Latest research in football - week 30 - 2013

Latest research in football

As previous literature updates, we have performed a PubCrawler search looking for football articles in NCBI Medline (PubMed) and GenBank databases.
Following studies were retrieved for this week:

#1 Strength and size ratios between reciprocal muscle groups in the thigh and lower leg of male collegiate soccer players
Authors: Akagi R, Tohdoh Y, Takahashi H.
Reference: Clin Physiol Funct Imaging. 2013 Jul 19. doi: 10.1111/cpf.12073. [Epub ahead of print]
Summary: This study investigated the strength and size ratios between reciprocal muscle groups in the thigh and lower leg of soccer players. In 11 male collegiate soccer players and 21 male non-soccer players, the joint torques developed during isometric maximal voluntary contractions of the knee extensors (TQKE ) and flexors (TQKF ), plantar flexors (TQPF ) and dorsiflexors (TQDF ) were determined using a dynamometer. The muscle volumes of the same muscles (i.e. MVKE , MVKF , MVPF and MVDF ) were determined by magnetic resonance imaging. There were no significant differences in any of the joint torques or muscle volumes between soccer and non-soccer players. TQKE /TQKF was significantly lower for soccer players than for non-soccer players. On the other hand, no significant difference in TQPF /TQDF was found between soccer and non-soccer players. MVKE /MVKF and MVPF /MVDF for soccer players were not significantly different from those for non-soccer players. Additionally, there were no significant correlations between MVKE /MVKF and TQKE /TQKF and between MVPF /MVDF and TQPF /TQDF of soccer players. Thus, the strength ratios appear not to be affected by the size ratios between the knee extensors and flexors and between the plantar flexors and dorsiflexors in soccer players.

#2 Comparison of Functional Outcome Measures After ACL Reconstruction in Competitive Soccer Players: A Randomized Trial
Authors: Mohammadi F, Salavati M, Akhbari B, Mazaheri M, Mohsen Mir S, Etemadi Y.
Reference: J Bone Joint Surg Am. 2013 Jul 17;95(14):1271-7. doi: 10.2106/JBJS.L.00724.
Summary: The choice of graft for anterior cruciate ligament (ACL) reconstruction remains controversial. Although many outcome studies comparing bone-patellar tendon-bone and semitendinosus and gracilis tendon grafts have been performed, most have not included tests of functional outcomes that challenge the graft. The objective of the current study was to compare the functional performances of soccer players after ACL reconstruction that was performed with either a bone-patellar tendon-bone or a semitendinosus and gracilis tendon graft. Forty-two soccer players with unilateral ACL injury were prospectively randomized to a bone-patellar tendon-bone group (BPTB group; twenty-one subjects) or a semitendinosus and gracilis tendon group (STG group; twenty-one subjects) and followed a rehabilitation protocol. At the time of return to sports, the patients were asked to execute strength, hop, and jump tests and the results were compared between the groups. Twenty-one healthy athletes were recruited as the control group to evaluate their performance of the same tests. The limbs with the ACL reconstruction in the STG group had greater values for quadriceps torque as well as on the triple-hop, crossover-hop, and jump-landing test (p &lt; 0.01), but the STG and BPTB groups showed similar results in terms of hamstrings peak torque and the results of two other hop tests (p< 0.05). Moreover, the subjects with ACL reconstruction had significantly lower values with regard to quadriceps and hamstrings peak torques and the results of the hop and jump tests compared with the healthy athletes (p<0.01). At the time of return to sports, the STG group had better performance in terms of quadriceps strength and the results of the triple-hop, crossover-hop, and jump-landing tests compared with the BPTB group. Compared with controls, soccer players who had undergone ACL reconstruction had less quadriceps and hamstrings strength and inferior hop performance and jump-landing strategy.

#3 Cardiac autonomic adaptations in elite spanish soccer players during preseason
Authors: Boullosa DA, Abreu L, Nakamura FY, Muñoz VE, Domínguez E, Leicht AS.
Reference: Int J Sports Physiol Perform. 2013 Jul;8(4):400-9.
Summary: The purpose of this study was to evaluate changes in autonomic control of heart rate (HR) and fitness in a group of elite soccer players during the preseason. Eight professional male soccer players competing in the Spanish First Division were evaluated in July (wk 1) and September (wk 8) with night-time HR variability (HRV) over 4 different days, ultra-short-term HR recovery (HRR) during a small-sided-games session, Yo-Yo Intermittent Recovery test level 1 (Yo-Yo IR1), and a field test for determination of maximum aerobic speed. Players exhibited a greater HRV and a faster ultra-short-term HRR at wk 8, with the players with a lower HRV at wk 1 exhibiting the greatest improvements at wk 8. However, there were unclear improvements in performance parameters, with maximum HR (HRmax) being reduced over the preseason period. This change in HRmax was correlated with the change in short-term HRV parameters (ρ= 0.829, P = .042). Large correlations were observed among HRV, ultra-short-term HRR, and field performance parameters only at wk 8. Furthermore, the variation (%) of the root-mean-square of successive differences between R-R intervals was increased during the preseason (12.95% ± 15.14% to 29.39% ± 21.93%, P = .013) and significantly correlated (r = .898, P = .006) with Yo-Yo IR1 performance (~2600 ± 786 m) at wk 8. The current results support the appropriateness and practicality of night-time HRV and ultra-short-term HRR for evaluation of autonomic adaptations in professional soccer players, despite the unclear improvements in specific field performance parameters.

#4 Brainwave entrainment for better sleep and post-sleep state of young elite soccer players - A pilot study
Authors: Abeln V, Kleinert J, Strüder HK, Schneider S.
Reference: Eur J Sport Sci. 2013 Jul 18. [Epub ahead of print]
Summary:  The effect of sleep deprivation on psychophysical performance and well-being is comprehensively investigated. Research investigating the effect of improved sleep is rare. Just as little exists about attempts to support athletic mental state and performance by improving sleep quality. This study aims to investigate whether sleep quality of top athletes can be improved by auditory brainwave entrainment and whether this leads to enhancements of post-sleep psychophysical states. In a pilot study, 15 young elite soccer players were stimulated for eight weeks during sleep with binaural beats around 2-8 Hz. Once a week after wake-up, participants completed three different questionnaires: a sleep diary, an adjective list for psychophysical and motivational state, and a self-assessment questionnaire for sleep and awakening quality. Fifteen sport students executed the same protocol sleeping on the same pillow, but without stimulation. Subjective ratings of sleep and awakening quality, sleepiness and motivational state were significantly improved only in the intervention group, but did not impact their perceived physical state. In summary, eight weeks of auditory stimulation with binaural beats improved perceived sleep quality and the post-sleep state of athletes, whereas the effect on physical level is assumed to occur in a time-delayed fashion. It seems to be worthwhile - to further elaborate long-time effects and consequences on physical and mental performance

#5 Variability of objective and subjective intensities during ball drills in youth soccer players
Authors: Los Arcos A, Martínez-Santos R, Yanci J, Martín J, Castagna C.
Reference: J Strength Cond Res. 2013 Jul 15. [Epub ahead of print]
Summary: The aim of this study was to examine the intra (intraclass coefficient correlation, ICC) and inter-subject variability (coefficient of variation, CV) of soccer ball drills (BD) involving or not opposition in male youth soccer. For this purpose a collective ball dribbling (DB) exercise and a 7-a-side ball game without coach encouragements were considered. Exercise intensity was assessed as heart rate (HR), training load (TL) and perceived exertion scales (PES). Fourteen U-14 male soccer players (age 14.79 ± 0.43 and experience 6.5 years) of a Spanish First Division club academy participated in the study. BD were examined for variability over 5 successive training sessions in similar field conditions. Results showed that 7-a-side was significantly (p=0.000) more demanding than DB. Indeed the TL, HRmax, HRmean, overall perceived exertion (OPE) and leg muscular perceived exertion (MPE) resulted 141, 8.7, 11, 56 and 72 %, higher in 7-a-side than in DB, respectively. In the 7-a-side condition good inter (CV<10%) and low intra-subject (ICC<0.7) variability were observed. In the DB condition CVs were below 10% CV only for HR variables and the ICC value were higher than 0.7 only for MPE. Despite the moderate reproducibility of BD not considering opponents this condition did not reveal to induce homogeneous physiological responses in young soccer players. Therefore the use of this kind of drills may be questionable when considered as alternative of moderate intensity generic aerobic training. Inter-players variability may be lower during team opposition drills despite the occurrence of frequent duel this suggesting their use as conditioning exercise when specific training (i.e. ball use during drills) is to be prescribed.

#6 Are professional soccer players at higher risk for ALS?
Author: Beghi E.
Reference: Amyotroph Lateral Scler Frontotemporal Degener. 2013 Jul 16. [Epub ahead of print]
Summary:  Since the observation of several deaths from amyotrophic lateral sclerosis (ALS) among Italian professional soccer players, an association between ALS and soccer has been postulated, supported by high rates of morbidity and mortality risks in large cohorts of professionals. Several factors may explain this. A history of repeated (head) injuries is reported more frequently by ALS patients than by individuals with other clinical conditions. An association between exercise and ALS has also been suggested, but results in animals and humans are conflicting. Some clinical and experimental observations suggest a relation between ALS and use of substances such as non-steroidal anti-inflammatory agents, and dietary supplements including branched-chain amino acids. Although Italian soccer players may be at higher risk of ALS than players in other countries, and higher than expected disease frequency seems soccer-specific, increased attention by the Italian lay press is an explanation that cannot be excluded. However, growing evidence points to the possibility that soccer players with ALS are susceptible individuals who develop the disease in response to combinations of environmental factors. Only cohort and case-control studies carried out with the same design in different European countries can provide a definite answer to this suspected but still unconfirmed association.

#7 Copenhagen hip and groin outcome score (HAGOS) in male soccer: reference values for hip and groin injury-free players
Authors: Thorborg K, Branci S, Stensbirk F, Jensen J, Hölmich P.
Reference: Br J Sports Med. 2013 Jul 12. doi: 10.1136/bjsports-2013-092607. [Epub ahead of print]
Summary:  Reference values are needed in order to interpret the Copenhagen Hip and Groin Outcome Score (HAGOS) in male soccer players with hip and groin pain. The aim of this study was to establish reference values for HAGOS in hip and groin injury-free male soccer players. We included 444 groin injury-free soccer players from 40 clubs (divisions 1-4) in Eastern Denmark, mean age (SD) 23.6 (4.4), training soccer 3.4 (1) times per week. All players were hip and groin injury-free at the time of inclusion (beginning of season, 2011). Of the 444 hip and groin injury-free players, 301 reported no hip and/or groin pain in either the present or the previous season, and 143 reported that they had experienced hip and/or groin pain in the previous season. Players (n=143) with hip and groin pain in the previous season displayed lower scores than players without (n=301), for all HAGOS subscales (p<0.001). Age and playing level were not related to HAGOS. The 95% reference ranges for HAGOS subscales in hip and groin injury-free soccer players, with no pain in the previous or present season (n=301), are: pain: 80.1-100, symptoms: 64.3-100, activities of daily living: 80.3-100, sport and recreational activities: 71.9-100, participation in physical activity: 75-100 and quality of living: 75-100. Lower HAGOS subscales are seen in soccer players who have experienced hip and/or groin pain in the previous season, compared with those who have not. Median HAGOS subscale scores in hip and groin injury-free soccer players are in proximity to the maximum score (100 points)

#8 Comprehensive Soccer-Specific Warm-Up and Lower Extremity Injury in Collegiate Male Soccer Players
Authors: Grooms DR, Palmer T, Onate JA, Myer G, Grindstaff T.
Reference: J Athl Train. 2013 Jul 12. [Epub ahead of print]
Summary:  A number of comprehensive injury-prevention programs have demonstrated injury risk-reduction effects but have had limited adoption across athletic settings. This may be due to program non-compliance, minimal exercise supervision, lack of exercise progression, and sport specificity. A soccer-specific program described as the "F-MARC 11+" was developed by an expert group in association with the Federation Internationale de Football Association (FIFA) Medical Assessment and Research Centre (F-MARC) to require minimal equipment and implementation as part of regular soccer training. The F-MARC 11+ has been shown to reduce injury risk in youth female soccer players but has not been evaluated in an American male collegiate population. The objective was to investigate the effects of a soccer-specific warm-up program (F-MARC 11+) on lower extremity injury incidence in male collegiate soccer players. One American collegiate soccer team was followed for 2 seasons - Forty-one male collegiate athletes aged 18-25 years. The F-MARC 11+ program were used as a comprehensive warm-up program targeting muscular strength, body kinesthetic awareness, and neuromuscular control during static and dynamic movements. Training sessions and program progression were monitored by a certified athletic trainer. Lower extremity injury risk and time lost to lower extremity injury was used to evaluate the efficiency of the program.  The injury rate in the referent season was 8.1 injuries per 1000 exposures with 291 days lost and 2.2 injuries per 1000 exposures and 52 days lost in the intervention season. The intervention season had reductions in the relative risk (RR) of lower extremity injury of 72% (RR = 0.28, 95% confidence interval = 0.09, 0.85) and time lost to lower extremity injury (P < .01). This F-MARC 11+ program reduced overall risk and severity of lower extremity injury compared with controls in collegiate-aged male soccer athletes

#9 Head Impact Exposure in Youth Football: High School Ages 14 to 18 Years and Cumulative Impact Analysis
Authors: Urban JE, Davenport EM, Golman AJ, Maldjian JA, Whitlow CT, Powers AK, Stitzel JD.
Reference: Ann Biomed Eng. 2013 Jul 18. [Epub ahead of print]
Summary: Sports-related concussion is the most common athletic head injury with football having the highest rate among high school athletes. Traditionally, research on the biomechanics of football-related head impact has been focused at the collegiate level. Less research has been performed at the high school level, despite the incidence of concussion among high school football players. The objective of this study is to twofold: to quantify the head impact exposure in high school football, and to develop a cumulative impact analysis method. Head impact exposure was measured by instrumenting the helmets of 40 high school football players with helmet mounted accelerometer arrays to measure linear and rotational acceleration. A total of 16,502 head impacts were collected over the course of the season. Biomechanical data were analyzed by team and by player. The median impact for each player ranged from 15.2 to 27.0 g with an average value of 21.7 (±2.4) g. The 95th percentile impact for each player ranged from 38.8 to 72.9 g with an average value of 56.4 (±10.5) g. Next, an impact exposure metric utilizing concussion injury risk curves was created to quantify cumulative exposure for each participating player over the course of the season. Impacts were weighted according to the associated risk due to linear acceleration and rotational acceleration alone, as well as the combined probability (CP) of injury associated with both. These risks were summed over the course of a season to generate risk weighted cumulative exposure. The impact frequency was found to be greater during games compared to practices with an average number of impacts per session of 15.5 and 9.4, respectively. However, the median cumulative risk weighted exposure based on combined probability was found to be greater for practices vs. games. These data will provide a metric that may be used to better understand the cumulative effects of repetitive head impacts, injury mechanisms, and head impact exposure of athletes in football

#10 Muscle injury rates in professional football increase with fixture congestion: an 11-year follow-up of the UEFA Champions League injury study
Authors: Bengtsson H, Ekstrand J, Hägglund M.
Reference: Br J Sports Med. 2013 Aug;47(12):743-7. doi: 10.1136/bjsports-2013-092383.
Summary: To study associations between recovery time and match load and injury rates and team performance in professional football. Exposure and time loss injuries were registered prospectively from 27 teams over 11 seasons. Matches were grouped according to recovery days before each match (≤3 vs >3 days, and ≤4 vs ≥6 days). Injury rates and team performance were compared between groups. Match load in match sequences containing five consecutive matches was determined by the number of days separating the first match and the last training session during that match sequence. Linear regression was used to study associations between match load and injury rates and team performance. Team performance showed no association with match load, or recovery days prior to matches, except for Europa League matches that indicated more matches lost with short recovery (≤3 days) (p=0.048). Total injury rates and muscle injury rates were increased in league matches with ≤4 days compared with ≥6 days' recovery (RR 1.09, 95% CI 1.00 to 1.18, and RR 1.32, 95% CI 1.15 to 1.51, respectively), specifically hamstring and quadriceps injuries. High match load was associated with increase in muscle injury rate in matches in the same match sequence (p=0.012), and increase in ligament injury rate in training in the subsequent match sequence (p=0.003). Fixture congestion was associated with increased muscle injury rates but had no, or very limited, influence on team performance

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