Latest research in football - week 28 - 2015

#1 Physiological and performance effects of carbohydrate gels consumed prior to the extra-time period of prolonged simulated soccer match-play
Reference: J Sci Med Sport. 2015 Jun 17. pii: S1440-2440(15)00134-6. doi: 10.1016/j.jsams.2015.06.009. [Epub ahead of print]
Authors: Harper LD, Briggs MA, McNamee G, West DJ, Kilduff LP, Stevenson E, Russell M
Summary: The physiological and performance effects of carbohydrate-electrolyte gels consumed before the 30min extra-time period of prolonged soccer-specific exercise were investigated. Eight English Premier League academy soccer players performed 120min of soccer-specific exercise on two occasions while consuming fluid-electrolyte beverages before exercise, at half-time and 90min. Carbohydrate-electrolyte (0.7±0.1gkg-1 BM) or energy-free placebo gels were consumed ∼5min before extra-time. Blood samples were taken before exercise, at half-time and every 15min during exercise. Physical (15-m and 30-m sprint speed, 30-m sprint maintenance and countermovement jump height) and technical (soccer dribbling) performance was assessed throughout each trial. Carbohydrate-electrolyte gels improved dribbling precision (+29±20%) and raised blood glucose concentrations by 0.7±0.8mmoll-1 during extra-time (both p<0.01). Supplementation did not affect sprint velocities (15m and 30m), 30-m sprint maintenance or dribbling speed as reductions compared to 0-15min values occurred at 105-120min irrespective of trial (all p<0.05). Plasma osmolality and blood sodium concentrations increased post-exercise vs. the opening 15min (p<0.05) but no effect of supplementation existed. Selected markers of physical performance (jump height, 30-m sprint velocity and 30-m repeated sprint maintenance) also reduced by >3% during half-time (all p<0.05). Carbohydrate-electrolyte gel ingestion raised blood glucose concentrations and improved dribbling performance during the extra-time period of simulated soccer match-play. Supplementation did not attenuate reductions in physical performance and hydration status that occurred during extra-time.


#2 Timing characteristics of body segments during the maximal instep kick in experienced football players
Reference: J Sports Med Phys Fitness. 2015 Jul 1. [Epub ahead of print]
Authors: Langhout R, Weber M, Tak I, Lenssen T
Summary: The first aim of this study was to describe duration and relative timing of the phases of the maximal instep kick. The second aim was to describe the concurrence of maximal range of motion, maximal angular acceleration, maximal angular deceleration and maximal angular velocity of body segments with four key points. Twenty experienced football players performed three maximal instep kicks. The kicks were analysed using a full body, three-dimensional motion capture system. Camera recordings determined kicking leg events. The concurrence of peak kinematics of body segments with four keypoints was calculated. Duration and timing of five phases were identified. Keypoint maximal hip extension (51.4±5.0%) concurred significantly with maximal range of motion (ROM) of shoulder extension. Keypoint maximal knee flexion (63.6±5.2%) concurred significantly with maximal angular acceleration of spine flexion and pelvis posterior tilt. Keypoint knee flexion 90 degrees (69.3±4.9%) concurred significantly with maximal angular velocity of shoulder flexion and spine flexion, maximal angular deceleration of hip flexion and maximal angular acceleration of knee extension. Keypoint ball impact (75.2±5.2%) concurred significantly with maximal ROM of hip deflexion and pelvis anterior rotation and with maximal angular deceleration of spine flexion and pelvis anterior rotation. This study demonstrated that eleven peak kinematics of upper body and kicking leg segments, significantly concurred with four kicking leg positions. These results provide keypoints for kicking coordination and stress the importance of dynamical coupling as a kicking mechanism.


#3 Heart Rate Variability: a Follow-up in Elite Soccer Players Throughout the Season
Reference: Int J Sports Med. 2015 Jul 3. [Epub ahead of print]
Authors: Naranjo J, De la Cruz B, Sarabia E, De Hoyo M, Domínguez-Cobo S
Summary: Heart rate variability (HRV) can provide useful information on physiological adaptations to training, but its role is unknown in professional soccer. The aim of this study was to determine an HRV profile in professional soccer over a season. A total of 504 records were made of the heart beat signal throughout a season from 22 professional soccer players. HRV was recorded in a sitting position, early morning and fasting for a period of 10 min. Standard deviation 1 and 2 (SD1, SD2), standard deviation of normal to normal R-R intervals (SDNN), Root Mean Square of the Successive Differences (rMSSD), percentage of RR intervals > 50 ms (pNN50), Sample Entropy (SampEn), Stress Score (SS) and sympathetic/parasympathetic ratio (S/PS ratio) were calculated. SDNN, rMSSD, pNN50, SD1 and SD2 showed an identical behaviour throughout the season, with lower values in the pre-season and the end of the season. SS and S/PS ratio indicated a sympathetic stress alert in the same periods. A weekly recording of the HRV over a 10 min period that includes a Poincaré plot with SS and S/PS ratio and at least one variable of the time domain is a useful tool for the follow-up of the individual assimilation of weekly workloads, including the game.


#4 Change of direction direction speed in soccer
Reference: Kinesiology 47(2015)1:67-74
Authors: Hader K, Palazzi D, Buchheit M
Summary: The aims of the present study were to examine: 1) the validity and reliability of a new timing system to assess running kinematics during change of direction (COD), and 2) the determinants of COD-speed. Twelve young soccer players performed three 20-m sprints, either in straight line or with one 45o- or 90o- COD. Sprints were monitored using timing gates and two synchronized 100-Hz laser guns, to track players’ velocities before, during and after the COD. The validity analysis revealed trivial-to-small biases and small- to-moderate typical errors of the estimate with the lasers compared with the timing gates. The reliability was variable-dependent, with trivial- (distance at peak speed) to-large (distance at peak deceleration) typical errors. Kinematic variables were angle-dependent, with likely lower peak speed, almost-certainly slower minimum speed during the COD and almost-certainly greater deceleration reached for 90o-COD vs. 45o- COD sprints. The minimum speed during the COD was largely correlated with sprint performance for both sprint angles. Correlations with most of the other independent variables were unclear. The new timing system showed acceptable levels of validity and reliability to assess some of the selected running kinematics during COD sprints. The ability to maintain a high speed during the COD may be the determinant of COD-speed.


#5 The relationship between lower-limb strength and match-related muscle damage in elite level professional European soccer players
Reference: J Sports Sci. 2015 Jul 9:1-6. [Epub ahead of print]
Authors: Owen A, Dunlop G, Rouissi M, Chtara M, Paul D, Zouhal H, Wong DP.
Summary: In professional soccer, the benefits of lower limb strength training have been advocated. However, from an aspect of performance development, specifically with respect to expression of fatigue and injury prevention, the advantages of increased lower body strength have received limited attention at the elite level of the game. The primary aim of this cross-sectional investigation was to examine the association between lower body strength and the expression of markers of fatigue as evaluated through muscle damage assessment following match play in professional soccer players. Ten male professional soccer players participated in this investigation (mean ± SD age 26.2 ± 4.3 years, height 181.6 ± 4.8 cm and body mass 78.7 ± 6.1 kg); creatine kinase (CK) was collected 2-days post-match for a 5-month period and at three different time points (Phase 1, Phase 2 and Phase 3); muscular strength (e.g. 4 repetition half-squat) was measured 3-day post-match. No significant change in CK and muscular force across three time points was found (F = 0.60, P = 0.56, η2 = 0.06 and F = 2.65, P = 0.10, η2 = 0.23, respectively). Muscular force was negatively correlated (moderate to very large) with CK. It can be concluded that players who produce greater lower body force as a result of being stronger in the lower limbs show reduced levels of CK 48 h post-match.


#6 Vitamin D receptor gene polymorphisms and musculoskeletal injuries in professional football players
Reference: Exp Ther Med. 2015 May;9(5):1974-1978. Epub 2015 Mar 16.
Authors: Massidda M, Corrias L, Bachis V, Cugia P, Piras F, Scorcu M, Calò CM
Summary: The aim of the present study was to investigate the association between vitamin D receptor (VDR) gene polymorphisms and musculoskeletal injury (MI) in elite football players. In total, 54 male professional football players were recruited from an official Italian professional championship team between 2009 and 2013. The cohort was genotyped for the ApaI, BsmI and FokI polymorphisms and MI data were collected over four football seasons. No significant differences were identified among the genotypes in the incidence rates or severity of MI (P=0.254). In addition, no significant associations were observed between VDR polymorphisms and MI phenotypes (P=0.460). However, the results of the casewise multiple regression analysis indicated that the ApaI genotypes accounted for 18% of injury severity (P=0.002). Therefore, while the BsmI and FokI polymorphisms did not appear to be associated with the severity or incidence of MI, the ApaI genotypes may have influenced the severity of muscle injury in top-level football players.


#7 Recovery-stress balance and injury risk in professional football players: a prospective study
Reference: J Sports Sci. 2015 Jul 13:1-9. [Epub ahead of print]
Authors: Laux P, Krumm B, Diers M, Flor H.
Summary: Professional football is a contact sport with a high risk of injury. This study was designed to examine the contribution of stress and recovery variables as assessed with the Recovery-Stress Questionnaire for Athletes (RESTQ-Sport) to the risk of injury in professional football players. In a prospective, non-experimental cohort design, 22 professional football players in the highest German football league were observed over the course of 16 months. From January 2010 until April 2011, the players completed the RESTQ-Sport a total of 222 times in monthly intervals. In addition, injury data were assessed by the medical staff of the club. Overall, 34 traumatic injuries and 10 overuse injuries occurred. Most of the injuries were located in the lower limb (79.5%), and muscle and tendon injuries (43.2%) were the most frequently occurring injury type. In a generalised linear model, the stress-related scales Fatigue (OR 1.70, P = 0.007), Disturbed Breaks (OR 1.84, P = 0.047) and Injury (OR 1.77, P < 0.001) and the recovery-related scale Sleep Quality (OR 0.53, P = 0.010) significantly predicted injuries in the month after the assessment. These results support the importance of frequent monitoring of recovery and stress parameters to lower the risk of injuries in professional football.