Latest research in football - week 4 - 2016

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 The Tracking of Morning Fatigue Status Across In-Season Training Weeks in Elite Soccer Players
Reference: Int J Sports Physiol Perform. 2016 Jan 27. [Epub ahead of print]
Authors: Thorpe RT, Strudwick AJ, Buchheit M, Atkinson G, Drust B, Gregson W
Summary: The purpose of the study was to quantify the mean daily changes in training and match load and any parallel changes in indicators of morning-measured fatigue across in-season training weeks in elite soccer players. Following each training session and match, ratings of perceived exertion (s-RPE) were recorded to calculate overall session load (RPE-TL) in 29 English Premier League players from the same team. Morning ratings of fatigue, sleep quality, delayed-onset muscle soreness (DOMS), as well as sub-maximal exercise heart rate (HRex), post-exercise heart rate recovery (HRR%) and variability (HRV) were also recorded pre-match day and one, two and four days post-match. Data were collected for a median duration of 3 weeks (range:1-13) and reduced to a typical weekly cycle including no mid-week match and a weekend match day. Data were analysed using within-subjects linear mixed models. RPE-TL was approximately 600 AU (95%CI: 546-644) higher on match-day vs the following day (P<0.001). RPE-TL progressively decreased by ≈ 60 AU per day over the 3 days prior to a match (P<0.05). Morning-measured fatigue, sleep quality and DOMS tracked the changes in RPE-TL, being 35-40% worse on post-match day vs pre-match day (P<0.001). Perceived fatigue, sleep quality and DOMS improved by 17-26% from post-match day to three days post-match with further smaller (7-14%) improvements occurring between four days post-match and pre-match day (P<0.01). There were no substantial or statistically significant changes in HRex, HRR% and HRV over the weekly cycle (P>0.05). Morning-measured ratings of fatigue, sleep quality and DOMS are clearly more sensitive than HR-derived indices to the daily fluctuations in session load experienced by elite soccer players within a standard in-season week.

#2 Comparative Effects of In-Season Full-Back Squat, Resisted Sprint Training, and Plyometric Training on Explosive Performance in U-19 Elite Soccer Players
Reference: J Strength Cond Res. 2016 Feb;30(2):368-77. doi: 10.1519/JSC.0000000000001094
Authors: de Hoyo M, Gonzalo-Skok O, Sañudo B, Carrascal C, Plaza-Armas JR, Camacho-Candil F, Otero-Esquina C
Summary: The aim of this study was to analyze the effects of 3 different low/moderate load strength training methods (full-back squat [SQ], resisted sprint with sled towing [RS], and plyometric and specific drills training [PLYO]) on sprinting, jumping, and change of direction (COD) abilities in soccer players. Thirty-two young elite male Spanish soccer players participated in the study. Subjects performed 2 specific strength training sessions per week, in addition to their normal training sessions for 8 weeks. The full-back squat protocol consisted of 2-3 sets × 4-8 repetitions at 40-60% 1 repetition maximum (∼1.28-0.98 m·s). The resisted sprint training was compounded by 6-10 sets × 20-m loaded sprints (12.6% of body mass). The plyometric and specific drills training was based on 1-3 sets × 2-3 repetitions of 8 plyometric and speed/agility exercises. Testing sessions included a countermovement jump (CMJ), a 20-m sprint (10-m split time), a 50-m (30-m split time) sprint, and COD test (i.e., Zig-Zag test). Substantial improvements (likely to almost certainly) in CMJ (effect size [ES]: 0.50-0.57) and 30-50 m (ES: 0.45-0.84) were found in every group in comparison to pretest results. Moreover, players in PLYO and SQ groups also showed substantial enhancements (likely to very likely) in 0-50 m (ES: 0.46-0.60). In addition, 10-20 m was also improved (very likely) in the SQ group (ES: 0.61). Between-group analyses showed that improvements in 10-20 m (ES: 0.57) and 30-50 m (ES: 0.40) were likely greater in the SQ group than in the RS group. Also, 10-20 m (ES: 0.49) was substantially better in the SQ group than in the PLYO group. In conclusion, the present strength training methods used in this study seem to be effective to improve jumping and sprinting abilities, but COD might need other stimulus to achieve positive effects.

#3 Reliability and Discriminative Ability of a New Method for Soccer Kicking Evaluation
Reference: PLoS One. 2016 Jan 26;11(1):e0147998. doi: 10.1371/journal.pone.0147998. eCollection 2016.
Authors: Radman I, Wessner B, Bachl N, Ruzic L, Hackl M, Baca A, Markovic G
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Summary: The study aimed to evaluate the test-retest reliability of a newly developed 356 Soccer Shooting Test (356-SST), and the discriminative ability of this test with respect to the soccer players' proficiency level and leg dominance. Sixty-six male soccer players, divided into three groups based on their proficiency level (amateur, n = 24; novice semi-professional, n = 18; and experienced semi-professional players, n = 24), performed 10 kicks following a two-step run up. Forty-eight of them repeated the test on a separate day. The following shooting variables were derived: ball velocity (BV; measured via radar gun), shooting accuracy (SA; average distance from the ball-entry point to the goal centre), and shooting quality (SQ; shooting accuracy divided by the time elapsed from hitting the ball to the point of entry). No systematic bias was evident in the selected shooting variables (SA: 1.98±0.65 vs. 2.00±0.63 m; BV: 24.6±2.3 vs. 24.5±1.9 m s-1; SQ: 2.92±1.0 vs. 2.93±1.0 m s-1; all p>0.05). The intra-class correlation coefficients were high (ICC = 0.70-0.88), and the coefficients of variation were low (CV = 5.3-5.4%). Finally, all three 356-SST variables identify, with adequate sensitivity, differences in soccer shooting ability with respect to the players' proficiency and leg dominance. The results suggest that the 356-SST is a reliable and sensitive test of specific shooting ability in men's soccer. Future studies should test the validity of these findings in a fatigued state, as well as in other populations.

#4 Cerebrovascular reactivity changes in asymptomatic female athletes attributable to high school soccer participation
Reference: Brain Imaging Behav. 2016 Jan 26. [Epub ahead of print]
Authors: Svaldi DO, McCuen EC, Joshi C, Robinson ME, Nho Y, Hannemann R, Nauman EA, Leverenz LJ, Talavage TM
Summary: As participation in women's soccer continues to grow and the longevity of female athletes' careers continues to increase, prevention and care for mTBI in women's soccer has become a major concern for female athletes since the long-term risks associated with a history of mTBI are well documented. Among women's sports, soccer exhibits among the highest concussion rates, on par with those of men's football at the collegiate level. Head impact monitoring technology has revealed that "concussive hits" occurring directly before symptomatic injury are not predictive of mTBI, suggesting that the cumulative effect of repetitive head impacts experienced by collision sport athletes should be assessed. Neuroimaging biomarkers have proven to be valuable in detecting brain changes that occur before neurocognitive symptoms in collision sport athletes. Quantifying the relationship between changes in these biomarkers and head impacts experienced by female soccer athletes may prove valuable to developing preventative measures for mTBI. This study paired functional magnetic resonance imaging with head impact monitoring to track cerebrovascular reactivity changes throughout a season and to test whether the observed changes could be attributed to mechanical loading experienced by female athletes participating in high school soccer. Marked cerebrovascular reactivity changes were observed in female soccer athletes, relative both to non-collision sport control measures and pre-season measures and were localized to fronto-temporal aspects of the brain. These changes persisted 4-5 months after the season ended and recovered by 8 months after the season. Segregation of the total soccer cohort into cumulative loading groups revealed that population-level changes were driven by athletes experiencing high cumulative loads, although athletes experiencing lower cumulative loads still contributed to group changes. The results of this study imply a non-linear relationship between cumulative loading and cerebrovascular changes with a threshold, above which the risk, of injury likely increases significantly.

#5 Comparison of two types of warm-up upon repeated sprint performance in experienced soccer players
Reference: J Strength Cond Res. 2016 Jan 20. [Epub ahead of print]
Authors: van den Tillaar R, von Heimburg E
Summary: The aim of the study was to compare the effects of a long warm-up and a short warm-up upon repeated sprint performance in soccer players. Ten male soccer players (age 21.9 ± 1.9 yr, body mass 77.7 ± 8.3 kg, body height 1.85 ± 0.03 m) conducted two types of warm-ups with one week in between: a long warm-up (20 minutes: LWup) and a short warm-up (10 minutes: SWup). Each warm-up was followed by a repeated sprint test consisting of 8 x 30 m sprints with a new start every 30<sup>th</sup> second. The best sprint time, total sprinting time and % decrease in time together with heart rate, lactate and rate of perceived exertion (RPE) were measured. No significant differences in performance were found for the repeated sprint test parameters (total sprint time: 35.99 ± 1.32 s [LWup] and 36.12 ± 0.96 s [SWup]; best sprint time: 4.32 ± 0.13 s [LWup] and 4.30 ± 0.10 s [SWup] and % sprint decrease: 4.16 ± 2.15 % [LWup] and 5.02 ± 2.07 % [SWup]). No differences in lactate concentration after the warm-up and after the repeated sprint test were found. However, RPE and heart rate were significantly higher after the long warm-up and the repeated sprint test compared with the short warm-up. It was concluded that a short warm-up is as effective as a long warm-up for repeated sprints in soccer. Therefore, in regular training less warm-up time is needed; the extra time could be used for important soccer skill training.

#6 The effect of vitamin d supplementation on training adaptation in well trained soccer players
Reference: J Strength Cond Res. 2016 Jan 20. [Epub ahead of print]
Authors: Jastrzębska M, Kaczmarczyk M, Jastrzębski Z
Summary: There is growing body of evidence implying that vitamin D may be associated with athletic performance, however, studies examining the effects of vitamin D on athletic performance are inconsistent. Moreover, very little literature exists about the vitamin D and training efficiency or adaptation, especially in high-level, well trained athletes. The purpose of the current study was to investigate the effect of vitamin D supplementation on training adaptation in well trained football players. The subjects were divided into two groups: the placebo group (PG) and the experimental group (SG, supplemented with vitamin D, 5000IU/day). Both groups were subjected to High Intensity Interval Training Program. The selection to the groups was based on peak power results attained before the experiment and position on the field. Blood samples for vitamin D level were taken from the players. In addition, total work, 5-10-20-30 m running speed, squat jump, and countermovement jump height were determined. There were no significant differences between SG and PG groups for any power-related characteristics at baseline. All power-related variables, except the 30 m sprint running time, improved significantly in response to interval training. However, the mean change scores (the differences between post- and pre-supplementation values) did not differ significantly between SG and PG groups. In conclusion, an 8-week vitamin D supplementation in highly trained football players was not beneficial in terms of response to high intensity interval training. Given the current level of evidence, the recommendation to use vitamin D supplements in all athletes to improve performance or training gains would be premature. To avoid a seasonal decrease in 25(OH)D level or to obtain optimal vitamin D levels, the combination of higher dietary intake and vitamin D supplementation may be necessary.

#6 Motion analysis of match play in New Zealand U13 to U15 age-group soccer players
Reference: J Strength Cond Res. 2016 Jan 20. [Epub ahead of print]
Authors: Atan SA, Foskett A, Ali A
Summary: The purpose of this study was to investigate motion analysis in 85 players (U13-U15 years) from Auckland's Metropolitan League during two competitive soccer matches. Five Hz GPS (with interpolated 10Hz output) units were used to measure total distance (absolute and relative) and time spent in standing, walking, low intensity running, medium intensity running, high intensity running and sprinting. Speed thresholds for each match activity were determined through mean 10-m flying sprint peak speed for each age-group. U15 (6600 ± 1480m) covered more absolute distance due to longer playing time than U14 (5385 ± 1296m, p=0.001) and U13 (4516 ± 702.6m, p=0.001). However, there were no differences in relative distances covered (U15: 94.5 ± 11.2 m·min, U14: 96.1 ± 11.9m·min, U15: 97.3 ± 17.6 m·min, p=0.685). Maximum speed attained during the match was faster for U15 (26.5 ± 1.68km·h) than U14 (25.4 ± 1.93 km·h, p=0.022) and U13 (23.5 ± 1.74 km·h, p=0.001); there were no differences in average distance per sprint, with all age groups covering ∼16m per sprint (p=0.603). The current findings provide useful information for developing specific training programmes for young soccer players and a framework for developing age-specific soccer simulation protocols.

#7 Technical and Physical Activities of Small-sided Games in Young Korean Soccer Players
Reference: J Strength Cond Res. 2016 Jan 20. [Epub ahead of print]
Authors: Joo CH, Hwang-Bo K, Jee H.
Summary: The aims of this study were to examine the 1) technical aspects and 2) physical demands during small-sided games (SSGs) with different sized pitches in young Korean soccer players. Participants were randomly selected during a nationally held youth competition. Three different game formats were used: SSG8 (8 vs. 8 played on a small-sized field (68 m × 47 m)), RSG8 (8 vs. 8 played on a regular-sized field (75 m × 47 m)), and RSG11 (11 vs. 11 played on a regular-sized field). 11 technical (ball touches, passes, and shots) and 6 physical demand variables (exercise frequency by intensity) were observed and analyzed. Same variables were also analyzed for the goalkeepers. As a result, SSG8 and RSG8 showed significantly greater numbers of technical plays in 5 and 4 variables in comparison to RSG11, respectively. In addition, although the exercise intensities increased slightly in both SSG formats, the amount was within the similar range as previous reports. In conclusion, the SSGs with reduced number of players may be referred in young players to effectively train technical aspects of the game by allowing greater ball exposure time without excessive physical demands. Various cofounding factors such as pitch dimension should be carefully considered for training specific technical and physical variables in young Korean players.

#8 Examining the external training load of an English Premier League football team with special reference to acceleration
Reference: J Strength Cond Res. 2016 Jan 22. [Epub ahead of print]
Authors: Akenhead R, Harley J, Tweddle S
Summary: Practitioners and coaches often use external training load variables such as distance run and the number of high speed running activities to quantify football training. However an important component of the external load may be overlooked when acceleration activities are not considered. The aim of this study was to describe the within-microcycle distribution of external load, including acceleration, during in-season 1-game weeks in an elite football team. Global positioning system technology (GPS) was used to collect time-motion data from twelve representative 7-day microcycles across a competitive season (48 training days, 295 data sets). Training time, total distance (TD), high-speed running distance (HSR [>5.8 m·s]), sprint running distance (SPR [>6.7 m·s]) and acceleration variables were recorded during each training session. Data were analysed for inter-day and inter-position differences using mixed linear modelling. The distribution of external load was characterised by the second training day of the microcycle (5 days pre-match) exhibiting the highest values for all variables of training load, with the fourth day (1 day pre-match) exhibiting the lowest values. Central midfield players covered ∼8%-16% greater TD than other positions excluding wide midfielders (P≤0.03, d=0.2-0.4) and covered ∼17% greater distance accelerating 1-2 m·s than central defenders (P=0.03, d=0.7). When expressed relative to training duration and TD, the magnitude of inter-day and inter-position differences were markedly reduced (P=0.03, d=0.2-0.3). When managing the distribution of training load practitioners should be aware of the intensity of training sessions and consider the density of external load within sessions.

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