Latest research in football - week 17 - 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 Talent identification and recruitment in youth soccer: Recruiter's perceptions of the key attributes for player recruitment
Reference: PLoS One. 2017 Apr 18;12(4):e0175716. doi: 10.1371/journal.pone.0175716. eCollection 2017.
Authors: Larkin P, O'Connor D
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Summary: Using the modified Delphi method, we aimed to understand the attributes youth coaches and recruiters perceive as important when identifying skilled youth performance at the entry level of representative soccer in Australia (i.e., Under 13 years). Furthermore, we also aimed to describe the current methods youth coaches and recruiters use to assess and identify these attributes in youth players. Australian regional youth technical directors and coaches (n = 20) completed a three stage process, including an initial interview and two subsequent questionnaires, whereby attributes and qualities associated with talent identification were rated and justified according to the importance for youth player performance and talent identification. Results indicate a hierarchy of attributes recruiters perceive as important for Under 13 soccer performance, including technical (i.e., first touch, striking the ball, one-versus-one ability, and technical ability under pressure), tactical (i.e., decision-making ability) and psychological attributes (i.e., coachability and positive attitude). In addition, the findings indicated attributes and qualities not emphasised within the talent identification process including, physiological, anthropometrical, sociological and several psychological attributes. It is suggested talent recruiters apply a holistic multidisciplinary approach to talent identification, with the current findings potentially providing initial evidence to suggest recruiters do consider numerous attributes when selecting and identifying youth players.

#2 Unilateral jumps in different directions: a novel assessment of soccer-associated power?
Reference: J Sci Med Sport. 2017 Mar 29. pii: S1440-2440(17)30348-1. doi: 10.1016/j.jsams.2017.03.016. [Epub ahead of print]
Authors: Murtagh CF, Vanrenterghem J, O'Boyle A, Morgans R, Drust B, Erskine RM
Summary: We aimed to determine whether countermovement jumps (CMJs; unilateral and bilateral) performed in different directions assessed independent lower-limb power qualities, and if unilateral CMJs would better differentiate between elite and non-elite soccer players than the bilateral vertical (BV) CMJ. Elite (n=23; age, 18.1±1.0years) and non-elite (n=20; age, 22.3±2.7years) soccer players performed three BV, unilateral vertical (UV), unilateral horizontal-forward (UH) and unilateral medial (UM) CMJs. Jump performance (height and projectile range), kinetic and kinematic variables from ground reaction forces, and peak activation levels of the vastus lateralis and biceps femoris (BF) muscles from surface electromyography, were compared between jumps and groups of players. Peak vertical power (V-power) was greater in BV (220.2±30.1W/kg) compared to UV (144.1±16.2W/kg), which was greater than UH (86.7±18.3W/kg) and UM (85.5±13.5W/kg) (all, p<0.05) but there was no difference between UH and UM (p=1.000). Peak BF EMG was greater in UH compared to all other CMJs (p≤0.001). V-power was greater in elite than non-elite for all CMJs (p≤0.032) except for BV (p=0.197). Elite achieved greater UH projectile range than non-elite (51.6±15.4 vs. 40.4±10.4cm, p=0.009). We have shown that UH, UV and UM CMJs assess distinct lower-limb muscular power capabilities in soccer players. Furthermore, as elite players outperformed non-elite players during unilateral but not BV CMJs, unilateral CMJs in different directions should be included in soccer-specific muscular power assessment and talent identification protocols, rather than the BV CMJ.

#3 Range limitation in hip internal rotation and fifth metatarsal stress fractures (Jones fracture) in professional football players
Reference: Knee Surg Sports Traumatol Arthrosc. 2017 Apr 25. doi: 10.1007/s00167-017-4552-4. [Epub ahead of print]
Authors: Saita Y, Nagao M, Kawasaki T, Kobayashi Y, Kobayashi K, Nakajima H, Takazawa Y, Kaneko K, Ikeda H
Summary: The purpose of the study was to identify unknown risk factors associated with fifth metatarsal stress fracture (Jones fracture). A case-controlled study was conducted among male Japanese professional football (soccer) players with (N = 20) and without (N = 40) a history of Jones fracture. Injury history and physical examination data were reviewed, and the two groups were compared. Univariate and multivariate logistic regression controlling for age, leg dominance and body mass index were used to obtain odds ratios (ORs) and 95% confidence intervals (CIs) to describe the association between physical examination data and the presence or absence of Jones fractures. From 2000 to 2014, among 162 professional football club players, 22 (13.6%; 21 Asians and one Caucasian) had a history of Jones fracture. Thirteen out of 22 (60%) had a Jones fracture in their non-dominant leg. The mean range of hip internal rotation (HIR) was restricted in players with a history of Jones fracture [25.9° ± 7.5°, mean ± standard deviation (SD)] compared to those without (40.4° ± 11.1°, P < 0.0001). Logistic regression analyses demonstrated that HIR limitation increased the risk of a Jones fracture (OR = 3.03, 95% CI 1.45-6.33, P = 0.003). Subgroup analysis using data prior to Jones fracture revealed a causal relationship, such that players with a restriction of HIR were at high risk of developing a Jones fracture [Crude OR (95% CI) = 6.66 (1.90-23.29), P = 0.003, Adjusted OR = 9.91 (2.28-43.10), P = 0.002]. In addition, right HIR range limitation increased the risks of developing a Jones fracture in the ipsilateral and the contralateral feet [OR = 3.11 (1.35-7.16) and 2.24 (1.22-4.12), respectively]. Similarly, left HIR range limitation increased the risks in the ipsilateral or the contralateral feet [OR (95% CI) = 4.88 (1.56-15.28) and 2.77 (1.08-7.08), respectively]. The restriction of HIR was associated with an increased risk of developing a Jones fracture. Since the HIR range is a modifiable factor, monitoring and improving the HIR range can lead to prevent reducing the occurrence of this fracture.

#4 Testosterone and cortisol responses in male soccer players: The effect of home and away venues
Reference: Physiol Behav. 2017 Apr 21. pii: S0031-9384(16)30994-5. doi: 10.1016/j.physbeh.2017.04.021. [Epub ahead of print]
Authors: Fothergill M, Wolfson S, Neave N
Summary: The present studies examined the influence of playing venue on psychobiological responses in male soccer players. Many studies have demonstrated the existence of a home advantage, wherein teams perform better at home than away. A recent focus has attempted to explain this advantage from a psychobiological perspective, with studies showing hormonal differences with regard to venue, game outcome, dominance and perceived stress. Two studies investigated testosterone and cortisol responses in relation to home and away venues. In an initial study of 18 male elite Premier League academy soccer players (age, 17.47, SD, 64), salivary cortisol levels were monitored in two competitive matches, both at home and away. Higher post-game cortisol levels were observed at home (p=0.002), with the team winning all its games. In a second study involving a 12 semi-professional group of players (age, 23.17, SD, 3.8), the same post-game cortisol findings at home were replicated (p=0.001), with this team losing all its games. No effects were observed for testosterone in either study. The results extend earlier research findings on the complex relationship which surrounds the psychobiological impact on the home advantage. The findings suggest that higher levels of stress are experienced by home players in their home matches.

#5 Wellbeing perception and the impact on external training output among elite soccer players
Reference: J Sci Med Sport. 2017 Apr 13. pii: S1440-2440(17)30360-2. doi: 10.1016/j.jsams.2017.03.019. [Epub ahead of print]
Authors: Malone S, Owen A, Newton M, Mendes B, Tiernan L, Hughes B, Collins K
Summary: The objective of the investigation was to observe the impact of player wellbeing on the training output of elite soccer players. Forty-eight soccer players (age: 25.3±3.1years; height: 183±7cm; mass: 72±7kg) were involved in this single season observational study across two teams. Each morning, pre-training, players completed customised perceived wellbeing questionnaires. Global positioning technology devices were used to measure external load (total distance, total high-speed running distance, high speed running, player load, player load slow, maximal velocity, maximal velocity exposures). Players reported ratings of perceived exertion using the modified Borg CR-10 scale. Integrated training load ratios were also analysed for total distance:RPE, total high speed distance:RPE player load:RPE and player load slow:RPE respectively. Mixed-effect linear models revealed significant effects of wellbeing Z-score on external and integrated training load measures. A wellbeing Z-score of -1 corresponded to a -18±2m (-3.5±1.1%), 4±1m (-4.9±2.1%,) 0.9±0.1kmh-1 (-3.1±2.1%), 1±1 (-4.6±2.9%), 25±3AU (-4.9±3.1%) and 11±0.5AU (-8.9±2.9%) reduction in total high speed distance, high speed distance, maximal velocity, maximal velocity exposures, player load and player load slow respectively. A reduction in wellbeing impacted external:internal training load ratios and resulted in -0.49±0.12mmin-1, -1.20±0.08mmin-1,-0.02±0.01AUmin-1 in total distance:RPE, total high speed distance:RPE and player load slow:RPE respectively. The results suggest that systematic monitoring of player wellbeing within soccer cohorts can provide coaches with information about the training output that can be expected from individual players during a training session.

#6 Reliability and Validity of a New Test of Agility and Skill for Female Amateur Soccer Players
Reference: J Hum Kinet. 2017 Mar 12;56:219-227. doi: 10.1515/hukin-2017-0039. eCollection 2017.
Authors: Kutlu M, Yapici H, Yilmaz A
Summary: The aim of this study was to evaluate the Agility and Skill Test, which had been recently developed to assess agility and skill in female athletes. Following a 10 min warm-up, two trials to test the reliability and validity of the test were conducted one week apart. Measurements were collected to compare soccer players' physical performance in a 20 m sprint, a T-Drill test, the Illinois Agility Run Test, change-of-direction and acceleration, as well as agility and skill. All tests were completed following the same order. Thirty-four amateur female soccer players were recruited (age = 20.8 ± 1.9 years; body height = 166 ± 6.9 cm; body mass = 55.5 ± 5.8 kg). To determine the reliability and usefulness of these tests, paired sample t-tests, intra-class correlation coefficients, typical error, coefficient of variation, and differences between the typical error and smallest worthwhile change statistics were computed. Test results showed no significant differences between the two sessions (p > 0.01). There were higher intra-class correlations between the test and retest values (r = 0.94-0.99) for all tests. Typical error values were below the smallest worthwhile change, indicating 'good' usefulness for these tests. A near perfect Pearson correlation between the Agility and Skill Test (r = 0.98) was found, and there were moderate-to-large levels of correlation between the Agility and Skill Test and other measures (r = 0.37 to r = 0.56). The results of this study suggest that the Agility and Skill Test is a reliable and valid test for female soccer players and has significant value for assessing the integrative agility and skill capability of soccer players.

#7 Analysis of Motor Activities of Professional Soccer Players during the 2014 World Cup in Brazil
Reference: J Hum Kinet. 2017 Mar 12;56:187-195. doi: 10.1515/hukin-2017-0036. eCollection 2017.
Authors: Chmura P, Andrzejewski M, Konefał M, Mroczek D, Rokita A, Chmura J
Summary: The aim of the present study was to analyze motor activities of soccer players in seven consecutive rounds of matches of the 2014 World Cup in Brazil and to compare the performance of the world champions, the German national team with other participating teams. The study sample comprised 905 observations of 340 soccer players, who played full-time matches in all seven rounds of the tournament. The study was conducted using data collected from the Castrol Performance Index, a kinematic game analysis system that records movements of players with semi-automatic cameras. The following variables were analyzed: total distance covered, the percentage of total distance covered at high intensity, the number of sprints, frequency of sprints and peak running speed. A statistically significant increase (p ≤ 0.01) was noted in total distance covered, the percentage of distance covered at high intensity and total number of sprints, between the quarter-finals and semi-finals of the World Cup tournament in Brazil. The German national team covered a significantly longer total distance (p ≤ 0.05) and had a greater percentage of distance covered at high intensity (p ≤ 0.001) than players from other teams. The obtained results point to the necessity of development of players' aerobic endurance and speed-endurance abilities while preparing for top-level soccer tournaments. Winning a soccer championship requires players to run longer mean total distances and longer distances at high intensity during a single match.

#8 Effects of the off-Season Period on Field and Assistant Soccer Referees `Physical Performance
Reference: J Hum Kinet. 2017 Mar 12;56:159-166. doi: 10.1515/hukin-2017-0033. eCollection 2017.
Authors: Castillo D, Cámara J, Castagna C, Yanci J
Summary: The evolution of referees' physical fitness has been studied over one or several seasons, however, the variation of the physical performance between the end of the competitive season (T1) and the start of the following pre-season (T2) has not been ascertained. Therefore, the aim of this study was to analyze the effects of the transition period on physical performance variables (i.e. linear straight sprint, change of direction ability and endurance) in National Soccer Division referees. Forty-five Spanish referees volunteered to participate in this study. Participants were classified according to competitive status, field referees (FR, n = 23) and assistant referees (AR, n = 22). A loss of performance (p < 0.05) was observed in the 20 and 30 m linear straight sprint between T1 and T2 in both FR (1.64-1.56%, d = 0.29 to 0.32) and AR (2.01-3.41%, d = 0.33 to 0.60). In T2 the FR significantly improved the distance covered (p < 0.05, 13.11%, d = 0.39) in the Yo-Yo Intermittent Recovery test (YYIR1). Besides, significant differences were observed between FR and AR in the distance covered (p < 0.05, -23.55%, d = -0.97) in the YYIR1 test in T2. More research may be necessary to focus on the off-season period in order to implement specific training programs and consequently reduce the loss of sprint ability in field and assistant referees and the decrease in cardiovascular fitness in assistant referees.

#9 Determination of Aerobic Performance in Youth Soccer Players: Effect of Direct And Indirect Methods
Reference: J Hum Kinet. 2017 Mar 11;56:109-118. doi: 10.1515/hukin-2017-0028. eCollection 2017.
Authors: Higino WP, Sorroche AS, de Mattos Falqueiro PG, Suzuki Lima YC, Higa CL
Summary: This study was conducted to correlate and compare values for variables determined in indirect tests with the values determined directly in youth soccer players. The study subjects were 27 youth soccer players (age 16.77 ± 0.75 years; body mass 63.29 ± 7.37 kg; body height 174.14 ± 8.46 cm) playing in the basic categories of a first division team at the regional level of Brazilian soccer. Each subject was evaluated with the following tests: a) a treadmill test to directly determine values of VO2max and Vamax (Treadmill); b) an indirect Shuttle Run Test (SRT); c) an indirect Carminatti's test (TCar). VO2max showed significantly different values in the Treadmill and the SRT (59.21 ± 5.88 and 50.67 ± 3.58 ml⋅kg-1⋅min-1, respectively). Similarly, values obtained for VPeak in the treadmill test and for Vamax in TCar were different from values for SRT VPeak (15.01 ± 1.10, 14.92 ± 0.87 and 12.64 ± 0.62 km⋅h-1, respectively). A correlation analysis showed a moderate relationship between values for VPeak TCar and VO2max determined on a treadmill (r = 0.46) and Vamax determined on a treadmill (r = 0.54). The analysis also showed a high correlation between values of VO2max determined on the treadmill and VO2max evaluated in the SRT (r = 0.69), as well as VPeak determined in the SRT and VO2max tested on the treadmill (r = 0.71), as well as between VPeak determined in the SRT and VO2max evaluated on the treadmill (r = 0.77). We concluded that the SRT underestimated values of VO2max and Vamax. Additionally, VPeak TCar showed no difference compared to Vamax, although it did show a low correlation with it. In addition the SRT, even with high correlations, did not seem to be a great predictor of aerobic fitness in youth soccer players.

#10 Effects of environmental temperature on physiological responses during submaximal and maximal exercises in soccer players
Reference: Integr Med Res. 2016 Sep;5(3):216-222. doi: 10.1016/j.imr.2016.06.002. Epub 2016 Jun 16.
Authors: No M, Kwak HB
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Summary: Although thermoregulation is effective in regulating body temperature under normal conditions, exercise or physical activity in extreme cold or heat exerts heavy stress on the mechanisms that regulate body temperature. The purpose of this study was to investigate the effects of environmental temperature on physiological responses and endurance exercise capacity during submaximal and maximal exercises in healthy adults. Nine male soccer players participated in this study. In this study, three environmental temperatures were set at 10 ± 1°C, 22 ± 1°C, and 35 ± 1°C with the same humidity (60 ± 10%). The participants cycled for 20 minutes at 60% maximum oxygen uptake (60% VO2max), and then exercise intensity was increased at a rate of 0.5 kp/2 min until exhaustion at three different environmental conditions. Oxygen uptake and heart rate were lower in a moderate environment (22 ± 1°C) than in a cool (10 ± 1°C) or hot (35 ± 1°C) environment at rest and during submaximal exercise, and were higher during maximal exercise (p < 0.05). Minute ventilation was lower at 22 ± 1°C than at 10 ± 1°C or 35 ± 1°C at rest and during submaximal exercise, and no significant differences were observed in minute ventilation during maximal exercise (p < 0.05). Blood lactate concentrations were lower at 22 ± 1 °C than at 10 ± 1°C or 35 ± 1°C at rest and during submaximal exercise, and were higher during maximal exercise (p < 0.05). Time to exhaustion during exercise was longer at 22 ± 1°C than at 10 ± 1°C or 35 ± 1°C (p < 0.05). It is concluded that physiological responses and endurance exercise capacity are impaired under cool or hot conditions compared with moderate conditions, suggesting that environmental temperature conditions play an important role for exercise performance.

#11 Enhancing the implementation of injury prevention exercise programmes in professional football
Reference: Br J Sports Med. 2017 May 3. pii: bjsports-2017-097539. doi: 10.1136/bjsports-2017-097539. [Epub ahead of print]
Authors: O'Brien J

American Football
#1 Game Times and Higher Winning Percentages of West Coast Teams of the National Football League Correspond With Reduced Prevalence of Regular Season Injury: Erratum.
Reference: J Strength Cond Res. 2017 May;31(5):e72. doi: 10.1519/JSC.0000000000001851.
Authors: no authors listed

#2 Acute Effect of Biomechanical Muscle Stimulation on the Counter-Movement Vertical Jump Power and Velocity in Division I Football Players
Reference: J Strength Cond Res. 2017 May;31(5):1259-1264. doi: 10.1519/JSC.0000000000001136.
Authors: Jacobson BH, Monaghan TP, Sellers JH, Conchola EC, Pope ZK, Glass RG
Summary: Research regarding whole body vibration (WBV) largely supports such training augmentation in attempts to increase muscle strength and power. However, localized biomechanical vibration has not received the same attention. The purpose of this study was to assess peak and average power before and after acute vibration of selected lower-body sites in division I athletes. Twenty-one subjects were randomly assigned to 1 of 2 conditions using a cross-over design. Pretest consisted of a counter-movement vertical jump (VJ) followed by either localized vibration (30 Hz) to 4 selected lower-body areas or 4 minutes of moderately low-resistance stationary cycling (70 rpm). Vibration consisted of 1 minute bouts at each lower-leg site for a total of 4 minutes followed by an immediate post-test VJ. Repeated measures analysis of variance yielded no significant differences (p > 0.05) in either peak power or peak velocity. Similarly, no significant differences were found for average power and velocity between conditions. It should be noted that, while not significant, the vibration condition demonstrated an increase in peak power and velocity while the bike condition registered slight decreases. Comparing each of the post-VJ repetitions (1, 2, and 3) the vibration condition experienced significantly greater peak power and velocity from VJ 1 to VJ 3 compared with the bike condition which demonstrated no significant differences among the post-test VJs. These results yielded similar, although not statistically significant outcomes to previous studies using WBV. However, the novelty of selected site biomechanical vibration merits further investigation with respect to frequency, magnitude, and duration of vibration.

#3 Epidemiology of Knee Sprains in Youth, High School, and Collegiate American Football Players
Reference: J Athl Train. 2017 Apr 17. doi: 10.4085/1062-6050-52.3.09. [Epub ahead of print]
Authors: Clifton DR, Onate JA, Schussler E, Djoko A, Dompier TP, Kerr ZY
Summary: Variations in knee-sprain incidence among competition levels are unclear but may help inform prevention strategies in American football players. The purpose of the study was to describe the epidemiology of knee sprains in youth, high school, and collegiate football players. Injury and athlete-exposure (AE) data were collected from 3 injury-surveillance programs at the youth, high school, and collegiate competition levels. Data from 310 youth, 184 high school, and 71 collegiate football team-seasons were collected during the 2012 through 2014 seasons. Knee-sprain rates and risks were calculated for each competition level. Injury rate ratios (IRRs) and risk ratios (RRs) compared knee-sprain rates by competition level. Injury proportion ratios (IPRs) compared differences in surgery needs, recurrence, injury mechanism, and injury activity by competition level. Knee-sprain rates in youth, high school, and collegiate football were 0.16/1000 AEs, 0.25/1000 AEs, and 0.69/1000 AEs, respectively. Knee-sprain rates increased as the competition level increased (high school versus youth: IRR = 1.60; 95% confidence interval [CI] = 1.12, 2.30; collegiate versus high school: IRR = 2.73; 95% CI = 2.38, 3.96). Knee-sprain risk was highest in collegiate (4.3%), followed by high school (2.0%) and youth (0.5%) athletes. Knee-sprain risk increased as the competition level increased (high school versus youth: RR = 3.73; 95% CI = 2.60, 5.34; collegiate versus high school: RR = 2.14; 95% CI = 1.83, 2.51). Collegiate football had the lowest proportion of knee sprains that were noncontact injuries (collegiate versus youth: IPR = 0.54; 95% CI = 0.31, 0.95; collegiate versus high school: IPR = 0.59; 95% CI = 0.44, 0.79) and the lowest proportion that occurred while being tackled (collegiate versus youth: IPR = 0.44; 95% CI = 0.26, 0.76; collegiate versus high school: IPR = 0.71; 95% CI = 0.51, 0.98). Knee-sprain incidence was highest in collegiate football. However, level-specific variations in the distributions of knee sprains by injury activity may highlight the need to develop level-specific policies and prevention strategies that ensure safe sports play.

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