Latest research in football - week 34 - 2020

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 Performance changes during the off-season period in football players - Effects of age and previous hamstring injury
Reference: J Sports Sci. 2020 Jul 13;1-11. doi: 10.1080/02640414.2020.1792160. Online ahead of print.
Authors: Jordi Vicens-Bordas, Ernest Esteve, Azahara Fort-Vanmeerhaeghe, Martí Casals, Thomas Bandholm, Lasse Ishøi, David Opar, Anthony Shield, Kristian Thorborg
Summary: The aims of this study were to investigate changes in selected performance measures during an off-season period, their association, and the potential role of age and previous hamstring injury in semi-professional and amateur football players. Seventy-four male players (age: 25 ± 4 years, stature: 178.0 ± 6.6 cm, body mass: 74.9 ± 8.1 kg) were assessed at the beginning and end of the off-season summer-period for sprint, change-of-direction performance and eccentric hamstring strength. Small to medium increases in sprint times were observed at 5 (d = 0.26, p = 0.057), 10 (d = 0.42, p < 0.001) and 30 m (d = 0.64, p < 0.001). Small (d = -0.23, p = 0.033) improvements were observed for COD performance, and no changes in eccentric hamstring strength (d = 0.10, p = 0.317). The changes in the outcomes were not affected by age (p = 0.449 to 0.928) or previous hamstring injury (p = 0.109 to 0.995). The impaired sprint performance was not related to changes in eccentric hamstring strength (r = -0.21 to 0.03, p = 0.213 to 0.856), instead, changes in COD performance were associated with changes in eccentric hamstring strength (r = -0.42, p = 0.008).

#2 Tattoos among professional football players in the 2018-2019 Spanish La Liga season
Reference: Ann Dermatol Venereol. 2020 Jul 9;S0151-9638(20)30249-0.
doi: 10.1016/j.annder.2020.03.008. Online ahead of print.
Authors: N Kluger , R Ahava
Summary: Data regarding tattoos among football players are limited. We investigated the prevalence and characteristics of tattoos among elite players over a full season in the Spanish La Liga. We assessed whether tattoos had any impact on the performance and behavior of players and teams on the pitch. Demographic (age, geographic origin, position), performance (goal-scoring) and disciplinary data (yellow/red cards received) for 476 players and overall team statistics over the 2018-2019 season were analyzed according to the presence of visible tattoos (head and neck, upper arms, lower limbs) for each player. Of the 472 players analyzed, 160 (36%) had visible tattoos (upper limbs, 99%; lower limbs, 18.5%; head and neck, 12%), most of which were in black ink (83%). Players from South and Central America had the highest prevalence of visible tattoos (50%) and significantly more head and neck tattoos than Europeans (19% vs. 10% P=0.02). Tattoos were not significantly related to players' age or position. The mean number of goals scored was higher in the tattooed player group (2.7±4.6 goals vs. 1.9±3.3; P=0.013). There was a correlation between having tattoos and number of goals (Spearman rho 0.103, P=0.034). Tattooed players were more likely to have received≥1 yellow card (91% vs. 83.5%, P=0.03). There was no difference regarding red cards received (15 vs. 14%, P>0.05). The mean number of yellow cards was higher among players with tattoos than those without (4.4±3.2 vs. 3.6±3.2; P=0.01). However, the proportion of tattooed players in a team did not influence the overall team outcomes. The results were no longer significant when we included only players taking part in at least in 22 matches. Among footballers in La Liga, 36% had visible tattoos, with individual variations attributable to differences in geographic, social, cultural and religious background. Having tattoos was associated with certain aspects of individual performance and discipline. The question whether this factor should be taken into consideration by players' agents and team managers remains open.

#3 Systematic Reductions in Differential Ratings of Perceived Exertion Across a 2-Week Repeated-Sprint-Training Intervention That Improved Soccer Players' High-Speed-Running Abilities
Reference: Int J Sports Physiol Perform. 2020 May 6;1-8. doi: 10.1123/ijspp.2019-0568. Online ahead of print.
Authors: Shaun J McLaren, Jonathan M Taylor, Tom W Macpherson, Iain R Spears, Matthew Weston
Summary: The purpose was to quantify changes in differential ratings of perceived exertion (dRPE) across a 2-wk repeated-sprint-training intervention that improved high-intensity intermittent-running ability and linear speed of semiprofessional soccer players.  Thirteen players completed 3 (sessions 1-3) or 4 (sessions 4-6) sets of 7 sprints (group 1 [n = 7]: 30-m straight; group 2 [n = 6]: 2 × 10-m shuttle), with 20 s and 4 min of recovery between sprints and sets, respectively. Postset perceptions of breathlessness (RPE-B) and leg-muscle exertion (RPE-L) were rated using the CR100 scale. Overall, RPE-B (mean [SD]: 46 [13] arbitrary units [AU], "hard") was most likely higher than RPE-L (39 [13] AU, "somewhat hard," mean difference: 8 AU; 90% confidence limits [CLs]: ±2). Set-to-set increases in dRPE (in AU; 90% CL: approximately ±2) were large in session 1 (RPE-B: 15; RPE-L: 14), moderate in sessions 2-5 (RPE-B: 7-10; RPE-L: 7-8), and small (RPE-B: 6) to moderate (RPE-L: 7) in session 6. Across the intervention, RPE-B reduced moderately in sets 3 (-13; 90% CL: ±4) and 4 (-12; 90% CL: ±12) and RPE-L reduced by a small magnitude in set 3 (-5; 90% CL: ±6). The set 4 change in RPE-L was unclear (-11; 90% CL: ±13). The authors observed systematic intrasession and intersession changes in dRPE across a 2-wk repeated-sprint-training intervention, with a fixed prescription of external load that improved semiprofessional soccer players' high-speed-running abilities. These findings could support dRPE as a measure of internal load and highlight its usefulness in evaluating repeated-sprint-training dose-response.

#4 The influence of thermal stress on the physical and technical activities of soccer players: lessons from the 2018 FIFA World Cup in Russia
Reference: Int J Biometeorol. 2020 Jul 16. doi: 10.1007/s00484-020-01964-3. Online ahead of print.
Authors: Marek Konefał, Paweł Chmura, Michał Zacharko, Jarosław Baranowski, Marcin Andrzejewski, Krzysztof Błażejczyk, Jan Chmura
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Summary: The present study attempts to assess changes in soccer players' physical and technical activity profiles due to thermal stress, measured with the Universal Thermal Climate Index (UTCI), in training centres and during matches of the 2018 FIFA World Cup in Russia. The study also verifies the theoretical models of soccer players' physiological parameters. The study sample consisted of 945 observations of 340 players of national teams taking part in the World Cup in Russia. The measured variables included physical activities: total distance covered, distances covered with an intensity of 20-25 km/h, number of sprints; technical activities: number of shots, number of passes, pass accuracy and physiological indicators: evaporative water loss and heart rate. In addition, the final ranking places of each national team were also used in the study. The UTCI was calculated based on meteorological data recorded at training centres and during matches. The UTCI records were then classified into two ranges: NTS-no thermal stress (UTCI 9-26 °C) and TS-thermal stress (UTCI > 26 °C). Climatic conditions at soccer training centres assessed as involving "no thermal stress" are found to be more beneficial for increasing the total distance covered and the number of sprints performed by players during a match. The theoretical models for determining soccer players' physiological parameters used in the study reduce the players' heart rate effort and evaporative water loss, which is in line with findings in studies by other authors. The climatic conditions at soccer training centres and during tournament matches should be taken into account in planning preparations for future World Cup tournaments, especially those in hotter countries.

#5 Multidirectional sprints in soccer: are there connections between linear, curved, and change-of-direction speed performances?
Reference: J Sports Med Phys Fitness. 2020 Jun 16. doi: 10.23736/S0022-4707.20.11155-1. Online ahead of print.
Authors: Tomás T Freitas, Ian Jeffreys , Valter P Reis , Victor Fernandes, Pedro E Alcaraz, Lucas A Pereira, Irineu Loturco
Summary: The aim of this study was to investigate the relationships between linear sprint, curve sprint (CS), and change of direction (COD) abilities and vertical jump performance in elite young soccer players. Twenty-nine players from the same soccer club participated in this study. On the same day, athletes performed countermovement jump (CMJ), 17-m linear sprint (with a 10- m split time), CS (for both sides), and COD tests. A Pearson product moment correlation was performed to determine the associations between the assessed variables. Significance level was set at P< 0.05. Linear sprint was significantly related to CS (r ranging from 0.67 and 0.76; P< 0.05) but not to COD performance (r = 0.23 and 0.33 for 10- and 17-m, respectively; P> 0.05). CS ability (for both good and weak sides) was significantly associated with COD performance (r = 0.60 and 0.54, respectively; P< 0.05). CMJ height was significantly correlated with both linear and CS velocities (r varying between 0.50 and 0.68; P< 0.05), but not with COD velocity (r =0.37; P> 0.05). Based on these findings, it is possible to suggest that training strategies designed to improve vertical jumping capacity may potentially improve both linear and curvilinear sprint abilities. Moreover, increases in COD velocity may also produce positive changes in CS performance.

#6 The effects of soccer training in aerobic capacity between trained and untrained adolescent boys of the same biological age
Reference: J Sports Med Phys Fitness. 2020 Jul 16. doi: 10.23736/S0022-4707.20.11117-4. Online ahead of print.
Authors: Athanasios Mandroukas, Thomas I Metaxas, Yiannis Michailidis, Kosmas Christoulas, Jan Heller
Summary: The purpose of this study was to examine the effects of soccer training on maximal oxygen uptake and anthropometric characteristics in different ages of soccer players and untrained adolescents of the same biological age. A total of one hundred and twenty six (n=126) young soccer players and untrained boys throughout the developmental ages of 12 (soccer players n=22; untrained boys=22) 14 (soccer players n=20; untrained boys= 18) and 16 (soccer players n=22; untrained boys=22) volunteered to participate in the study. Sexual maturation was classified according to Tanner's stages. Soccer players participated both in their school's physical education program and in a soccer training program, while the untrained participated only in their school's physical education program. All participants underwent anthropometric measurements and performed a maximal exercise test on a motor driven treadmill to determine maximal oxygen uptake (V̇O2max) and cardiorespiratory indices. Blood lactate (BL) concentration was determined in the 5th minute of recovery using a lactate photometer. The trained group showed significantly higher VO2max, in absolute and relative values (P<0.001), BLmax (P<0.05) and maximal respiratory exchange ratio (RERmax) (P<0.05) compared to the untrained group. Resting heart rate (HR) and systolic blood pressure were significantly lower (P<0.05) for the trained compared to untrained. The results of this study showed that systematic soccer training has a positive effect in the central cardiovascular system expressed as VO2max, HR and blood pressure.

#7 Peak torque angle, acceleration time and time to peak torque as additional parameters extracted from isokinetic test in professional soccer players: a cross-sectional study
Reference: Sports Biomech. 2020 Jul 16;1-12. doi: 10.1080/14763141.2020.1784260. Online ahead of print.
Authors: Daniel Germano Maciel, Glauko André Figueiredo Dantas, Mikhail Santos Cerqueira, Jean Artur Mendonça Barboza, Vinícius Vieira De Alencar Caldas, Alef Cavalcanti Matias de Barros, Ronan Romeno Varela, Diego Helps Magalhães, Wouber Hérickson de Brito Vieira
Summary: This study investigated additional and traditional variables from isokinetic test of thigh muscles in soccer players across different field positions. One hundred and eighty-nine athletes performed maximal concentric isokinetic knee contractions on dominant (DL) and non-dominant limb (NDL) at 60º/s and 240º/s. The additional outcomes peak torque angle (AngPT), acceleration time (AcT) and time to peak torque (TPT) and traditional outcomes Peak torque (PT), total work (TW) and power (Pw) were extracted from the exam. Goalkeepers (GK), side backs (SB), central backs (CB), central defender midfielders (CDM), central attacking midfielders (CAM) and forwards (FW) were considered. Comparisons between limbs and positions demonstrated that SB extensors of the DL presented TPT lower (p = 0.006) and AngPT higher (p = 0.011) than NDL at 60°/s. CDM extensors of the DL showed lower TPT at 60°/s (p = 0.003) and 240°/s (p = 0.024). CAM flexors of the DL showed lower TPT (p = 0.026) and AcT (p = 0.021) at 240°/s than NDL. CB, CDM and CAM extensors of the NDL showed higher PT, TW and Pw than DL (p < 0.05). In conclusion, there are muscle imbalances between limbs in SB, CDM and CAM and across different field positions.

#8 Walking Soccer: A Systematic Review of a Modified Sport
Reference: Scand J Med Sci Sports. 2020 Jul 15. doi: 10.1111/sms.13772. Online ahead of print.
Authors: Rekesh Corepal, Jia Yu Zhang, Sanya Grover, Harry Hubball, Maureen C Ashe
Summary: Walking soccer (football) is an emerging modified sport gaining recognition globally. The aim was to synthesize current evidence for walking soccer, and provide a summary of global walking soccer organizations. We searched for studies published across all years and all languages within multiple databases for studies focused on walking soccer (football) in adults (18+ years). Two authors independently screened citations at Level 1 and 2. We also conducted a forward citation search and reviewed the reference lists for included studies. We searched the grey literature to identify walking soccer organizations. We conducted the last database search in December 2019. We conducted a standard systematic review following established guidelines. We also summarized findings from a limited search for walking soccer organizations. For peer-reviewed literature, we used the Mixed Methods Appraisal Tool (MMAT) to assess methodological quality, and conducted a narrative synthesis of the evidence. We identified nine peer-reviewed studies (with 117 participants). Most studies included small sample sizes and interventions with short duration. Walking soccer is an emerging modified sport that is popular across the United Kingdom (UK), with its reach extending to other countries. Limited published evidence exists for walking soccer, despite is global popularity. For the studies identified, generalizability was limited to predominately older men from the UK. Based on preliminary findings, walking soccer has the potential to confer health benefits and build social connections.

#9 Individual versus team heart rate variability responsiveness analyses in a national soccer team during training camps
Reference: Sci Rep. 2020 Jul 16;10(1):11726. doi: 10.1038/s41598-020-68698-5.
Authors: Alejandro Muñoz-López , José Naranjo-Orellana
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Summary: Heart rate variability (HRV) analyses can be performed using group or individual changes. Individual changes could be of potential interest during training camps for national soccer teams. The purpose of this study was to compare whether analysis of individual daily HRV could detect changes in cardiac autonomic responses during training camps for national soccer teams. During two different training camps, 34 professional soccer players were monitored daily over 9 days, using heart rate monitors. Players were divided into First Eleven (those who participated in the main squad) or Reserves. Daily HRV was individually analyzed using a day-to-day method or a baseline (days prior to first match) method, using the smallest worthwhile change and the typical error in the estimate to establish a trivial (random change) zone. Group changes were also analyzed using an ANOVA one-way repeated measures test. Players' responsiveness was classified as High-, Low- or Non-response depending on individual changes. Both analyses showed substantial daily individual changes after playing a soccer match, regardless of the group. However, group changes showed that only First Eleven players had significant changes after playing a soccer match. In conclusion, individual daily HRV analyses are useful in detecting individual changes in professional soccer players.

#10 Prevalence and Incidence of Microhemorrhages in Adolescent Football Players
Reference: AJNR Am J Neuroradiol. 2020 Jul;41(7):1263-1268.
doi: 10.3174/ajnr.A6618.
Authors: B R Shah, J M Holcomb, E M Davenport, C M Lack, J M McDaniel, D M Imphean, Y Xi, D A Rosenbaum, J E Urban, B C Wagner, A K Powers, C T Whitlow, J D Stitzel, J A Maldjian
Summary: SWI is an advanced imaging modality that is especially useful in cerebral microhemorrhage detection. Such microhemorrhages have been identified in adult contact sport athletes, and the sequelae of these focal bleeds are thought to contribute to neurodegeneration. The purpose of this study was to utilize SWI to determine whether the prevalence and incidence of microhemorrhages in adolescent football players are significantly greater than those of adolescent noncontact athletes. Preseason and postseason SWI was performed and evaluated on 78 adolescent football players. SWI was also performed on 27 adolescent athletes who reported no contact sport history. Two separate one-tailed Fisher exact tests were performed to determine whether the prevalence and incidence of microhemorrhages in adolescent football players are greater than those of noncontact athlete controls. Microhemorrhages were observed in 12 football players. No microhemorrhages were observed in any controls. Adolescent football players demonstrated a significantly greater prevalence of microhemorrhages than adolescent noncontact controls (P = .02). Although 2 football players developed new microhemorrhages during the season, microhemorrhage incidence during 1 football season was not statistically greater in the football population than in noncontact control athletes (P = .55). Adolescent football players have a greater prevalence of microhemorrhages compared with adolescent athletes who have never engaged in contact sports. While microhemorrhage incidence during 1 season is not significantly greater in adolescent football players compared to adolescent controls, there is a temporal association between playing football and the appearance of new microhemorrhages.

#11 Soccer Heading and Subclinical Neuropsychiatric Symptomatology in Professional Soccer Players
Reference: Neurology. 2020 Jul 10;10.1212/WNL.0000000000010244. doi: 10.1212/WNL.0000000000010244. Online ahead of print.
Authors: Nadav Amitay, Yair Zlotnik, Tara Coreanu, Lior Zeller, Ibrahim Abu-Salameh, Victor Novack, Gal Ifergane
Summary: The aim was to evaluate the association between post-concussive symptomatology and heading in professional soccer players, overcoming the bias of self-reported exposure, we evaluated several clinical neuropsychiatric symptoms using questionnaires after a thorough objective follow-up of players heading-exposure throughout an entire season We collected heading data for all Israeli Premier League players for an entire season using a web-based platform for performance analysis, which enabled us to quantify the exact number of headers per player. Players filled questionnaires regarding post-concussion symptoms, depression, anxiety and sleep disorders. We tested the association between the number of headers and each outcome using a negative binomial regression corrected for the hours played. 159 players were included, of which 79 considered in the high heading exposure group (49%) defined as more than median number of headings (1.34 per game hour). Among players without any past head injury, those with higher heading exposure were less likely to suffer post-concussion symptoms compared to players with low heading exposure (RR per heading per hour=0.94, 95%CI [0.912;0.963]). Players with high heading exposure suffered less from depression symptoms (RR=0.98, 95%CI [0.961;0.997]), anxiety (RR=0.98, 95%CI [0.958;0.997]) and sleep disorders (RR=0.98, 95%CI [0.961;0.996]). Professional soccer players with high heading rate do not display higher post-concussive symptomatology severity. Symptoms among players with low heading exposure might be explained by low resilience, possibly associated with an inferior heading technique. Alternatively, it can reflect heading avoidant behavior.

#12 Reliability of a musculoskeletal profiling test battery in elite academy soccer players
Reference: PLoS One. 2020 Jul 23;15(7):e0236341. doi: 10.1371/journal.pone.0236341. eCollection 2020.
Authors: Neval Grazette, Scot McAllister, Chin Wei Ong, Caroline Sunderland, Mary E Nevill, John G Morris
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Summary: The study aimed to quantify the measurement error / reliability of a musculoskeletal profiling test battery administered in young, elite academy soccer players, and to examine if the order in which the test battery was administered, and who it was administered by, influenced reliability. Players (n = 75; age 12-20 years; stature 1.47-1.95 m; body mass 36-89 kg) from U-12 to U-23 age groups were assigned to either: 1) intra-rater-fixed order; 2) intra-rater-non-fixed order; 3) inter-rater-fixed order; or, 4) inter-rater-non-fixed order groups. On two separate occasions separated by 3 to 7 days, 12 raters conducted a musculoskeletal profiling test battery comprising 10 tests (Supine Medial Hip Rotation, Supine Lateral Hip Rotation, Hamstring 90/90, Prone Medial Hip Rotation [degrees]; Combined Elevation, Thoracic Rotation, Weight-Bearing Dorsiflexion, Y-Balance [centimetres]; Beighton, Lumbar Quadrant [categorical]). The measurement error / reliability for tests measured in degrees and centimetres was evaluated using the intraclass correlation (relative reliability), coefficient of variation and ratio limits of agreement (absolute reliability). Intraclass correlations varied from 0.04 ("poor") to 0.95 ("excellent"), coefficient of variation from 2.9 to 43.4%, and the ratio limits of agreement from 1.058 (*/÷ 1.020) to 2.026 (*/÷ 1.319) for the tests measured in degrees and centimetres. The intraclass correlation, coefficient of variation and ratio limits of agreement were smallest for five out of eight tests measured in degrees and centimetres when the tests were administered in an intra-rater-fixed test order. These findings emphasise that different testing methods, and the administration of a musculoskeletal profiling test battery using a less than optimal design, will influence measurement error and hence test reliability. These observations need to be considered when investigating musculoskeletal function and age, injury, training or asymmetry in young, elite academy soccer players.

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