Latest research in football - week 43 - 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 Is there a correlation between coaches' leadership styles and injuries in elite football teams? A study of 36 elite teams in 17 countries
Reference: Br J Sports Med. 2017 Oct 22. pii: bjsports-2017-098001. doi: 10.1136/bjsports-2017-098001. [Epub ahead of print]
Authors: Ekstrand J, Lundqvist D, Lagerback L, Vouillamoz M, Papadimitiou N, Karlsson J
Summary: Do coaches' leadership styles affect injury rates and the availability of players in professional football? Certain types of leadership behaviour may cause stress and have a negative impact on players' health and well-being. The aim of the study was to investigate the transformational leadership styles of head coaches in elite men's football and to evaluate the correlation between leadership styles, injury rates and players' availability. Medical staff from 36 elite football clubs in 17 European countries produced 77 reports at four postseason meetings with a view to assessing their perception of the type of leadership exhibited by the head coaches of their respective teams using the Global Transformational Leadership scale. At the same time, they also recorded details of individual players' exposure to football and time-loss injuries. There was a negative correlation between the overall level of transformational leadership and the incidence of severe injuries (rho=-0.248; n=77; p=0.030); high levels of transformational leadership were associated with smaller numbers of severe injuries. Global Transformational Leadership only explained 6% of variation in the incidence of severe injuries (r2=0.062). The incidence of severe injuries was lower at clubs where coaches communicated a clear and positive vision, supported staff members and gave players encouragement and recognition. Players' attendance rates at training were higher in teams where coaches gave encouragement and recognition to staff members, encouraged innovative thinking, fostered trust and cooperation and acted as role models. There is an association between injury rates and players' availability and the leadership style of the head coach.

#2 Sex and age differences in head acceleration during purposeful soccer heading
Reference: Res Sports Med. 2017 Oct 25:1-11. doi: 10.1080/15438627.2017.1393756. [Epub ahead of print]
Authors: Caccese JB, Buckley TA, Tierney RT, Rose WC, Glutting JJ, Kaminski TW
Summary: Differences in head-neck segment mass, purposeful heading technique, and cervical strength and stiffness may contribute to differences in head accelerations across sex and age. The purpose of this study was to compare head acceleration across sex and age (youth [12-14 years old], high school and collegiate) during purposeful soccer heading. One-hundred soccer players (42 male, 58 female, 17.1 ± 3.5 years, 168.5 ± 20.3 cm, 61.5 ± 13.7 kg) completed 12 controlled soccer headers at an initial ball velocity of 11.2 m/s. Linear and rotational accelerations were measured using a triaxial accelerometer and gyroscope and were transformed to the head centre-of-mass. A MANOVA revealed a significant multivariate main effect for sex (Pillai's Trace = .165, F(2,91) = 11.868, p < .001), but not for age (Pillai's Trace = .033, F(4,182) = 0.646, p = .630). Peak linear and rotational accelerations were higher in females (40.9 ± 13.3 g; 3279 ± 1065 rad/s2) than males (27.6 ± 8.5 g, 2219 ± 823 rad/s2). These data suggest that under controlled soccer heading conditions, females may be exposed to higher head accelerations than males.

#3 Multifactorial examination of sex-differences in head injuries and concussions among collegiate soccer players: NCAA ISS, 2004-2009
Reference: Inj Epidemiol. 2017 Oct 25;4(1):28. doi: 10.1186/s40621-017-0127-6.
Authors: Chandran A, Barron MJ, Westerman BJ, DiPietro L
Summary: While head injuries and concussions are major concerns among soccer players, the multifactorial nature of head injury observations in this group remains relatively undefined. We aim to extend previous analyses and examine sex-differences in the incidence of head injuries, odds of head injuries within an injured sample, and severity of head injuries, among collegiate soccer players between 2004 and 2009. Data collected within the National Collegiate Athletic Association (NCAA) Injury Surveillance System (ISS) between the years of 2004 and 2009, were analyzed in this study. Unadjusted rate ratios (RR), compared incidence rates between categories of sex, injury mechanism, setting and competition level. We also examined sex-differences in head injury incidence rates, across categories of the other covariates. Multivariable logistic regression and negative binomial regression modeling tested the relation between sex and head injury corollaries, while controlling for contact, setting, and competition level. Between 2004 and 2009, head injuries accounted for approximately 11% of all soccer-related injuries reported within the NCAA-ISS. The rate of head injuries among women was higher than among men (RR = 1.23, 95% CI = [1.08, 1.41]). The rate of head injuries due to player-to-player contact was comparable between women and men (RR = 0.95, 95% CI = [0.81, 1.11]). Whereas, the rate of injury due to contact with apparatus (ball/goal) was nearly 2.5 times higher (RR = 2.46, 95% CI = [1.76, 3.44]) and the rate due to contact with a playing surface was over two times higher (RR = 2.29, 95% CI = [1.34, 3.91]) in women than in men. In our multifactorial models, we also observed that the association between sex and head injury corollaries varied by injury mechanism. Sex-differences in the incidence, odds (given an injury), and severity (concussion diagnosis, time-loss) of head injuries varied by injury mechanism (player-to-player contact vs. all other mechanisms) in this sample.

#4 Leg Stiffness in Female Soccer Players: Intersession Reliability and the Fatiguing Effects of Soccer-Specific Exercise
Reference: J Strength Cond Res. 2017 Nov;31(11):3052-3058. doi: 10.1519/JSC.0000000000001715.
Authors: De Ste Croix MBA, Hughes JD, Lloyd RS, Oliver JL, Read PJ
Summary: Low levels of leg stiffness and reduced leg stiffness when fatigue is present compromise physical performance and increase injury risk. The purpose of this study was to (a) determine the reliability of leg stiffness measures obtained from contact mat data and (b) explore age-related differences in leg stiffness after exposure to a soccer-specific fatigue protocol in young female soccer players. Thirty-seven uninjured female youth soccer players divided into 3 subgroups based on chronological age (under 13 [U13], under 15 [U15], and under 17 [U17] year-olds) volunteered to participate in the study. After baseline data collection, during which relative leg stiffness, contact time, and flight time were collected, participants completed an age-appropriate soccer-specific fatigue protocol (SAFT). Upon completion of the fatigue protocol, subjects were immediately retested. Intersession reliability was acceptable and could be considered capable of detecting worthwhile changes in performance. Results showed that leg stiffness decreased in the U13 year-olds, was maintained in the U15 age group, and increased in the U17 players. Contact times and flight times did not change in the U13 and U15 year-olds, but significantly decreased and increased, respectively, in the U17 age group. The data suggest that age-related changes in the neuromuscular control of leg stiffness are present in youth female soccer players. Practitioners should be aware of these discrepancies in neuromuscular responses to soccer-specific fatigue, and should tailor training programs to meet the needs of individuals, which may subsequently enhance performance and reduce injury risk.

#5 Biomechanical Differences of Multidirectional Jump Landings Among Female Basketball and Soccer Players
Reference: J Strength Cond Res. 2017 Nov;31(11):3034-3045. doi: 10.1519/JSC.0000000000001785.
Authors: Taylor JB, Ford KR, Schmitz RJ, Ross SE, Ackerman TA, Shultz SJ
Summary: Anterior cruciate ligament (ACL) injury prevention programs are less successful in basketball than soccer and may be due to distinct movement strategies that these athletes develop from sport-specific training. The purpose of this study was to identify biomechanical differences between female basketball and soccer players during multidirectional jump landings. Lower extremity biomechanics of 89 female athletes who played competitive basketball (n = 40) or soccer (n = 49) at the middle- or high-school level were analyzed with 3-dimensional motion analysis during a drop vertical jump, double- (SAG-DL) and single-leg forward jump (SAG-SL), and double- (FRONT-DL) and single-leg (FRONT-SL) lateral jump. Basketball players landed with either less hip or knee, or both hip and knee excursion during all tasks (p ≤ 0.05) except for the SAGSL task, basketball players landed with greater peak hip flexion angles (p = 0.04). The FRONT-SL task elicited the most distinct sport-specific differences, including decreased hip adduction (p < 0.001) angles, increased hip internal rotation (p = 0.003), and increased relative knee external rotation (p = 0.001) excursions in basketball players. In addition, the FRONT-SL task elicited greater forces in knee abduction (p = 0.003) and lesser forces in hip adduction (p = 0.001) and knee external rotation (p < 0.001) in basketball players. Joint energetics were different during the FRONT-DL task, as basketball players exhibited less sagittal plane energy absorption at the hip (p < 0.001) and greater hip (p < 0.001) and knee (p = 0.001) joint stiffness. Sport-specific movement strategies were identified during all jump landing tasks, such that soccer players exhibited a more protective landing strategy than basketball players, justifying future efforts toward sport-specific ACL injury prevention programs.

#6 Head Impact Exposure in Youth Soccer and Variation by Age and Sex
Reference: Clin J Sport Med. 2017 Oct 20. doi: 10.1097/JSM.0000000000000497. [Epub ahead of print]
Authors: Chrisman SPD, Ebel BE, Stein E, Lowry SJ, Rivara FP
Summary: The purpose was to examine variation in head impact exposure (HIE) by age and sex in youth soccer. Youth soccer athletes (11-14 years old) in local clubs participated in this study.  Head impact exposure measured using adhesive-mounted accelerometers during 1 month of soccer. Forty-six youth athletes (54% female) participated. No athlete reported a concussion during the study. More males than females had at least 1 head impact ≥15 g (P = 0.02). Of those who sustained a head impact above the 15-g threshold (57%), females sustained HIE of greater magnitude than males (median 47.4 g vs 33.3 g, P = 0.04). Eighty-five percent of athletes on U14 teams had at least 1 head impact ≥15 g compared with 15% of athletes on U12 teams (P < 0.001). Poisson regression stratified by sex and controlling for team-suggested age effects were significant only for females (P = 0.02). There was significant variation in HIE by team. There were no decrements in concussion symptoms, health-related quality of life, or neuropsychological testing after 1 month of soccer play. There is significant variation in HIE in youth soccer, which seems to be influenced by age and sex. Further studies are needed to better understand potential significance for injury prevention.

#7 Combined visual and dribbling performance in young soccer players of different expertise
Reference: Res Sports Med. 2017 Oct 23:1-8. doi: 10.1080/15438627.2017.1393751. [Epub ahead of print]
Authors: Bekris E, Gissis I, Ispyrlidis I, Mylonis E, Axeti G
Summary: We aimed to evaluate dribbling performance in terms of technique and visual skills assessment of both young experienced (EX, n = 24) and novice (NO, n = 24) soccer players. Both groups performed two dribbling tests with four levels of difficulty in visual signals (A1-A4 and B1-B4; B - half distance of A; 1 - no visual signal; 4 - signal with the shorter flashing time). All players performed slower when visual signals were added to the testing process (~2.5 s; p < 0.01). EX completed all tests faster than NO (~3 s, p < 0.01). The average number of visual mistakes was significantly lower for EX than NO in all tests (p < 0.01). These results demonstrated the importance of evaluating dribbling along with visual stimuli in young soccer EX and NO players.

#8 Positional synchronization affects physical and physiological responses to preseason in professional football (soccer)
Reference: Res Sports Med. 2017 Oct 23:1-13. doi: 10.1080/15438627.2017.1393754. [Epub ahead of print]
Authors: Folgado H, Goncalves B, Sampaio J
Summary: This study aimed to identify changes in tactical, physical and physiological performances in large-sided games during the preseason of elite footballers. Thirty professional football players participated in several GK+8vs.8+GK large-sided games across the first four weeks of the season. Players were monitored by GPS units and heart rate monitors to quantify physical, physiological and tactical performances. The variables were compared according to the preseason period, players' positioning and professional experience. The training situation promoted similar physiological responses during the first and the last training period. However, players were revealed to have higher levels of positional synchronization during the last preseason period, indicating an improved tactical performance. Tactical variables seem to reflect the improvement of players' performance during the preseason, measured in large-sided games situation, while affecting both physical and physiological demands. These results highlight the potential of positioning derived variables, concurrently to physical and physiological variables, for football training optimization.

American Football
#1 Risk and Causes of Death among Former National Football League Players (1986-2012)
Reference: Med Sci Sports Exerc. 2017 Oct 26. doi: 10.1249/MSS.0000000000001466. [Epub ahead of print]
Authors: Lincoln AE, Vogel RA, Allen TW, Dunn RE, Alexander K, Kaufman ND, Tucker AM
Summary: Previous research identified decreased overall and cardiovascular mortality for National Football League (NFL) players from the 1959-1988 era. The present study explored the mortality risk among recent NFL players who played in an era of heavier linemen and nearly year-round physical conditioning. This cohort study included 9778 former NFL players with at least one year in the NFL whose last season was between 1986 and 2012. Players' pension fund records were matched to the National Death Index (NDI) to determine vital status, date of death, and cause of death. Standardized mortality ratios (SMRs) compared player mortality through 2014 with US men of the same age, race, and calendar year. Cox proportional hazards models assessed the effect of player characteristics on overall and cardiovascular mortality. Two percent (n=227) of players were deceased with a median age at death of 38 years (range: 23-61). The most common major causes of death were diseases of the heart (n=47, 21%), violence (n=39, 17%), and transportation injuries (n=34, 15%). Risk of death was significantly lower than the general population for overall mortality (SMR 0.46, 95% CI 0.40-0.52), cardiovascular disease (SMR 0.65, 95% CI 0.50-0.84), and other major causes. Players with playing-time BMI>35 kg/m had significantly higher cardiovascular disease mortality (SMR 2.20, 95% CI 1.32-3.44) than the general population and higher overall mortality risk (SRR 3.84, 95% CI 2.66-5.54) than players with BMI<30 kg/m. Consistent with an earlier NFL cohort and other elite athlete populations, the overall and cardiovascular mortality risk of this NFL cohort was significantly lower than the general US male population, likely attributable to a healthy worker effect and less smoking.However, players with the highest playing-time BMI exhibited elevated cardiovascular mortality risk.

#2 Acute changes in plasma total Tau levels are independent of subconcussive head impacts in college football players
Reference: J Neurotrauma. 2017 Oct 26. doi: 10.1089/neu.2017.5376. [Epub ahead of print]
Authors: Kawata K, Rubin LH, Wesley L, Lee J, Sim T, Takahagi M, Bellamy A, Tierney R, Langford D
Summary: Athletes in contact sports sustain repetitive subconcussive head impacts in a brief window, yet neurophysiological sequelae from repetitive subconcussion remain unclear. This prospective longitudinal study examined a relationship between changes in plasma Tau protein levels and subconcussive impact kinematic data in 23 Division-I collegiate football players during a series of pre-season practices. Plasma measures for Tau and S100β proteins, symptom scores, and near point of convergence were obtained at preseason baseline and pre-post practices. During each practice, impact frequency and linear and rotational head accelerations were recorded via an accelerometer-embedded mouth guard. There were significant elevations in plasma Tau levels at all post-practice time points compared to those of pre-practice and baseline levels. However, the highest degree of elevation in plasma Tau was observed after the first practice, for which players sustained the lowest number of hits and magnitudes for these hits. Subconcussive impact exposure during practice (e.g., head impact frequency and magnitude) did not predict increased plasma Tau levels. Concussion history and years of football experience were also unrelated to changes in plasma Tau levels. Increases in plasma Tau levels were associated with increases in S100β levels only after the first practice. There were no significant aassociations between changes in Tau levels, symptom scores, or near point of convergence. These data suggest that the changes in levels of circulating Tau protein were independent from subconcussive head impact exposure, pointing to the possibility that other factors may have played roles in changes in plasma Tau levels.

#3 Why Professional Football Players Chose Not to Reveal Their Concussion Symptoms During a Practice or Game
Reference: Clin J Sport Med. 2017 Oct 20. doi: 10.1097/JSM.0000000000000495. [Epub ahead of print]
Authors: Delaney JS, Caron JG, Correa JA, Bloom GA
Summary: The purpose was to determine why professional football players in Canada decided not to seek medical attention during a game or practice when they believed they had suffered a concussion. Four hundred fifty-four male professional football players participated in this study. Reasons athletes did not seek medical attention for a presumed concussion during the previous season, how often this occurred and how important these reasons were in the decision process were used as outcome measured. One hundred six of the 454 respondents (23.4%) believed they had suffered a concussion during their previous football season and 87 of the 106 (82.1%) did not seek medical attention for a concussion at least once during that season. The response "Did not feel the concussion was serious/severe and felt you could still continue to play with little danger to yourself" was the most commonly listed reason (49/106) for not seeking medical attention for a presumed concussion. Many players answered that they did not seek medical attention because they did not want to be removed from a game (42/106) and/or they did not want to risk missing future games (41/106) by being diagnosed with a concussion. Some professional football players who believed they had suffered a concussion chose not to seek medical attention at the time of injury. Players seemed educated about the concussion evaluation process and possible treatment guidelines, but this knowledge did not necessarily translate into safe and appropriate behavior at the time of injury.

Gaelic Football
#1 Lower limb injuries in men's elite Gaelic football: A prospective investigation among division one teams from 2008 to 2015
Reference: J Sci Med Sport. 2017 Sep 6. pii: S1440-2440(17)31028-9. doi: 10.1016/j.jsams.2017.08.023. [Epub ahead of print]
Authors: Roe M, Murphy JC, Gissane C, Blake C
Summary: The purpose was to prospectively investigate incidence and associated time-loss of lower limb injuries in elite Gaelic football. Additionally, to identify sub-groups of elite players at increased risk of sustaining a lower limb injury. Team physiotherapists provided exposure and injury on a weekly basis to the National Gaelic Athletic Association (GAA) Injury Surveillance Database. Injury was defined using a time-loss criterion. Fifteen different teams participated throughout the 8-year study providing 36 team datasets from 2008 to 2015. Lower limb injuries (n=1239) accounted for 83.5% (95% CI 82.0-85.0) and 77.6% (95% CI 75.8-79.4) of training and match-play injuries, respectively. Injury incidence was 4.5 (95% CI 3.7-5.2) and 38.4 (95% CI 34.3-42.60) per 1000 training and match-play hours, respectively. One-in-four (25.0%, 95% CI 22.4-27.0) lower limb injuries were recurrent. Non-contact injuries accounted for 80.9% (95% CI 79.2-82.6) of cases. The median team rate was 30 (IQR 24-43) lower limb injuries per season resulting in 840.8 (95% CI 773.3-908.2) time-loss days. Previously injured players had a 2.5-times (OR 95% CI 2.2-2.8) greater risk of sustaining a lower limb injury. Overall, 56.8% of players with a previous lower limb injury sustained another. Incidence was higher for forward players and those aged >25years. Lower limb injuries are the most common injury among elite division one Gaelic football teams. Injury risk management should become an ongoing component of a player's development programme and consider injury history, age, and playing position.

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