Latest research in football - week 37 - 2017

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 Strategies for injury prevention in Brazilian football: Perceptions of physiotherapists and practices of premier league teams
Reference: Phys Ther Sport. 2017 Jul 25;28:1-8. doi: 10.1016/j.ptsp.2017.07.004. [Epub ahead of print]
Authors: Meurer MC, Silva MF, Baroni BM
Summary: The purpose was to describe the physiotherapists perceptions and the current practices for injury prevention in elite football (soccer) clubs in Brazil. 16 of the 20 football clubs involved in the Brazilian premier league 2015 were involved in this study. Most physiotherapists (∼88%) were active in design, testing and application of prevention programs. Previous injury, muscle imbalance, fatigue, hydration, fitness, diet, sleep/rest and age were considered "very important" or "important" injury risk factors by all respondents. The methods most commonly used to detect athletes' injury risk were: monitoring of biochemical markers (100% of teams), isokinetic dynamometry (81%), questionnaires (75%), functional movement screen (56%), fleximetry (56%) and horizontal jump tests (50%). All clubs used strength training, functional training, core exercises and balance/proprioception exercises in their injury prevention program; and Nordic hamstring exercise and other eccentric exercises were used by 94% of clubs. "FIFA 11+" prevention program was adapted by 88% of clubs. Physiotherapists perceptions and current practices of injury prevention within Brazilian elite football clubs were similar to those employed in developed countries. There remains a gap between clinical practice and scientific evidence in high performance football.

#2 Prevention of Head and Brain Injuries in Football: Is there a Need to Ban Headings?
Reference: Sportverletz Sportschaden. 2017 Sep;31(3):143-153. doi: 10.1055/s-0043-113208. Epub 2017 Sep 4. [Article in German; Abstract available in German from the publisher]
Authors: Krutsch V, Krutsch W, Jansen P, Hoffmann H, Angele P, Lehmann J, Loose O, Gansslen A, Nerlich M, Weber J.
Summary: Football, the most popular sports worldwide, is associated with a high number of injuries. Head and brain injuries in football are less frequent, but may result in severe long-term damage. The mechanisms of these injuries in football are multifactorial, and the ball is rarely the main cause of a head injury. Short-term, medium-term and long-term consequences of headings in football are insufficiently examined, and there are hardly any case reports about severe episodes. Heading has been banned in US junior football since November 2015, which is understandable considering the higher risk for head injuries in popular American sports such as ice hockey or American football and people's fear associated with this risk. However, the decision to ban heading in football is not based on scientific results, and different experiences have been made in European football. Therefore, this decision should not simply be transferred to Europe. In fact, other injury prevention strategies for head injuries in junior football are necessary, such as the adaptation of ball sizes, which has already been implemented. It is not necessary to change match rules in the form of a general ban on heading in football. Further scientific studies are needed for standardisation in football worldwide.

#3 Does player unavailability affect football teams' match physical outputs? A two-season study of the UEFA champions league
Reference: J Sci Med Sport. 2017 Aug 24. pii: S1440-2440(17)30995-7. doi: 10.1016/j.jsams.2017.08.007. [Epub ahead of print]
Authors: Windt J, Ekstrand J, Khan KM, McCall A, Zumbo BD
Summary: Player unavailability negatively affects team performance in elite football. However, whether player unavailability and its concomitant performance decrement is mediated by any changes in teams' match physical outputs is unknown. We examined whether the number of players injured (i.e. unavailable for match selection) was associated with any changes in teams' physical outputs. Between-team variation was calculated by correlating average team availability with average physical outputs. Within-team variation was quantified using linear mixed modelling, using physical outputs - total distance, sprint count (efforts over 20km/h), and percent of distance covered at high speeds (>14km/h) - as outcome variables, and player unavailability as the independent variable of interest. To control for other factors that may influence match physical outputs, stage (group stage/knockout), venue (home/away), score differential, ball possession (%), team ranking (UEFA Club Coefficient), and average team age were all included as covariates. Teams' average player unavailability was positively associated with the average number of sprints they performed in matches across two seasons. Multilevel models similarly demonstrated that having 4 unavailable players was associated with 20.8 more sprints during matches in 2015/2016, and with an estimated 0.60-0.77% increase in the proportion of total distance run above 14km/h in both seasons. Player unavailability had a possibly positive and likely positive association with total match distances in the two respective seasons. Having more players injured and unavailable for match selection was associated with an increase in teams' match physical outputs.

#4 Hypernetworks Reveal Compound Variables That Capture Cooperative and Competitive Interactions in a Soccer Match
Reference: Front Psychol. 2017 Aug 28;8:1379. doi: 10.3389/fpsyg.2017.01379. eCollection 2017.
Authors: Ramos J, Lopes RJ, Marques P, Araujo D
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Summary: The combination of sports sciences theorization and social networks analysis (SNA) has offered useful new insights for addressing team behavior. However, SNA typically represents the dynamics of team behavior during a match in dyadic interactions and in a single cumulative snapshot. This study aims to overcome these limitations by using hypernetworks to describe illustrative cases of team behavior dynamics at various other levels of analyses. Hypernetworks simultaneously access cooperative and competitive interactions between teammates and opponents across space and time during a match. Moreover, hypernetworks are not limited to dyadic relations, which are typically represented by edges in other types of networks. In a hypernetwork, n-ary relations (with n > 2) and their properties are represented with hyperedges connecting more than two players simultaneously (the so-called simplex-plural, simplices). Simplices can capture the interactions of sets of players that may include an arbitrary number of teammates and opponents. In this qualitative study, we first used the mathematical formalisms of hypernetworks to represent a multilevel team behavior dynamics, including micro (interactions between players), meso (dynamics of a given critical event, e.g., an attack interaction), and macro (interactions between sets of players) levels. Second, we investigated different features that could potentially explain the occurrence of critical events, such as, aggregation or disaggregation of simplices relative to goal proximity. Finally, we applied hypernetworks analysis to soccer games from the English premier league (season 2010-2011) by using two-dimensional player displacement coordinates obtained with a multiple-camera match analysis system provided by STATS (formerly Prozone). Our results show that (i) at micro level the most frequently occurring simplices configuration is 1vs.1 (one attacker vs. one defender); (ii) at meso level, the dynamics of simplices transformations near the goal depends on significant changes in the players' speed and direction; (iii) at macro level, simplices are connected to one another, forming "simplices of simplices" including the goalkeeper and the goal. These results validate qualitatively that hypernetworks and related compound variables can capture and be used in the analysis of the cooperative and competitive interactions between players and sets of players in soccer matches.

#5 Monitoring training load and fatigue in soccer players with physiological markers
Reference: Physiol Behav. 2017 Sep 5. pii: S0031-9384(17)30278-0. doi: 10.1016/j.physbeh.2017.09.004. [Epub ahead of print]
Authors: Djaoui L, Haddad M, Chamari K, Dellal A
Summary: The quantification and monitoring of training load (TL) has been the topic of many scientific works in the last fifteen years. TL monitoring helps coaches to individually prescribe, follow-up, analyse, adjust and programme training sessions. In particular, the aim of the present review was to provide a critical literature report regarding different physiological markers of TL monitoring, particularly in soccer, as the load is specific to individual sports. Therefore, the interests and limitations of heart rate (HR), HR variability (HRV) and biochemical variables (blood, urinary and hormonal variations) were analysed, with a special focus on daily measures (before, during and after training) and monitoring throughout a whole season. It appears that the most relevant markers were the resting HR before training, HR reserve during training, HRV during rest days, blood lactate, and blood and salivary immunological status in follow-ups throughout the season. Urinary markers indicative of the players' hydration status also deserve attention. However, these objective markers should be considered with a subjective marker of TL such as the rating of perceived exertion to give a more precise quantification of TL and its perception. Future research could be directed towards urinary marker analysis and the analysis of specific markers of TL, which could be related to injury occurrence and to performance during competition.

#6 Perception, Action, and Cognition of Football Referees in Extreme Temperatures: Impact on Decision Performance
Reference: Front Psychol. 2017 Aug 29;8:1479. doi: 10.3389/fpsyg.2017.01479. eCollection 2017.
Authors: Gaoua N, de Oliveira RF, Hunter S
Summary: Different professional domains require high levels of physical performance alongside fast and accurate decision-making. Construction workers, police officers, firefighters, elite sports men and women, the military and emergency medical professionals are often exposed to hostile environments with limited options for behavioral coping strategies. In this (mini) review we use football refereeing as an example to discuss the combined effect of intense physical activity and extreme temperatures on decision-making and suggest an explicative model. In professional football competitions can be played in temperatures ranging from -5°C in Norway to 30°C in Spain for example. Despite these conditions, the referee's responsibility is to consistently apply the laws fairly and uniformly, and to ensure the rules are followed without waning or adversely influencing the competitiveness of the play. However, strenuous exercise in extreme environments imposes increased physiological and psychological stress that can affect decision-making. Therefore, the physical exertion required to follow the game and the thermal strain from the extreme temperatures may hinder the ability of referees to make fast and accurate decisions. Here, we review literature on the physical and cognitive requirements of football refereeing and how extreme temperatures may affect referees' decisions. Research suggests that both hot and cold environments have a negative impact on decision-making but data specific to decision-making is still lacking. A theoretical model of decision-making under the constraint of intense physical activity and thermal stress is suggested. Future naturalistic studies are needed to validate this model and provide clear recommendations for mitigating strategies.

#7 Trends in aggressive play and refereeing among the top five European soccer leagues
Reference: J Sports Sci. 2017 Sep 12:1-9. doi: 10.1080/02640414.2017.1377911. [Epub ahead of print]
Authors: Sapp RM, Spangenburg EE, Hagberg JM
Summary: Current trends suggest professional soccer is becoming less aggressive, with England often argued to have the most aggressive of the top European leagues. The purpose of this study was to investigate differences in fouls and cards as indicators of aggressive play in the first divisions of England, France, Germany, Italy, and Spain over the past decade. Number of fouls per match and per yellow card has decreased in all leagues since 2007/08, though attempted tackles per foul has not changed or has increased. A lack of substantial rule changes suggests players have become less aggressive in tackling as opposed to referees becoming more lenient. Total number of fouls and cards per match were consistently lower in the English Premier League, however attempted tackles per foul was higher. The data also demonstrate the notions of home advantage and potentially referee bias, since referees tended to call more fouls and award more cards to away teams. Lastly, number of attempted tackles per foul and yellow cards received exhibited the strongest correlations with final league position across the leagues. In conclusion, our data support that elite European soccer has become less aggressive and the English Premier League is the most aggressive league.

#8 Effects of repeated-sprints with changes of direction on youth soccer player's performance: Impact of initial fitness level
Reference: J Strength Cond Res. 2017 Sep 6. doi: 10.1519/JSC.0000000000002232. [Epub ahead of print]
Authors: Nakamura FY, Sanchez-Sanchez J, Ramirez-Campillo R, Petisco C, Gonzalo-Skok O, Rodriguez-Fernandez A, Minano J
Summary: The aim of this study was to assess the effect of an 8-week repeated sprint with change of direction training program on repeated sprint, intermittent endurance and change of direction performance in youth soccer players with different aerobic fitness levels. Athletes were randomly assigned into a soccer-only (n=9; age, 14.9±0.4 yr) and soccer plus repeated sprint with change of direction (RS-COD) training programs for players with high (n=10; age, 14.4±0.5 yr) and low (n=10; age, 14.4±0.5 yr) aerobic fitness. RS-COD was completed two days per week, involving three sets of ten 18-m sprints with two changes of direction of 90°. The soccer-only group achieved greater improvements in intermittent endurance (ES = 0.61) and sprint decrement during RS ability test (ES = 0.77) compared to the RS-COD groups (ES = 0.19 to 0.24; ES = -0.27 to 0.02; respectively). RS-COD training improved repeated sprint (ES = 0.16 to 0.38) and change of direction (ES = 0.48) performance in high, but not in low fitness players (ES = 0.03 to 0.13; ES = 0.16; respectively). Moreover, improvements in repeated sprint and change of direction were (possibly) greater in high compared to low fitness and soccer-only trained players (possibly to very likely). In conclusion, although RS-COD training may positively affect repeated sprint and change of direction performance, its effects may vary according to the initial aerobic fitness of youth soccer players, with trivial effect on intermittent endurance in high-aerobic fitness players and possible beneficial effect on low-aerobic fitness players.

#9 A Cross-Continental Study on Children's Drawings of Football Players: Implications for Understanding Key Issues and Controversies in Human Figure Drawings
Reference: Eur J Psychol. 2017 Aug 31;13(3):455-471. doi: 10.5964/ejop.v13i3.1237. eCollection 2017 Aug.
Authors: Baluch B, Duffy LJ, Badami R, Pereira ECA
Summary: Professionals examine various aspects of girls' and boys' drawings as a way of understanding their intelligence, personality and emotional state. However, the extent to which such measures could be universally generalised or attributed to a specific cultural norm is still a debatable issue. In the present study five key features of children's drawings namely: the size (height) of the drawings, profile or full face, figure in action or static, shaded or non-shaded and the nature of additional details were examined from a cross-cultural perspective, and by providing a topic (football) for which children's drawing of a human figure could provide opportunities for the latter indices to manifest and flourish. Children from three countries; England, Iran and Brazil, representing three continents took part in this study. The participants were asked to draw a football player from their own country and from the other participating countries. The results showed that Brazilian children differ from Iranian and English children by drawing significantly smaller figures and putting more football action in the drawings. Shading of the figure drawn was more prevalent amongst English children. Such findings have implications for the interpretation of key aspects of children's drawings in educational, clinical and therapeutic settings and from a universal vs. culturally-specific viewpoint.

American Football
#1 What is the Value of a National Football League Draft Pick? An Analysis Based on Changes Made in the Collective Bargaining Agreement
Reference:  J Strength Cond Res. 2017 Jun 2. doi: 10.1519/JSC.0000000000002045. [Epub ahead of print]
Authors: Kraeutler MJ, Carver TJ, Belk JW, McCarty EC
Summary: The purpose of this study was to analyze and compare the value of players drafted in early rounds of the NFL Draft since the new collective bargaining agreement (CBA) began in 2011. The NFL's player statistics database and database of player contract details were searched for players drafted in the first three rounds of the 2011 to 2013 NFL Drafts. Performance outcomes specific to each position were divided by each player's salary to calculate a value statistic. Various demographics, NFL Combine results, and total number of games missed due to injury were also recorded for each player. These statistics were compared within each position between players selected in the 1st round of the NFL Draft (Group A) versus those drafted in the 2nd or 3rd round (Group B). A total of 147 players were included (Group A 35, Group B 112). Overall, players in Group A were significantly taller (p < 0.01) and heavier (p = 0.037) than players in Group B. Group B demonstrated significantly greater value statistics than Group A for quarterbacks (p = 0.028), wide receivers (p < 0.001), defensive tackles (p = 0.019), and cornerbacks (p < 0.001). No significant differences were found between groups with regard to number of games missed due to injury. Players drafted in the 2nd or 3rd rounds of the NFL Draft often carry more value than those drafted in the 1st round. NFL teams may wish to more frequently trade down in the Draft rather than trading up.

#2 Nano-Composite Foam Sensor System in Football Helmets
Reference: Ann Biomed Eng. 2017 Sep 7. doi: 10.1007/s10439-017-1910-9. [Epub ahead of print]
Authors: Merrell AJ, Christensen WF, Seeley MK, Bowden AE, Fullwood DT
Summary: American football has both the highest rate of concussion incidences as well as the highest number of concussions of all contact sports due to both the number of athletes and nature of the sport. Recent research has linked concussions with long term health complications such as chronic traumatic encephalopathy and early onset Alzheimer's. Understanding the mechanical characteristics of concussive impacts is critical to help protect athletes from these debilitating diseases and is now possible using helmet-based sensor systems. To date, real time on-field measurement of head impacts has been almost exclusively measured by devices that rely on accelerometers or gyroscopes attached to the player's helmet, or embedded in a mouth guard. These systems monitor motion of the head or helmet, but do not directly measure impact energy. This paper evaluates the accuracy of a novel, multifunctional foam-based sensor that replaces a portion of the helmet foam to measure impact. All modified helmets were tested using a National Operating Committee Standards for Athletic Equipment-style drop tower with a total of 24 drop tests (4 locations with 6 impact energies). The impacts were evaluated using a headform, instrumented with a tri-axial accelerometer, mounted to a Hybrid III neck assembly. The resultant accelerations were evaluated for both the peak acceleration and the severity indices. These data were then compared to the voltage response from multiple Nano Composite Foam sensors located throughout the helmet. The foam sensor system proved to be accurate in measuring both the HIC and Gadd severity index, as well as peak acceleration while also providing additional details that were previously difficult to obtain, such as impact energy.

#3 Examining Play Counts and Measurements of Injury Incidence in Youth Football
Reference: J Athl Train. 2017 Sep 7. doi: 10.4085/1062-6050-52.7.06. [Epub ahead of print]
Authors: Kerr ZY, Yeargin SW, Djoko A, Dalton SL, Niceley MM, Dompier TP
Summary: Whereas researchers have provided estimates for the number of head impacts sustained within a youth football season, less is known about the number of plays across which such impact exposure occurs.  The objective was to estimate the number of plays in which youth football players participated during the 2013 season and to estimate injury incidence through play-based injury rates. Youth football players (N = 2098; age range, 5-15 years) from 105 teams in 12 recreational leagues across 6 states participated in this study.  We calculated the average number of athlete-plays per season and per game using independent-samples t tests to compare age groups (5-10 years old versus 11-15 years old) and squad sizes (<20 versus ≥20 players); game injury rates per 1000 AE and per 10 000 athlete-plays; and injury rate ratios (IRRs) with 95% confidence intervals (CIs) to compare age groups. On average, youth football players played 333.9 ± 178.5 plays per season and 43.9 ± 24.0 plays per game. Age groups (5- to 10-year-olds versus 11- to 15-year-olds) did not differ in the average number of plays per season (335.8 versus 332.3, respectively; t2086.4 = 0.45, P = .65) and per game (44.1 versus 43.7, respectively; t2092.3 = 0.38, P = .71). However, players from smaller teams participated in more plays per season (373.7 versus 308.0; t1611.4 = 8.15, P < .001) and per game (47.7 versus 41.4; t1523.5 = 5.67, P < .001). Older players had a greater game injury rate than younger players when injury rates were calculated per 1000 AEs (23.03 versus 17.86/1000 AEs; IRR = 1.29; 95% CI = 1.04, 1.60) or per 10 000 athlete- plays (5.30 versus 4.18/10 000 athlete-plays; IRR = 1.27; 95% CI = 1.02, 1.57). A larger squad size was associated with a lower average number of plays per season and per game. Increasing youth football squad sizes may help reduce head-impact exposure for individual players. The AE-based injury rates yielded effect estimates similar to those of play-based injury rates.

#4 Editorial: Do Orthopaedic Surgeons Belong on the Sidelines at American Football Games?
Reference: Clin Orthop Relat Res. 2017 Sep 5. doi: 10.1007/s11999-017-5483-6. [Epub ahead of print]
Authors: Leopold SS, Dobbs MB, Gebhardt MC, Gioe TJ, Rimnac CM, Wongworawat MD
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Australian Football
#1 Australian Football League concussion guidelines: what do community players think?
Reference: BMJ Open Sport Exerc Med. 2016 Nov 28;2(1):e000169. doi: 10.1136/bmjsem-2016-000169. eCollection 2016.
Authors: White PE, Donaldson A, Sullivan SJ, Newton J, Finch CF
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Summary: Preventing concussion in sport is a global challenge. To assess community-level adult male Australian Football players' views on following the Australian Football League's (AFL) concussion guidelines. 3 focus groups, each comprising 6 players from 1 regional league, were conducted until saturation of issues raised. Discussions followed a semistructured script and were audio-recorded and transcribed verbatim. Thematic analysis was conducted by 2 coders independently. Identified advantages of the guidelines included highlighting the seriousness of concussion; changing the culture around playing with concussion and shifting return-to-play decision responsibility from players to others. Disadvantages included players being removed from play unnecessarily; removal of players' rights to decide if they are fit to play and players changing their behaviours to avoid being removed from play. Identified facilitators to guideline use included local league enforcement; broad information dissemination and impartial medically trained staff to assess concussion. Identified barriers to guideline use included players' desire to play at all costs; external pressure that encouraged players to return to play prematurely; and inconvenience and cost. Players generally understand that the AFL concussion guidelines protect their long-term welfare. However, their desire to play at all costs and help their team win is a common barrier to reporting concussion and adhering to guidelines. Leagues should take a lead role by mandating and enforcing the use of the guidelines and educating coaches, game day medical providers and players. The return-to-play component of the guidelines is complex and needs further consideration in the context of community sport.

#2 Effect of Prior Injury on Changes to Biceps Femoris Architecture across an Australian Football League Season
Reference: Med Sci Sports Exerc. 2017 Oct;49(10):2102-2109. doi: 10.1249/MSS.0000000000001333.
Authors: Timmins RG, Bourne MN, Hickey JT, Maniar N, Tofari PJ, Williams MD, Opar DA
Summary: The purpose was to assess in-season alterations of biceps femoris long head (BFlh) fascicle length in elite Australian footballers with and without a history of unilateral hamstring strain injury (HSI) in the past 12 months. Thirty elite Australian football players were recruited. Twelve had a history of unilateral HSI. Eighteen had no HSI history. All had their BFlh architecture assessed at approximately monthly intervals, six times across a competitive season. The previously injured limb's BFlh fascicles increased from the start of the season and peaked at week 5. Fascicle length gradually decreased until the end of the season, where they were shortest. The contralateral uninjured limb's fascicles were the longest when assessed at week 5 and showed a reduction in-season where weeks 17 and 23 were shorter than week 1. Control group fascicles were longest at week 5 and reduced in-season. The previously injured limb's BFlh fascicles were shorter than the control group at all weeks and the contralateral uninjured limb at week 5. Compared with the control group, the contralateral uninjured limb had shorter fascicles from weeks 9 to 23. Athletes with a history of HSI end the season with shorter fascicles than they start. Limbs without a history of HSI display similar BFlh fascicle lengths at the end of the season as they begin with. All athletes increase fascicle length at the beginning of the season; however, the extent of the increase differed based on history of HSI. These findings show that a HSI history may influence structural adaptation of the BFlh in-season.

#3 The validity and reliability of the sphygmomanometer for hip strength assessment in Australian football players
Reference: Physiother Theory Pract. 2017 Sep 11:1-6. doi: 10.1080/09593985.2017.1374492. [Epub ahead of print]
Authors: Toohey LA, de Noronha M, Taylor C, Thomas J
Summary: The purpose was to investigate the intra-rater and inter-rater reliability of the sphygmomanometer for the assessment of the adductor squeeze test and isometric hip abduction strength and to investigate the concurrent validity of the sphygmomanometer for the assessment of hip muscular strength. Thirty-two healthy adult male community Australian football players (age 23.9 ± 4.5 years) were assessed by two blinded raters that measured the strength of the adductor squeeze test and isometric hip abduction, using a commercially available sphygmomanometer. Concurrent validity was calculated using handheld dynamometry as the reference standard. Moderate to high intra-rater reliability (ICC = 0.61 to 0.92) and high inter-rater reliability (ICC = 0.77 to 0.91) were found. High concurrent validity (Pearson's r = 0.77 to 0.91) was established. Sixteen of the participants reached the maximal reading of the sphygmomanometer, demonstrating a ceiling effect. A sphygmomanometer is a cost-efficient device that appears to be both reliable and valid for the assessment of hip strength, offering clinicians an alternate and easily accessible option to obtain objective strength data. A ceiling effect may limit the application of the sphygmomanometer as a strength measurement device in stronger individuals.

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