Latest research in football - week 4 - 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 Head impact magnitudes that occur from purposeful soccer heading depend on the game scenario and head impact location
Reference: Musculoskelet Sci Pract. 2019 Jan 24;40:53-57. doi: 10.1016/j.msksp.2019.01.009. [Epub ahead of print]
Authors: Harriss A, Johnson AM, Walton DM, Dickey JP
Summary: This study quantified the linear and angular kinematics that result from purposeful heading during youth soccer games, and the influence of game scenario and head impact location on these magnitudes. This observational study recruited thirty-six female soccer players (13.4 ± 0.9 years old) from three elite youth soccer teams (U13, U14, U15) and followed for an entire soccer season. Players wore wireless sensors during each game to quantify head impact magnitudes. A total of 60 regular season games (20 games per team) were video recorded, and purposeful heading events were categorized by game scenario (e.g. throw in), and head impact location (e.g. front of head). Game scenario had a statistically significant effect on the linear head acceleration, and rotational head velocity, that resulted from purposeful headers. Rotational velocity from purposeful headers varied significantly between head impact locations, with impacts to the top of the head (improper technique) resulting in larger peak rotational velocities than impacts to the front of the head (proper technique); this was also the case for the linear acceleration for punts. Our findings suggest that the magnitude for both linear and angular head impact kinematics depend on the game scenario and head impact location. Headers performed with the top of the head (improper technique) result in larger rotational velocities compared to the front of the head (proper technique). Accordingly, youth players should be educated on how to execute proper heading technique to reduce head impact accelerations.

#2 Jumping Asymmetries Are Associated With Speed, Change of Direction Speed, and Jump Performance in Elite Academy Soccer Players
Reference: J Strength Cond Res. 2019 Jan 30. doi: 10.1519/JSC.0000000000003058. [Epub ahead of print]
Authors: Bishop C, Brashill C, Abbott W, Read P, Lake J, Turner A
Summary: The aim of this study was to establish interlimb asymmetries across different age groups in elite academy male soccer players and to examine any relationships between asymmetry and measures of physical performance. Fifty-one players from an English Premier League soccer academy were split into under-23 (n = 21), under-18 (n = 14), and under-16 (n = 16) groups and performed bilateral and unilateral countermovement jumps, 5-, 10-, and 20-m sprints, and a 505 change of direction speed tests. All tests showed low variability (coefficient of variation ≤ 2.5%) and good to excellent reliability (intraclass correlation coefficient = 0.80-0.99). A 1-way analysis of variance showed that the under-23 group was significantly faster than the under-16 group during the 20-m sprint (2.90 vs. 2.98 s; p = 0.02; effect size = 0.94). No other significant differences were present between groups. Interlimb asymmetry was quantified from the single-leg countermovement jump, and no significant differences in the magnitude of asymmetry were present between groups. However, multiple significant correlations were present in each age group between asymmetry and physical performance tests, all of which were indicative of reduced athletic performance. Results from this study show that although interlimb asymmetry scores are comparable across age groups in elite academy soccer players, differences as low as 5% are associated with reduced physical performance during jumping, sprinting, and change of direction speed tasks. This study suggests the importance of monitoring jump height asymmetries in elite academy soccer players.

#3 Coaching Efficacy, Player Perceptions of Coaches' Leadership Styles, and Team Performance in Premier League Soccer
Reference: Res Q Exerc Sport. 2019 Feb 1:1-9. doi: 10.1080/02701367.2018.1563277. [Epub ahead of print]
Authors: Keatlholetswe L, Malete L
Summary: The coaching competency research has demonstrated the role of coaching efficacy and coaching behaviors on various athlete outcomes. However, athlete perceptions of these relationships and how they affect performance are less understood. This study examined if coaching efficacy is predictive of player perceptions of coaches' leadership styles, team atmosphere, and team performance in a soccer season. Fifteen male premier league soccer coaches (Mage = 45.27, SD = 6.07) and 226 players (Mage = 25.66, SD = 3.96) from Botswana participated in the study. All participants completed a background information questionnaire. Coaches completed the Coaching Efficacy Scale. Players rated their coaches' leadership styles using the Revised Leadership Scale for Sports as well as team atmosphere. Team performance was based on position in the league log and player ratings of the teams' performance. Findings showed that coaches' self-ratings on technique efficacy predicted player perceptions of the coaches' use of all six leadership styles. Game strategy efficacy predicted higher team atmosphere and team performance. Motivation efficacy was not significantly associated with player perceptions of the coaches' use of any of the leadership styles, while character building efficacy was negatively associated with the various leadership styles. Findings provide support to previous research evidence linking higher coaching efficacy, leadership styles, and team outcomes. The study expands the emergent research within the coaching competency literature that examines player perceptions of coaches' behaviors and leadership styles.

#4 The Effect of In-Season Traditional and Explosive Resistance Training Programs on Strength, Jump Height, and Speed in Recreational Soccer Players
Reference: Res Q Exerc Sport. 2019 Feb 1:1-8. doi: 10.1080/02701367.2018.1563276. [Epub ahead of print]
Authors: Griffiths B, Grant J, Langdown L, Gentil P, Fisher J, Steele J
Summary: Resistance training is often performed in a traditional training style using deliberate relatively longer repetition durations or in an explosive training style using maximal intended velocities and relatively shorter repetition durations. Both improve strength, "power" (impulsivity), and speed. This study compared explosive and traditional training over a 6-week intervention in 30 healthy young adult male recreational soccer players. Full body supervised resistance training was performed 2 times a week using 3 sets of each exercise at 80% of one repetition maximum to momentary failure. Outcomes were Smith machine squat 1 repetition maximum, 10 meter sprint time, and countermovement jump. Both groups significantly improved all outcomes based on 95% confidence intervals not crossing zero. There were no between-group differences for squat 1 RM (TRAD = 6.3[5.1 to 7.6] kg, EXP = 5.2[3.9 to 6.4] kg) or 10 meter sprint (TRAD = -0.05[-0.07 to -0.04] s, EXP = -0.05[-0.06 to -0.03] s). Explosive group had a significantly greater increase in countermovement jump compared to the traditional group (TRAD = 0.7[0.3 to 1.1] cm, EXP = 1.3[0.9 to 1.7] cm). Both the traditional training and explosive training performed to momentary failure produced significant improvements in strength, speed, and jump performance. Strength gains are similar independent of intended movement speed. However, speed and jump performance changes are marginal with resistance training.

#5 A match-day analysis of the movement profiles of substitutes from a professional soccer club before and after pitch-entry
Reference:  PLoS One. 2019 Jan 31;14(1):e0211563. doi: 10.1371/journal.pone.0211563. eCollection 2019.
Authors: Hills SP, Barrett S, Feltbower RG, Barwood MJ, Radcliffe JN, Cooke CB, Kilduff LP, Cook CJ, Russell M
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Summary: Whilst the movement demands of players completing a whole soccer match have been well-documented, comparable information relating to substitutes is sparse. Therefore, this study profiled the match-day physical activities performed by soccer substitutes, focusing separately on the pre and post pitch-entry periods. Seventeen English Championship soccer players were monitored using 10 Hz Micromechanical Electrical Systems (MEMS) devices during 13 matches in which they participated as substitutes (35 observations). Twenty physical variables were examined and data were organised by bouts of warm-up activity (pre pitch-entry), and five min epochs of match-play (post pitch-entry). Linear mixed modelling assessed the influence of time (i.e., 'bout' and 'epoch'), playing position, and match scoreline. Substitutes performed 3±1 rewarm-up bouts∙player-1∙match-1. Compared to the initial warm-up, each rewarm-up was shorter (-19.7 to -22.9 min) and elicited less distance (-606 to -741 m), whilst relative total distances were higher (+26 to +69 m∙min-1). Relative total (+13.4 m∙min-1) and high-speed (+0.4 m∙min-1) distances covered during rewarm-ups increased (p <0.001) with proximity to pitch-entry. Players covered more (+3.2 m; p = 0.047) high-speed distance per rewarm-up when the assessed team was losing compared with when winning at the time of pitch-entry. For 10 out of 20 variables measured after pitch-entry, values reduced from 0-5 min thereafter, and substitutes covered greater (p ˂0.05) total (+67 to +93 m) and high-speed (+14 to +33 m) distances during the first five min of match-play versus all subsequent epochs. Midfielders covered more distance (+41 m) per five min epoch than both attackers (p ˂0.001) and defenders (p = 0.016). Acknowledging the limitations of a solely movement data approach and the potential influence of other match-specific factors, such findings provide novel insights into the match-day demands faced by substitute soccer players. Future research opportunities exist to better understand the match-day practices of this population.

#6 Effect of Heavy Resisted Sled Sprint Training During the Competitive Season on Sprint and Change-of-Direction Performance in Professional Soccer Players
Reference: Int J Sports Physiol Perform. 2019 Jan 31:1-25. doi: 10.1123/ijspp.2018-0592. [Epub ahead of print]
Authors: McMorrow BJ, Ditroilo M, Egan B
Summary: Resisted sled sprinting (RSS) is an effective tool for improving sprint performance over short distances, but the effect on change-of-direction (COD) performance is largely unknown. The present study investigated the effect of heavy RSS training during the competitive season on sprint and COD performance in professional soccer players. Over six weeks in-season, a RSS training group (n=6) performed RSS at a sled load of 30% of body mass for a total programme running distance of 800 m, while an unresisted sprint (URS) training group (n=7) performed the same distance of unresisted sprinting. A 20 m maximal sprint with split times measured at 5, 10 and 20 m, and the sprint 9-3-6-3-9 m with 180° turns COD test were performed before and after the intervention. Sprint performance (mean; 95% confidence limits; qualitative inference) was improved in both groups over 5 m (URS, 5.1%; -2.4, 12.7; likely moderate; RSS, 5.4%; 0.5, 10.4; likely moderate), 10 m (URS, 3.9%; -0.3, 8.1; very likely moderate; RSS, 5.0%; 1.8, 8.0; very likely large), and 20 m (URS, 2.0%; -0.6, 4.5; likely moderate; RSS (3.0%; 1.7, 4.4; very likely moderate). COD was improved in both groups (URS, 3.7%; 2.2, 5.2; most likely large; RSS, 3.3%; 1.6, 5.0; most likely moderate). Between-group differences were unclear. Heavy RSS or URS training matched for running distance were similarly effective at improving sprint and COD performance in professional soccer players when performed in the competitive phase of the season.

#7 Predicting Future Perceived Wellness in Professional Soccer: The Role of Preceding Load and Wellness
Reference: Int J Sports Physiol Perform. 2019 Jan 31:1-25. doi: 10.1123/ijspp.2017-0864. [Epub ahead of print]
Authors: Jaspers A, De Beéck TO, Brink MS, Frencken WGP, Staes F, Davis JJ, Helsen WF
Summary: The influence of preceding load and perceived wellness on the future perceived wellness of professional soccer players is unexamined. This paper simultaneously evaluates the external and internal load for different time frames in combination with pre-session wellness to predict future perceived wellness using machine learning techniques. Training and match data were collected from a professional soccer team. The external load was measured using global positioning system technology and accelerometry. The internal load was obtained using the RPE multiplied by duration. Predictive models were constructed using gradient boosted regression trees (GBRT) and one naive baseline method. The individual predictions of future wellness items (i.e., fatigue, sleep quality, general muscle soreness, stress levels, and mood) were based on a set of external and internal load indicators in combination with pre-session wellness. The external and internal load was computed for acute and cumulative time frames. The GBRT model's performance on predicting the reported future wellness was compared to the naive baseline's performance by means of absolute prediction error and effect size. The GBRT model outperformed the baseline for the wellness items fatigue, general muscle soreness, stress levels and mood. Additionally, only the combination of external load, internal load, and pre-session perceived wellness resulted in non-trivial effects for predicting future wellness. Including the cumulative load did not improve the predictive performances. The findings may indicate the importance of including both acute load and pre-session perceived wellness in a broad monitoring approach in professional soccer.

#8 A tactical comparison of the 4-2-3-1 and 3-5-2 formation in soccer: A theory-oriented, experimental approach based on positional data in an 11 vs. 11 game set-up
Reference: PLoS One. 2019 Jan 30;14(1):e0210191. doi: 10.1371/journal.pone.0210191. eCollection 2019.
Authors: Memmert D, Raabe D, Schwab S, Rein R
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Summary: The presented field experiment in an 11 vs. 11 soccer game set-up is the first to examine the impact of different formations (e.g. 4-2-3-1 vs. 3-5-2) on tactical key performance indicators (KPIs) using positional data in a controlled experiment. The data were gathered using player tracking systems (1 Hz) in a standardized 11 vs. 11 soccer game. The KPIs were measured using dynamical positioning variables like Effective Playing Space, Player Length per Width ratio, Team Separateness, Space Control Gain, and Pressure Passing Efficiency. Within the experimental positional data analysis paradigm, neither of the team formations showed differences in Effective Playing Space, Team Separateness, or Space Control Gain. However, as a theory-based approach predicted, a 3-5-2 formation for the Player Length per Width ratio and Pressure Passing Efficiency exceeded the 4-2-3-1 formation. Practice task designs which manipulate team formations therefore significantly influence the emergent behavioral dynamics and need to be considered when planning and monitoring performance. Accordingly, an experimental positional data analysis paradigm is a useful approach to enable the development and validation of theory-oriented models in the area of performance analysis in sports games.

#9 Time Trends of Head Injuries Over Multiple Seasons in Professional Male Football (Soccer)
Reference: Sports Med Int Open. 2019 Jan 28;3(1):E6-E11. doi: 10.1055/a-0808-2551. eCollection 2019 Jan.
Authors: Beaudouin F, der Fünten KA, Tröß T, Reinsberger C, Meyer T
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Summary: The present study aimed to investigate time trends of head injuries and their injury mechanisms since a rule change as monitoring may help to identify causes of head injuries and may advance head injury prevention efforts. Based on continuously recorded data from the German football magazine "kicker Sportmagazin ® " as well as other media sources, a database of head injuries in the 1 st German male Bundesliga was generated comprising 11 seasons (2006/07-2016/17). Injury mechanisms were analysed from video recordings. Injury incidence rates (IR) and 95% confidence intervals (95% CI) were calculated. Time trends were analysed via linear regression. Two hundred thirty-eight match head injuries occurred (IR 1.77/1000 match hours, 95% CI 1.56-2.01). There were no significant seasonal changes, expressed as annual average year-on-year change, in IRs over the 11-year period for total head injuries (p=0.693), facial/head fractures (p=0.455), lacerations/abrasions (p=0.162), and head contusions (p=0.106). The annual average year-on-year increase for concussion was 6.4% (p=0.004). Five head injury mechanisms were identified. There were no seasonal changes in injury mechanisms over the study period. The concussion subcategory increased slightly over the seasons, which may either be a result of increasing match dynamics or raised awareness among team physicians and players.

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