 0.2 ml-1·kg-1·min-1) and strikers (49.8 ± 6.2 ml-1·kg-1·min-1). No significant (p > 0.05) differences were observed for all other variables, with the exception of body height, body mass, and VO2max. It was therefore concluded that sports scientists and coaches should tailor conditioning programmes in soccer according to players' positions in view of the implications for successful performance.

#3 Effects of Passive and Active Rest on Physiological Responses and Time Motion Characteristics in Different Small Sided Soccer Games
Reference: J Hum Kinet. 2017 Dec 28;60:123-132. doi: 10.1515/hukin-2017-0095. eCollection 2017 Dec.
Authors: Arslan E, Alemdaroglu U, Koklu Y, Hazir T, Muniroglu S, Karakoc B
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Summary: The purpose of this study was to investigate the effects of resting regimes on physiological responses and time motion characteristics between bouts during small sided games (SSGs) in young soccer players. Sixteen players (average age 16.87 ± 0.34 years; body height 176.69 ± 3.21 cm; body mass 62.40 ± 2.59 kg; training experience 3.75 ± 0.44 years) performed four bouts 2-a-side, 3-a-side and 4-a-side games with three minutes active (SSGar: Running at 70% of HRmax) and passive (SSGpr) rest between bouts at two-day intervals. The heart rate (HR) along with total distance covered in different speed zones - walking (W, 0-6.9 km·h-1), low-intensity running (LIR, 7.0-12.9 km·h-1), moderate-intensity running (MIR, 13.0-17.9 km·h-1) and high-intensity running (HIR, >18km·h-1), were monitored during all SSGs, whereas the rating of perceived exertion (RPE, CR-20) and venous blood lactate (La-) were determined at the end of the last bout of each SSG. The results demonstrated that all SSGpr elicited significantly higher physiological responses compared to SSGar in terms of the RPE and La- (p < 0.05). In addition, 2-a-side SSGpr induced significantly lower %HRmax responses and total distance covered than 2-a-side SSGar (p < 0.05). Moreover, the distance covered at HIR was significantly higher in 4-a-side SSGar than 4-side SSGpr. The results of this study indicate that both SSGs with passive and active rest can be used for soccer specific aerobic endurance training. Furthermore, all SSGs with active recovery should be performed in order to increase players and teams' performance capacity for subsequent bouts.

#4 Multivariate Profiles of Selected versus Non-Selected Elite Youth Brazilian Soccer Players
Reference: J Hum Kinet. 2017 Dec 28;60:113-121. doi: 10.1515/hukin-2017-0094. eCollection 2017 Dec.
Authors: Aquino R, Alves IS, Padilha MB, Casanova F, Puggina EF, Maia J
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Summary: This study determined whether a multivariate profile more effectively discriminated selected than non-selected elite youth Brazilian soccer players. This examination was carried out on 66 youth soccer players (selected, n = 28, mean age 16.3 ± 0.1; non-selected, n = 38, mean age 16.7 ± 0.4) using objective instruments. Multivariate profiles were assessed through anthropometric characteristics, biological maturation, tactical-technical skills, and motor performance. The Student's t-test identified that selected players exhibited significantly higher values for height (t = 2.331, p = 0.02), lean body mass (t = 2.441, p = 0.01), and maturity offset (t = 4.559, p < 0.001), as well as performed better in declarative tactical knowledge (t = 10.484, p < 0.001), shooting (t = 2.188, p = 0.03), dribbling (t = 5.914, p < 0.001), speed - 30 m (t = 8.304, p < 0.001), countermovement jump (t = 2.718, p = 0.008), and peak power tests (t = 2.454, p = 0.01). Forward stepwise discriminant function analysis showed that declarative tactical knowledge, running speed -30 m, maturity offset, dribbling, height, and peak power correctly classified 97% of the selected players. These findings may have implications for a highly efficient selection process with objective measures of youth players in soccer clubs.

#5 High-Intensity Small-Sided Games versus Repeated Sprint Training in Junior Soccer Players
Reference: J Hum Kinet. 2017 Dec 28;60:101-111. doi: 10.1515/hukin-2017-0104. eCollection 2017 Dec.
Authors: Eniseler N, Şahan C, Ozcan I, Dinler K
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Summary: The aim of this study was to compare the effects of high-intensity small-sided games training (SSGT) versus repeated-sprint training (RST) on repeated-sprint ability (RSA), soccer specific endurance performance and short passing ability among junior soccer players. The junior soccer players were recruited from of a professional team (age 16.9 ± 1.1 years). The tests included the repeated-shuttle-sprint ability test (RSSAT), Yo-Yo Intermittent Recovery Test level 1 (Yo-Yo IR1) and Loughborough Soccer Passing Test (LSPT). Nineteen participants were randomly assigned to either the small-sided games training (SSGTG) (n = 10) or repeated-sprint training group (RSTG) (n = 9). Small-sided games or repeated-sprint training were added to the regular training sessions for two days of the regular practice week. The Wilcoxon signed-rank and Mann-Whitney U tests were used to examine differences in groups and training effects. A time x training group effect was found in the improvement of short-passing ability for the smallsided games training group which showed significantly better scores than the repeated-sprint training group (p ≤ 0.05). Both groups showed similar improvements in RSAdecrement (p < 0.05). Only the repeated-sprint training group improved in the Yo-Yo IR1 (p < 0.05). This study clearly shows that high-intensity small-sided games training can be used as an effective training mode to enhance both repeated sprint ability and short-passing ability.

#6 Association between Match Activity Variables, Measures of Fatigue and Neuromuscular Performance Capacity Following Elite Competitive Soccer Matches
Reference: J Hum Kinet. 2017 Dec 28;60:93-99. doi: 10.1515/hukin-2017-0093. eCollection 2017 Dec.
Authors: Varley I, Lewin R, Needham R, Thorpe RT, Burbeary R
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Summary: The aim of the study was to assess the relationships between match activity variables, subsequent fatigue and neuromuscular performance capacity in elite soccer players. Subjects (n = 10) were professional soccer players participating in the English Championships. Match activity variables and markers of fatigue status were measured before and following two matches. Creatine kinase (CK) and muscle soreness were measured at baseline, immediately following, as well as 40 and 64 h post-match. Countermovement jump performance and perceived ratings of wellness were measured at baseline, then 40 and 64 h post-match. Relationships were shown between CK and the total number of accelerations and decelerations immediately (r = 0.63; large), 40 h (r = 0.45; moderate) and 64 h post-match (r = 0.35; moderate) (p < 0.05). Relationships between CK and total sprint distance (r = 0.39; moderate) and the number of sprints (r = 0.35; moderate) 40 h post-match (p < 0.05) were observed. Furthermore, relationships were shown between the perceived rating of wellness and number of accelerations 40 (r = 0.52; large) and 64 h (r = 0.40; moderate) post-match, sprint distance 40 h post-match (r = 0.40; moderate) and the total number of sprints 40 h post-match (r = 0.51; large) (p < 0.05). The quantification of match activity variables, particularly the total number of accelerations and decelerations and the number of sprints, provides insights into the fatigue status in elite soccer players 40 and 64 h post-match.

#7 Physiological Demands, Morphological Characteristics, Physical Abilities and Injuries of Female Soccer Players
Reference: J Hum Kinet. 2017 Dec 28;60:77-83. doi: 10.1515/hukin-2017-0091. eCollection 2017 Dec.
Authors: Milanovic Z, Sporis G, James N, Trajkovic N, Ignjatovic A, Sarmento H, Trecroci A, Mendes BMB
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Summary: The popularity of female soccer is increasing as well as the number of females playing soccer. Similarly, over the last twenty or so years, research in soccer has increased significantly, but a large disparity exists in the volume of studies involving male and female players. As a consequence of this, female players remain less well understood compared to males. The purpose of the present narrative review was to describe morphological characteristics, physiological demands, physical abilities and injuries in female soccer players. Physiological demands are similar between men's and women's soccer, but competitive women's matches were characterized by nearly 33% less distance covered, although at higher intensity levels (maximum speeds greater than 15 km/h) than typically found in the men's game. Sub-elite female players also tended to run less at higher intensity levels at the end of both halves in comparison with elite female players. High intensity running is an important factor of success in soccer since many critical moments of the game occur under this condition. The ability to rapidly change direction also determined elite, sub-elite and amateur levels. The implementation of functional training, which focused on soccer-specific drills and plyometric exercises, to improve explosive power, may improve conditioning in female soccer players as well as decrease the risk of injuries which was 3-8 times higher in females compared to males. This review presents an in-depth overview of the most influential factors for determining success in female soccer.

#8 Effects of Plyometric and Resistance Training on Muscle Strength, Explosiveness and Neuromuscular Function in Young Adolescent Soccer Players
Reference: J Strength Cond Res. 2018 Jan 4. doi: 10.1519/JSC.0000000000002428. [Epub ahead of print]
Authors: McKinlay BJ, Wallace P, Dotan R, Long D, Tokuno C, Gabriel D, Falk B
Summary: This study examined the effect of 8-weeks of free-weight-resistance (RT) and plyometric (PLYO) training on maximal strength, explosiveness and jump performance compared with no added training (CON), in young male soccer players. Forty-one 11[FIGURE DASH]13-year-old soccer players were divided into three groups (RT, PLYO, CON). All participants completed isometric and dynamic (240°/s) knee extensions pre- and post-training. Peak torque (pT), peak rate of torque development (pRTD), electromechanical-delay (EMD), rate of muscle activation (Q50), m. vastus-lateralis thickness (VLT), and jump performance were examined. pT, pRTD and jump performance significantly improved in both training groups. Training resulted in significant (p<0.05) increases in isometric pT (23.4 vs. 15.8%) and pRTD (15.0 vs. 17.6%), in RT and PLYO, respectively. During dynamic contractions, training resulted in significant increases in pT (12.4 and 10.8% in RT and PLYO, respectively), but not pRTD. Jump performance increased in both training groups (RT=10.0%, PLYO=16.2%), with only PLYO significantly different from CON. Training resulted in significant increases in VLT (RT=6.7%. PLYO=8.1%). There were no significant EMD changes. In conclusion, 8-week free-weight resistance and plyometric training resulted in significant improvements in muscle strength and jump performance. Training resulted in similar muscle hypertrophy in the two training modes, with no clear differences in muscle performance. Plyometric training was more effective in improving jump performance, while free-weight resistance training was more advantageous in improving peak torque, where the stretch reflex was not involved.

#9 Comparison of step-by-step kinematics in repeated 30m sprints in female soccer players
Reference: J Strength Cond Res. 2018 Jan 4. doi: 10.1519/JSC.0000000000002429. [Epub ahead of print]
Authors: van den Tillaar R
Summary: The aim of this study was to compare kinematics in repeated 30m sprints in female soccer players. Seventeen subjects performed seven 30m sprints every 30s in one session. Kinematics were measured with an infrared contact mat and laser gun, and running times with an electronic timing device. The main findings were that sprint times increased in the repeated sprint ability test. The main changes in kinematics during the repeated sprint ability test were increased contact time and decreased step frequency, while no change in step length was observed. The step velocity increased in almost each step until the 14, which occurred around 22m. After this, the velocity was stable until the last step, when it decreased. This increase in step velocity was mainly caused by the increased step length and decreased contact times. It was concluded that the fatigue induced in repeated 30m sprints in female soccer players resulted in decreased step frequency and increased contact time. Employing this approach in combination with a laser gun and infrared mat for 30m makes it very easy to analyse running kinematics in repeated sprints in training. This extra information gives the athlete, coach and sports scientist the opportunity to give more detailed feedback and help to target these changes in kinematics better to enhance repeated sprint performance.

#10 Mild jugular compression collar ameliorated changes in brain activation of working memory after one soccer season in female high school athletes
Reference: J Neurotrauma. 2018 Jan 15. doi: 10.1089/neu.2017.5262. [Epub ahead of print]
Authors: Yuan W, Dudley J, Barber-Foss K, Ellis JD, Thomas S, Galloway RT, DiCesare C, Leach J, Adams J, Maloney T, Gadd B, Smith D, Epstein J, Grooms DR, Logan K, Howell DR, Altaye M, Myer GD
Summary: Recent neuroimaging studies have suggested that repetitive sub-concussive head impacts, even after only one sport season, may lead to pre- to post-season structural and functional alterations in male high school football athletes. However, data on female atheletes is limited. In the current investigation, we aimed to (1) assess the longitudinal pre- to post-season changes in fMRI of working memory and working memory performance, (2) quantify the association between the pre- to post-season change in fMRI of working memory and the exposure to head impact and working memory performance, and (3) assess whether wearing a neck collar designed to reduce intracranial slosh via mild compression of the jugular veins can ameliorate the changes in fMRI brain activation observed in the non-collar group after a full soccer season. A total of 48 female high school soccer athletes (age range: 14.00 - 17.97 years) were included in the study. These athletes were assigned to the non-collar group (n=21) or to the collar group (n=27). All athletes undewent MRI at both pre-season and post-season. In each session, a fMRI verbal N-Back task was used to engage working memory. A significant pre- to post-season increase in fMRI BOLD signal was demonstrated when performing the N-back working memory task in the non-collar group but not in the collar group, despite the comparable exposure of head impacts during the season between the two groups. The collar group demonstrated significantly smaller pre- to post-season change in fMRI BOLD signal than the non-collar group, suggesting a potential protective effect from the collar device. Significant correlations were also found between the pre- to post-season increase in fMRI brain activation and the decrease in task accuracy in the non-collar group, indicating an association between the compensatory mechanism in underlying neurophysiology and the alteration in the behavioral outcomes.

#11 Effects of resisted sprint training on sprinting ability and change of direction speed in professional soccer players
Reference: J Sports Sci. 2018 Jan 15:1-7. doi: 10.1080/02640414.2018.1426346. [Epub ahead of print]
Authors: Gil S, Barroso R, Crivoi do Carmo E, Loturco I, Kobal R, Tricoli V, Ugrinowitsch C, Roschel H
Summary: Resisted sprint training consists of performing overloaded sprints, which may produce greater effects than traditional sprint training. We compared a resisted sprint training with overload control versus an unresisted sprint training program on performance in soccer players. Eighteen elite athletes were randomly assigned to resisted (RST) or unresisted sprint training protocol (UR). Before and after a 6-week training period, sprinting ability, change of direction speed (COD), vertical jumps (SJ and CMJ), mean power (MP) and mean propulsive power (MPP) at distinct loads were assessed. Both groups improved sprinting ability at all distances evaluated (5m: UR = 8%, RST = 7%; 10m: UR = 5%, RST = 5%; 15m: UR = 4%, RST = 4%; 20m: UR = 3%, RST = 3%; 25m: UR = 2%, RST = 3%;), COD (UR = 6%; RST = 6%), SJ (UR = 15%; RST = 13%) and CMJ (UR = 15%; RST = 15%). Additionally, both groups increased MP and MPP at all loads evaluated. The between-group magnitude-based inference analysis demonstrated comparable improvement ("trivial" effect) in all variables tested. Finally, our findings support the effectiveness of a short-term training program involving squat jump exercise plus sprinting exercises to improve the performance of soccer players.

#12 Does inside passing contribute to the high incidence of groin injuries in soccer? A biomechanical analysis
Reference: J Sports Sci. 2018 Jan 15:1-9. doi: 10.1080/02640414.2017.1423193. [Epub ahead of print]
Authors: Dupre T, Funken J, Muller R, Mortensen KRL, Lysdal FG, Braun M, Krahl H, Potthast W
Summary: Groin injuries are common in soccer and often cause time-loss from training. While groin injuries have been linked to full effort kicking, the role of inside passing is unclear. Therefore, the purpose of this study was to investigate hip joint kinematics and muscle force, stress and contraction velocity for adductor longus and gracilis during inside passing. 3D kinematics of ten soccer players (23.4 yrs; 77.5 kg; 1.81 m) were captured with a motion capture system inside a Footbonaut. Muscle force and contraction velocity were determined with AnyBody Modelling System. Gracilis muscle forces were 9% lower compared to adductor longus (p = 0.005), but muscle stress was 183% higher in gracilis (p = 0.005). Contraction velocity reveals eccentric contraction of gracilis in the last quarter of the swing phase. Considering the combination of eccentric contraction, high muscle stress and the repetitive nature of inside passing, gracilis accumulates high loads in matches and training. These results indicate that the high incidence of groin injuries in soccer could be linked to isolated pass training. Practitioners need to be aware of the risk and refrain from sudden increases in the amount of pass training. This gives the musculoskeletal system time to adapt and might avoid career threatening injuries.


American Football
#1 Making Football Safer: Assessing the current NFL policy on the type of helmets allowed on the playing field
Reference: J Neurotrauma. 2018 Feb 8. doi: 10.1089/neu.2017.5446. [Epub ahead of print]
Authors: Colello R D.Phil., Colello IA, AbdelHameid D, Cresswell KG, Merchant R, Beckett E
Summary: In an effort to reduce concussions in football, a helmet safety-rating system was developed in 2011 that rated helmets based on their ability to reduce g-forces experienced by the head across a range of impact forces measured on the playing field. Although this was considered a major step in making the game safer, the NFL continues to allow players the right to choose what helmet to wear during play. This prompted us to ask: what helmets do NFL players wear and does this helmet policy make the game safer? Accordingly, we identified the helmets worn by nearly 1000 players on Weeks 13 and 1 of the 2015-2016 and 2016-2017 seasons, respectively. Using stop-motion footage, we found that players wore a wide range of helmets with varying safety ratings influenced, in part, by the player's position and age. Moreover, players wearing lower safety-rated helmets were more likely to receive a concussion than those wearing higher safety-rated helmets. Interestingly, many players suffering a concussion in 2015 did not switch to a higher safety-rated helmet in 2016. Using a helmet-to-helmet impactor, we found that the g-forces experienced in the highest safety-rated helmets were roughly 30% less than that for the lowest safety-rated helmets. These results suggest that the current NFL helmet policy puts players at increased risk of receiving a concussion as many players are wearing low safety-rated helmets, which transmits more energy to the brain than higher-safety-rated helmets, following collision. Thus, the NFL should mandate that players only wear helmets that receive the highest safety rating. This policy change would likely represent the simplest and most straightforward way to reduce concussions in football.


Gaelic Football
#1 An Acceleration Profile of Elite Gaelic Football with Special Reference to Position of Play
Reference: J Strength Cond Res. 2018 Feb 1. doi: 10.1519/JSC.0000000000002479. [Epub ahead of print]
Authors: Ryan M, Malone S, Donnellan A, Collins K
Summary: The current study aimed to characterize the positional match-play demands of elite Gaelic football players with special reference to acceleration utilizing predetermined 5- min periods (epochs). Thirty-five male Gaelic players (Mean ± SD, age: 24 ± 6 years; height: 180 ± 7 cm; mass: 81 ± 7 kg) across five playing positions (full-back, half-back, midfield, half-forward, full-forward) were monitored during the investigation. Player movement was recorded during nineteen matches using 4-Hz global positioning system technology (GPS; VXSport, New Zealand) resulting in 154 player observations. GPS was used to record total distance (m), high-speed running (HSR; m; ≥17 kmh), very high-speed running distance, (VHSR; m; ≥22 kmh), the number of accelerations (n), duration of accelerations (s), peak acceleration (m), and distance of accelerations (m). Acceleration profiles were position dependent with midfielders found to have a high accumulation of acceleration movements when compared to all other positions (p < 0.05). Declines of -2% to -32% for acceleration distance (m) depending on positional line of play were observed during match-play. Less HSR and VHSR, was performed by the full-back line (HSR; -39%, VHSR; -36%) and full-forward line (-35%; -29%) when compared to half-back, midfielders and half-forwards (p=0.01, d = 1.35 to 1.77). Similar trends were reported for peak acceleration distance (p=0.01, d = 1.15 to 1.93). The current investigation provides a greater understanding of temporal differences in acceleration profiles of playing position. We show that half-back, midfield and half-forwards have the highest acceleration movements these data can assist coaches in appropriately preparing players for the required acceleration distances required during match-play.

#2 The Pre-Competition Macronutrient Intake of Elite Gaelic Football Players
Reference: Int J Sport Nutr Exerc Metab. 2018 Feb 6:1-20. doi: 10.1123/ijsnem.2017-0292. [Epub ahead of print]
Authors: Cassidy C, Collins K, Shortall M
Summary: Competition related dietary intake has not yet been investigated in Gaelic football. The present study examined the pre-competition macronutrient intake of elite male Gaelic football players. Forty players from two teams completed a food diary on the two days preceding competition (DAY-1 & DAY-2) and on match date pre-match (MATCH-DAY). Carbohydrate intake was significantly greater on DAY-2 compared to DAY-1, for both absolute [295 ± 98 vs. 318 ± 77 g] (p = 0.048; -23.6 g [-47.3 to 0.2]; Cohen's d = 0.27) and relative intake [3.4 ± 1.1 vs. 3.7 ± 1.0] (p = 0.027; -0.3 [-0.6 to -0.03]; Cohen's d = 0.32). The number of players in accordance with and not in accordance with the guidelines for carbohydrate intake on DAY-2 was significantly different to an expected frequency distribution [χ2 (1) = 32.400; p = <0.001; ϕ = 0.9] with a greater number of players not meeting the guidelines [observed N = 2 vs. 38]. The number of players in accordance with and not in accordance with the recommendations for carbohydrate intake on MATCH-DAY was significantly different to an expected frequency distribution [χ2 (1) = 8.100; p = 0.004; ϕ = 0.45] with a greater number of players meeting the guidelines [observed N = 29 vs. 11]. The major finding from the current investigation was that a significantly greater number of players did not meet carbohydrate intake guidelines on the day before competition. Individualised nutritional interventions are required in order to modify current pre-match dietary intake.

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