Latest research in football - week 4 - 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 Symptoms from repeated intentional and unintentional head impact in soccer players
Reference: Neurology. 2017 Feb 1. pii: 10.1212/WNL.0000000000003657. doi: 10.1212/WNL.0000000000003657. [Epub ahead of print]
Authors: Stewart WF, Kim N, Ifrah CS, Lipton RB, Bachrach TA, Zimmerman ME, Kim M, Lipton ML
Summary: The purpose was to determine the rate and differential contribution of heading vs unintentional head impacts (e.g., head to head, goal post) to CNS symptoms in adult amateur soccer players. Amateur soccer players completed baseline and serial on-line 2-week recall questionnaires (HeadCount) and reported (1) soccer practice and games, (2) heading and unintentional soccer head trauma, and (3) frequency and severity (mild to very severe) of CNS symptoms. For analysis, CNS symptoms were affirmed if one or more moderate, severe, or very severe episodes were reported in a 2-week period. Repeated measures logistic regression was used to assess if 2-week heading exposure (i.e., 4 quartiles) or unintentional head impacts (i.e., 0, 1, 2+) were associated with CNS symptoms. A total of 222 soccer players (79% male) completed 470 HeadCount questionnaires. Mean (median) heading/2 weeks was 44 (18) for men and 27 (9.5) for women. One or more unintentional head impacts were reported by 37% of men and 43% of women. Heading-related symptoms were reported in 20% (93 out of 470) of the HeadCounts. Heading in the highest quartile was significantly associated with CNS symptoms (odds ratio [OR] 3.17, 95% confidence interval [CI] 1.57-6.37) when controlling for unintentional exposure. Those with 2+ unintentional exposures were at increased risk for CNS symptoms (OR 6.09, 95% CI 3.33-11.17) as were those with a single exposure (OR 2.98, 95% CI 1.69-5.26) when controlling for heading. Intentional (i.e., heading) and unintentional head impacts are each independently associated with moderate to very severe CNS symptoms.

#2 No Effect of Generalized Joint Hypermobility on Injury Risk in Elite Female Soccer Players
Reference: Am J Sports Med. 2017 Feb;45(2):286-293. doi: 10.1177/0363546516676051. Epub 2016 Dec 14.
Authors: Blokland D, Thijs KM, Backx FJ, Goedhart EA, Huisstede BM
Summary: Although it has been suggested that generalized joint hypermobility (GJH) is a risk factor for injury in soccer players, it remains unclear whether this applies to elite female soccer players. The purpose was to investigate whether GJH is a risk factor for injury in elite female soccer players. Elite female soccer players in the Netherlands were screened at the start of the 2014-2015 competitive season. GJH was assessed using the Beighton score. Soccer injuries and soccer exposure were registered throughout the entire season. Poisson regression was performed to calculate incidence risk ratios (IRRs) using different cutoff points of the Beighton score (≥3, ≥4, and ≥5) to indicate GJH. Of the 114 players included in the study, 20 were classified as hypermobile (Beighton score ≥4). The mean (±SD) injury incidence per player was 8.40 ± 9.17 injuries/1000 hours of soccer, with no significant difference between hypermobile and nonhypermobile players. GJH was not a risk factor for injuries when using Beighton score cutoff points of ≥3 (IRR = 1.06 [95% CI, 0.74-1.50]; P = .762), ≥4 (IRR = 1.10 [95% CI, 0.72-1.68]; P = .662), or ≥5 (IRR = 1.15 [95% CI, 0.68-1.95]; P = .602). Similarly, GJH was not a significant risk factor for thigh, knee, or ankle injuries evaluated separately. This study indicates that GJH is not a risk factor for injuries in elite female soccer players, irrespective of Beighton score cutoff point. Hypermobile players at this elite level might have improved their active stability and/or used braces to compensate for joint laxity.

#3 Functional Performance Among Active Female Soccer Players After Unilateral Primary Anterior Cruciate Ligament Reconstruction Compared With Knee-Healthy Controls
Reference: Am J Sports Med. 2017 Feb;45(2):377-385. doi: 10.1177/0363546516667266. Epub 2016 Oct 7.
Authors: Faltstrom A, Hagglund M, Kvist J
Summary: Good functional performance with limb symmetry is believed to be important to minimize the risk of injury after a return to pivoting and contact sports after anterior cruciate ligament reconstruction (ACLR). This study aimed to investigate any side-to-side limb differences in functional performance and movement asymmetries in female soccer players with a primary unilateral anterior cruciate ligament (ACL)-reconstructed knee and to compare these players with knee-healthy controls from the same soccer teams. This study included 77 active female soccer players at a median of 18 months after ACLR (interquartile range [IQR], 14.5 months; range, 7-39 months) and 77 knee-healthy female soccer players. The mean age was 20.1 ± 2.3 years for players with an ACL-reconstructed knee and 19.5 ± 2.2 years for controls. We used a battery of tests to assess postural control (Star Excursion Balance Test) and hop performance (1-legged hop for distance, 5-jump test, and side hop). Movement asymmetries in the lower limbs and trunk were assessed with the drop vertical jump and the tuck jump using 2-dimensional analyses. The reconstructed and uninvolved limbs did not differ in any of the tests. In the 5-jump test, players with an ACL-reconstructed knee performed worse than controls (mean 8.75 ± 1.05 m vs 9.09 ± 0.89 m; P = .034). On the drop vertical jump test, the ACL-reconstructed limb had significantly less knee valgus motion in the frontal plane (median 0.028 m [IQR, 0.049 m] vs 0.045 m [IQR, 0.043 m]; P = .004) and a lower probability of a high knee abduction moment (pKAM) (median 69.2% [IQR, 44.4%] vs 79.8% [IQR, 44.8%]; P = .043) compared with the control players' matched limb (for leg dominance). Results showed that 9% to 49% of players in both groups performed outside recommended guidelines on the different tests. Only 14 players with an ACL-reconstructed knee (18%) and 15 controls (19%) had results that met the recommended guidelines for all 5 tests ( P = .837). The reconstructed and uninvolved limbs did not differ, and players with an ACL-reconstructed knee and controls differed only minimally on the functional performance tests, indicating similar function. It is worth noting that many players with an ACL-reconstructed knee and controls had movement asymmetries and a high pKAM pattern, which have previously been associated with an increased risk for both primary and secondary ACL injury in female athletes.

#4 The influence of football shoe characteristics on athletic performance and injury risk – a review
Reference: Footwear Science, Received 21 Oct 2016, Accepted 16 Jan 2017, Published online: 03 Feb 2017
Author: Kulessa DJ, Gollhofer A, Gehring D
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Summary: Footwear is the most relevant tool of a footballer's gear and has to cope with the individual sport-specific demands of the athlete. The challenge in the development of a football shoe is to fine-tune its functional parameters to meet criteria of injury prevention and performance enhancement. There exists a contradiction between those two set-targets that needs to be addressed. The objective of the review is to provide a literature survey about the scientific knowledge on how football shoe characteristics influence performance and injury risk. Almost 100 scientific publications are included in a qualitative synthesis. The outsole configuration and its influence on traction can be regarded as the most studied functional shoe parameter and has been scientifically proven to affect the player's performance in translational locomotion and the injury risk in rotational movements. The second main aspect consists of the interaction between shoe and ball, which can be divided into the influence of shoe construction on kicking velocity, accuracy and ball control. Footwear properties, such as bending and torsional stiffness, cushioning and comfort, have barely been studied, if at all. The present literature survey shows a substantial influence of football footwear on the athlete. Currently available literature suggests that outsole design influences shoe–surface interaction, which has implications regarding performance and injury risk. Additionally, shoe upper seems to play a crucial role with regard to the performance in ball interactions. However, further research is needed on shoe–foot interactions and on footwear requirements for female, adolescent and indoor players.

#5 Evaluating injury risk in first and second league professional Portuguese soccer: muscular strength and asymmetry
Reference: J Hum Kinet. 2016 Jul 2;51:19-26. doi: 10.1515/hukin-2015-0166. eCollection 2016
Authors: Carvalho A, Brown S, Abade E
Summary: Strength imbalances between the hamstrings and quadriceps are an essential predictor for hamstring strain in soccer. The study aimed to investigate and compare the muscle strength imbalances of professional soccer players of different performance levels. One hundred and fifty nine senior male professional soccer players from first (n = 75) and second league (n = 84) Portuguese clubs participated in this study. Muscle strength was evaluated with a REV9000 isokinetic dynamometer. Maximal peak torque data were used to calculate quadriceps and hamstrings strength during concentric and eccentric actions, bilateral asymmetry, conventional strength ratios and dynamic control ratios. Second league athletes produced slightly lower conventional strength ratios in the right and left legs (ES = 0.22, p = 0.17 and ES = 0.36, p = 0.023, respectively) compared to the first league athletes. No significant differences were found in dynamic control ratios or in bilateral asymmetry among first and second league athletes. These findings do not show a clear link between the competitive level and injury risk in soccer players. However, some of the differences found, particularly in conventional strength ratios, highlight the importance of performing off-season and pre-season strength assessments to prescribe and adjust individual strength training programs among professional soccer players.

#6 Muscle Strength and Speed Performance in Youth Soccer Players
Reference: J Hum Kinet. 2016 Apr 13;50:203-210. doi: 10.1515/hukin-2015-0157. eCollection 2016
Authors: Penailillo L, Espildora F, Jannas-Vela S, Mujika I, Zbinden-Foncea H
Summary: This study aimed to examine the relationship between maximum leg extension strength and sprinting performance in youth elite male soccer players. Sixty-three youth players (12.5 ± 1.3 years) performed 5 m, flying 15 m and 20 m sprint tests and a zigzag agility test on a grass field using timing gates. Two days later, subjects performed a one-repetition maximum leg extension test (79.3 ± 26.9 kg). Weak to strong correlations were found between leg extension strength and the time to perform 5 m (r = -0.39, p = 0.001), flying 15 m (r = -0.72, p < 0.001) and 20 m (r = -0.67, p < 0.001) sprints; between body mass and 5 m (r = -0.43, p < 0.001), flying 15 m (r = -0.75, p < 0.001), 20 m (r = -0.65, p < 0.001) sprints and agility (r =-0.29, p < 0.001); and between height and 5 m (r = -0.33, p < 0.01) and flying 15 m (r = -0.74, p < 0.001) sprints. Our results show that leg muscle strength and anthropometric variables strongly correlate with sprinting ability. This suggests that anthropometric characteristics should be considered to compare among youth players, and that youth players should undergo strength training to improve running speed.

#7 Acute Effect of Different Combined Stretching Methods on Acceleration and Speed in Soccer Players
Reference: J Hum Kinet. 2016 Apr 13;50:179-186. doi: 10.1515/hukin-2015-0154. eCollection 2016
Authors: Amiri-Khorasani M, Calleja-Gonzalez J, Mogharabi-Manzari M
Summary: The purpose of this study was to investigate the acute effect of different stretching methods, during a warm-up, on the acceleration and speed of soccer players. The acceleration performance of 20 collegiate soccer players (body height: 177.25 ± 5.31 cm; body mass: 65.10 ± 5.62 kg; age: 16.85 ± 0.87 years; BMI: 20.70 ± 5.54; experience: 8.46 ± 1.49 years) was evaluated after different warm-up procedures, using 10 and 20 m tests. Subjects performed five types of a warm-up: static, dynamic, combined static + dynamic, combined dynamic + static, and no-stretching. Subjects were divided into five groups. Each group performed five different warm-up protocols in five non-consecutive days. The warm-up protocol used for each group was randomly assigned. The protocols consisted of 4 min jogging, a 1 min stretching program (except for the no-stretching protocol), and 2 min rest periods, followed by the 10 and 20 m sprint test, on the same day. The current findings showed significant differences in the 10 and 20 m tests after dynamic stretching compared with static, combined, and no-stretching protocols. There were also significant differences between the combined stretching compared with static and no-stretching protocols. We concluded that soccer players performed better with respect to acceleration and speed, after dynamic and combined stretching, as they were able to produce more force for a faster execution.

#8 Preseason Strategies of Italian First League Soccer Clubs in Relation to their Championship Ranking: A Five-Year Analysis
Reference: J Hum Kinet. 2016 Apr 13;50:145-155. doi: 10.1515/hukin-2015-0151. eCollection 2016
Authors: Francioni FM, Figueiredo AJ, Lupo C, Conte D, Capranica L, Tessitore A
Summary: This study is focused on the strategies adopted by Italian 'Serie A' soccer clubs during the non-competitive period. Thus, duration (i.e., number of days) of the specific non-competitive periods (i.e., off- plus pre-season, off-season, pre-season, summer camp), the number of games (i.e., friendly, official and polled games) and days between games (i.e., ≤ 3, 4-5, or ≥ 6 days between two consecutive games) of the entire non-competitive period were recorded and compared by clubs participating in the European Championships (EU), only 'Serie A' (A) and promoted from 'Serie B' (B) during five and single seasons (i.e., 2009/10-2013/14). Due to the short B off-season duration (2009/10-2013/14, 2010/11, 2011/12, 2012/13, 2013/14, p≤.001), effects between clubs emerged also for the off- plus preseason (2009/10-2013/14, EU vs A, p≤.01, EU vs B, p≤.01, A vs B, p≤.001; 2011/12, 2012/13, 2013/14, p≤.001). Nevertheless, no difference between clubs resulted for the pre-season. Reduced duration of summer camps was reported by the EU (i.e., 2009/10-2013/14, 2009/10, 2013/14, p≤.001). A higher number of official games were played by EU than A (i.e., 2009/10-2013/14, p≤.001; 2010/11, p≤.01). No effect emerged for the days between games. Therefore, despite the longer 'Serie B' schedule and EU preliminary UEFA games that determined the off-season restriction, clubs demonstrated the tendency to guarantee satisfactory pre-season duration.

#9 Prevalence of Dehydration Before Training Sessions, Friendly and Official Matches in Elite Female Soccer Players
Reference: J Hum Kinet. 2016 Apr 13;50:79-84. doi: 10.1515/hukin-2015-0145. eCollection 2016
Authors: Castro-Sepulveda M, Astudillo J, Letelier P, Zbinden-Foncea H
Summary: This study aimed to evaluate and compare the hydration states prior to different sporting events (training sessions, friendly and official matches) in elite female soccer players and relate that to the importance that the player attached to the hydration state as a determinant of sports performance. The hydration state of 17 female elite soccer players (age: 21.5 ± 3 years; body mass: 62 ± 6 kg; body height: 165 ± 9 cm) was determined by measuring their urine specific gravity (USG) prior to three different sports events: training sessions (PT), friendly (PF) and official (PO) matches. The importance that each player attached to the hydration state as a determinant of sports performance was evaluated through a simple questionnaire. An average of 47.05% of the soccer players were severely dehydrated (USG > 1.030), 33.33% were significantly dehydrated (USG > 1.020), 17.64% were mildly dehydrated (USG > 1.010) and 1.96% were euhydrated (USG < 1.010). The average USG was 1.027 ± 0.007 (PT = 1.029 ± 0.009; PF = 1.023 ± 0.010 and PO = 1.030 ± 0.006). Differences were found between urine specific gravity prior to a friendly and an official match (p = 0.03). No relationship was found between urine specific gravity and the importance each player attached to the hydration state as a determinant of sports performance. The results show that dehydration is the most prevalent hydration state of elite soccer players before training sessions, friendly and official matches. Players were most dehydrated prior to official matches, which was unlinked to the players' perceived importance of hydration for sports performance.

#10 Comparison of Lateral Abdominal Muscle Thickness and Cross Sectional Area of Multifidus in Adolescent Soccer Players with and without Low Back Pain: A Case Control Study
Reference: Asian J Sports Med. 2016 Oct 4;7(4):e38318. doi: 10.5812/asjsm.38318. eCollection 2016
Authors: Noormohammadpour P, Hosseini Khezri A, Linek P, Mansournia MA, Hassannejad A, Younesian A, Farahbakhsh F, Kordi R
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Summary: Low back pain (LBP) is a common complaint amongst adolescent athletes. While different studies have shown association between LBP and trunk muscle thickness in the general population, few articles have studied it in adolescent athletes. The aim of this study is to compare lateral abdominal muscle thickness and function, and cross sectional area (CSA) of lumbar multifidus (LM) in adolescent soccer players with and without LBP. In total, 28 adolescent soccer players with and without LBP, from the premier league participated in this study. The thickness of external oblique, internal oblique and transversus abdominis and the CSA of the LM muscles at L4 level on both sides were measured at rest and contraction via ultrasound imaging (USI). In addition, leg length discrepancy, hamstring flexibility, active lumbar forward flexion, and isometric muscle endurance of trunk extensors were measured in both groups. (study design/setting: case control study). The mean (SD) age in LBP group and non-LBP group were 14.0 (1.1) and 14.1 (0.9) years, respectively. There was no significant difference in baseline characteristics of participants between groups. Findings showed no significant difference between LBP and non-LBP groups comparing all measured variables. The data obtained support that there is not a correlation between abdominal muscle thickness and CSA of the lumbar multifidi and LBP in adolescent soccer players. These findings suggest that other factors rather than the thickness of deep trunk muscles may play a more significant role in the etiology of LBP in adolescent soccer players.

#11 Temporal Changes in Technical and Physical Performances During a Small-Sided Game in Elite Youth Soccer Players
Reference: Asian J Sports Med. 2016 Oct 16;7(4):e35411. doi: 10.5812/asjsm.35411. eCollection 2016.
Authors: Moreira A, Saldanha Aoki M, Carling C, Alan Rodrigues Lopes R, Felipe Schultz de Arruda A, Lima M, Cesar Correa U, Bradley PS
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Summary: There have been claims that small-sided games (SSG) may generate an appropriate environment to develop youth players' technical performance associated to game-related problem solving. However, the temporal change in technical performance parameters of youth players during SSG is still unknown. The aim of this study was to examine temporal changes in technical and physical performances during a small-sided game (SSG) in elite soccer players. Sixty elite youth players (age 14.8 ± 0.2 yr; stature 177 ± 5 cm; body mass 66.2 ± 4.7 kg) completed a 5 v 5 SSG using two repetitions of 8 minutes interspersed by 3 minutes of passive recovery. To evaluate temporal changes in performance, the data were analysed across 4 minutes quarters. Physical performance parameters included the total distance covered (TDC), the frequency of sprints (>18 km•h-1), accelerations and decelerations (> 2.0 m•s-2 and - 2.0 m•s-2), metabolic power (W•kg-1), training impulse (TRIMP), TDC: TRIMP, number of impacts, and body load. Technical performance parameters included goal attempts, total number of tackles, tackles and interceptions, total number of passes, and passes effectiveness. All physical performance parameters decreased from the first to the last quarter with notable declines in TDC, metabolic power and the frequency of sprints, accelerations and decelerations (P < 0.05; moderate to very large ES: 1.08 - 3.30). However, technical performance parameters did not vary across quarters (P > 0.05; trivial ES for 1st v 4th quarters: 0.15 - 0.33). The data demonstrate that technical performance is maintained despite substantial declines in physical performance during a SSG in elite youth players. This finding may have implications for designing SSG's for elite youth players to ensure physical, technical and tactical capabilities are optimized. Modifications in player number, pitch dimensions, rules, coach encouragement, for instance, should be included taking into account the main aim of a given session and then focused on overloading physical or technical elements.

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