Latest research in football - week 11 - 2015

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 The Effect of Two Generic Aerobic Interval Training Methods on Laboratory and Field Test Performance in Soccer Players
Reference: J Strength Cond Res. 2015 Mar 9. [Epub ahead of print]
Authors: Fernandes da Silva J, Nakamura FY, Carminatti LJ, Dittrich N, Cetolin T, Guglielmo LG
Summary: The purpose of this study was to compare the effects of two generic aerobic training (GAT) models, based on PVT-CAR in U-20 elite soccer players. Seventeen soccer players (age 17.9±1.0 years; 178.6±5.0 cm; 73.6±6.6 kg; 11.1±1.3%) from a team competing in a national junior league took part in the study. The athletes performed a series of pre and post training tests (incremental test on a treadmill to determine the maximal oxygen uptake (VO2max), velocity at maximal oxygen uptake (vVO2), the lactate threshold (LT) and T-CAR). The interval training models applied were with 180° direction change (T12:12; n=9) and without direction change (T6:6; n=8). No significant interaction (time vs. group) was observed for the majority of variables analyzed (p > 0.05), although significant main effects in time were evident regarding PVTREAD (F=56.3; P<0.0001), vVO2max (F=35.8; p<0.0001), LT (F=57.7; p<0.0001) and PVT-CAR (F=52.9; p<0.0001). Moreover, there was no significant change in VO2max between pre and post training period (F=4.26; p=0,056) in both training groups. Thus, it can be concluded that the prescribed training with and without direction change in the intensity of the PVT-CAR, increases the PVTREAD, the vVO2max, the LT and the PVT-CAR similarly.

#2 Muscle fatigue induced by a soccer match-play simulation in amateur Black South African players
Reference: J Sports Sci. 2015 Mar 12:1-7. [Epub ahead of print]
Authors: Jones RI, Ryan B, Todd AI
Summary: The purpose of the current study was to investigate the effects of a soccer-specific fatigue protocol on the temporal changes in torque producing abilities of the thigh within African soccer players. Twenty amateur Black South African soccer players performed the SAFT90 soccer match-play simulation protocol, while isokinetic measurements were obtained pre-exercise (T0), after the 1st half (T45), after half time (T60) and after the 2nd half (T105). During SAFT90 performance, significant overall concentric quadriceps peak torque changes were observed (1.05 rad · s-1 = 16.6%, 3.14 rad · s-1 = 9.5%). Eccentric hamstring peak torque also decreased significantly over time (1.05 rad · s-1 = 17.4%, 3.14 rad · s-1 = 18.5%), with significant reductions occurring during both halves. The functional strength ratio (eccH:conQ) at 3.14 rad · s-1 was observed to significantly decrease by 10.1% overall. The indicated time-dependent changes in Black South African players have implications for competitive performance and increased predisposition to hamstring muscle injuries. Because of muscle fatigue, the hamstrings may have insufficient eccentric strength during the late swing phase when sprinting, resulting in eccentric overload and damage to the muscle. The changes in strength found in the current study help explain the increased predisposition to hamstring strains during the latter stages of both halves of match-play as reported by epidemiological studies.

#3 Match Running Performance and Success Across a Season in German Bundesliga Soccer Teams
Reference: Int J Sports Med. 2015 Mar 11. [Epub ahead of print]
Authors: Hoppe MW, Slomka M, Baumgart C, Weber H, Freiwald J
Summary: The aim of the study was to quantify the association between match running performance and success across a season in soccer teams competing within a European top league. We analyzed the match running performance data of all soccer teams from the German Bundesliga across the season 2012/13 (306 matches). The following match running performance data were used: total distance covered as well as number of running activities>18.0 km/h and > 22.7 km/h. Depending on the team's ball possession status, all match running performance data were also analyzed as those with and without ball possession. The success across the season was defined as the final competition points accumulated. The match running performance alone was not significantly correlated with the final points accumulated (best r=0.24; p=0.34). In contrast, positive-significant correlations were observed for the match running performance with ball possession (best r=0.77; p<0.01). However, of these latter correlations, only the total distance covered with ball possession was a significant predictor (p<0.01) and accounted for 60% of the variance (R2=0.60) in the final points accumulated. It is concluded that it is not the match running performance alone that is important for achieving success in German Bundesliga soccer teams, but rather its relation to technical/tactical skills with respect to ball possession.

#4 Smartphone-derived Heart Rate Variability and Training Load in a Female Soccer Team
Reference: Int J Sports Physiol Perform. 2015 Mar 10. [Epub ahead of print]
Authors: Flatt AA, Esco MR
Summary:  The purpose of the study was to evaluated the 7-day mean and CV of supine and standing ultra-short log transformed root mean square of successive R-R intervals multiplied by 20 (lnRMSSDx20) obtained with a smartphone application (app) in response to varying weekly training load (TL). Additionally, we aimed to determine if these values could be accurately assessed in as few as 5 or 3 days per week. 9 females from a collegiate soccer team performed daily HRV measures with an app in supine and standing positions over 3 weeks of moderate, high and low TL. The mean and CV over 7, 5, and 3 days were compared within and between each week. The 5 and 3-day measures within each week provided very good to near perfect intraclass correlations (ICCs ranging from 0.74 - 0.99) with typical errors ranging from 0.64 - 5.65 when compared with the 7-day criteria. The 7, 5, and 3-day supine CV and the 7-day standing CV were moderately lower during the low load compared to the high load week (p values ranged from 0.003 - 0.045 and effect sizes ranged from 0.86 - 0.92), with no significant changes occurring in the other measures. This study supports the use of the mean and CV of lnRMSSD measured across at least 5 days for reflecting weekly values. The supine lnRMSSDx20 CV as measured across 7, 5, and 3 days was the most sensitive marker to the changes in TL within the 3-week period.

#5 Plyometric Training Effects on Athletic Performance in Youth Soccer Athletes: A Systematic Review Plyometrics and Youth Soccer Performance
Reference: J Strength Cond Res. 2015 Mar 9. [Epub ahead of print]
Authors: Bedoya AA, Miltenberger MR, Lopez RM.
Summary: The purpose of this systematic review was to critically analyze the literature to determine the effectiveness of plyometric training on athletic performance in youth soccer athletes. A total of seven studies were included in this review after meeting the following criteria: a) used plyometric training programs to assess athletic performance, b) subjects were soccer athletes aged pre-adolescent up to 17 years, and c) were published from 2000 to January 2014. Study methods were assessed using the PEDro scale with scores ranging from 4 to 6. Results showed similarities and differences in methodologies and procedures among the included studies. Athletic performance consisting of kicking distance, speed, jumping ability, and agility significantly improved because of plyometric training interventions. The current evidence suggests plyometric training should be completed two days per week for 8-10 weeks during soccer practice with a 72-hour rest period between plyometric training days. The initial number of foot contacts should be 50-60 per session and increase to no more than 80-120 foot contacts per session for this age group to prevent overuse injuries. A total of 3-4 plyometric training exercises should be performed 2-4 sets for 6-15 reps per training session. The evidence and the literature suggest that plyometric training for this age group should only be implemented using recommended safety guidelines such as those published by the Canadian Society for Exercise Physiology and the National Strength and Conditioning Association and under appropriate supervision by trained personnel.

#6 A low-cost method for estimating energy expenditure during soccer refereeing
Reference: J Sports Sci. 2015 Mar 9:1-6. [Epub ahead of print]
Authors: Ardigò LP, Padulo J, Zuliani A, Capelli C
Summary: This study aimed to apply a validated bioenergetics model of sprint running to recordings obtained from commercial basic high-sensitivity global positioning system receivers to estimate energy expenditure and physical activity variables during soccer refereeing. We studied five Italian fifth division referees during 20 official matches while carrying the receivers. By applying the model to the recorded speed and acceleration data, we calculated energy consumption during activity, mass-normalised total energy consumption, total distance, metabolically equivalent distance and their ratio over the entire match and the two halves. Main results were as follows: (match) energy consumption = 4729 ± 608 kJ, mass normalised total energy consumption = 74 ± 8 kJ · kg-1, total distance = 13,112 ± 1225 m, metabolically equivalent distance = 13,788 ± 1151 m and metabolically equivalent/total distance = 1.05 ± 0.05. By using a very low-cost device, it is possible to estimate the energy expenditure of soccer refereeing. The provided predicting mass-normalised total energy consumption versus total distance equation can supply information about soccer refereeing energy demand.

#7 The impact of in-season national team soccer play on injury and player availability in a professional club
Reference: J Sports Sci. 2015 Mar 9:1-7. [Epub ahead of print]
Authors: Carling C, McCall A, Le Gall F, Dupont G.
Summary: This study investigated the impact of in-season national team duty on injury rates and player availability in a professional soccer club. Time-loss injuries and exposure time during club and national team duties were recorded prospectively over 5 seasons (2009-2014). A time-loss injury was sustained by 37.7% of squad members participating in national duty, all injuries occurring in match-play. The incidence (per 1000 h exposure) for national team player match-play injuries did not differ (P = 0.608) to that for all players in club competitions: 48.0 (95% CI 20.9-75.5) vs. 41.9 (95% CI 36.5-47.4), incidence rate ratio = 1.2 (CI: 0.8-2.4). The majority (58%) of national team injuries resulted in a layoff ≤1 week. Of all working days lost to injury generally, 5.2% were lost through injury on national duty. Injury incidence in the week following national duty was comparable (P = 0.818) in players participating or not: 7.8 (95% CI 3.6-12.0) vs. 7.1 (95% CI: 4.6-9.6), incidence rate ratio = 1.1 (CI: 0.7-2.7). While approximately 40% of participating players incurred a time-loss injury on national duty, no training injuries were sustained and injuries made up a negligible part of overall club working days lost to injury. Following duty, players had a similar injury risk to peers without national obligations.

#8 Pattern recall skills of talented soccer players: Two new methods applied
Reference: Hum Mov Sci. 2015 Mar 3;41C:59-75. doi: 10.1016/j.humov.2015.02.007. [Epub ahead of print]
Authors: van Maarseveen MJ, Oudejans RR, Savelsbergh GJ
Summary: In this study we analyzed the pattern recall skills of talented soccer players by means of two innovative methods of analysis and gaze behavior data. Twenty-two young female soccer players watched video clips of 3 vs. 3 small-sided games and, after occlusion, had to reproduce the positions of the players. Recall performance was measured by calculating the spatial error of the recalled player positions at the moment of occlusion and at consecutive 33ms increments. We analyzed player positions relative to each other, by assessing geometric pattern features in terms of angles between players, and we transformed the data into real-world coordinates to exclude the effects of the 2D perspective in the video clips. The results showed that the participants anticipated the movements of the patterns. In real-world coordinates, the more experienced players anticipated the pattern further in advance than the less experienced players and demonstrated a higher search rate, a shorter fixation duration and a higher fixation order. The differences in recall accuracy between the defensive and offensive elements were not consistent across the methods of analysis and, therefore, we propose that perspective effects of the video clip should be taken into account in further research.

#9 Impact of limited hamstring flexibility on vertical jump, kicking speed, sprint, and agility in young football players
Reference: J Sports Sci. 2015 Mar 12:1-5. [Epub ahead of print]
Authors: García-Pinillos F, Ruiz-Ariza A, Moreno Del Castillo R, Latorre-Román PÁ.
Summary: This study aims to analyse the impact of limited hamstring flexibility (HF) on specific football skills, such as sprinting and jumping ability, agility, and kicking speed in young football players. Forty-three male football players (aged 14-18) from a semi-professional football academy participated voluntarily in this study. Data about anthropometric measurements, HF (unilateral passive straight-leg raise test: PSLR), vertical jumping ability (countermovement jump: CMJ), sprinting ability (5, 10, 20 m: S5 m, S10 m, S20 m), agility (Balsom agility test: BAT), and kicking speed in terms of ball speed (dominant and non-dominant leg: KSdom and KSnon-dom) were collected. Cluster analysis grouped according to HF, dividing participants into a flexible group (FG, n = 24) and a non-flexible group (NFG, n = 19) in relation to performances on the PSLR test. Despite finding no significant differences between groups in body composition and age, the FG performed better in terms of sprint scores (S5 m: 6.12%, S10 m: 4.09%, S20 m: 3.29%), BAT score (4.11%), CMJ score (10.49%), and scores for KSdom (6.86%) and KSnon-dom (8%) than the NFG. The results suggest that HF is a key factor for performing football-specific skills, such as sprinting, jumping, agility, and kicking in young football players. These results support the rationale that muscle flexibility must be specifically trained in football players beginning at early ages.

#10 Epidemiological and clinical outcome comparison of indirect ('strain') versus direct ('contusion') anterior and posterior thigh muscle injuries in male elite football players: UEFA Elite League study of 2287 thigh injuries (2001-2013)
Reference: Br J Sports Med. 2015 Mar 9. pii: bjsports-2014-094285. doi: 10.1136/bjsports-2014-094285. [Epub ahead of print]
Authors: Ueblacker P, Mueller-Wohlfahrt HW, Ekstrand J
Summary: Data regarding direct athletic muscle injuries (caused by a direct blunt or sharp external force) compared to indirect ones (without the influence of a direct external trauma) are missing in the current literature-this distinction has clinical implications. The purpose of the study was to compare incidence, duration of absence and characteristics of indirect and direct anterior (quadriceps) and posterior thigh (hamstring) muscle injuries. 30 football teams and 1981 players were followed prospectively from 2001 until 2013. The team medical staff recorded individual player exposure and time-loss injuries. Muscle injuries were defined as indirect or direct according to their injury mechanism. In total, 2287 thigh muscle injuries were found, representing 25% of all injuries. Two thousand and three were valid for further analysis, of which 88% were indirect and 12% direct. The incidence was eight times higher for indirect injuries (1.48/1000 h) compared to direct muscle injuries (0.19/1000 h) (p<0.01). Indirect muscle injuries caused 19% of total absence, and direct injuries 1%. The mean lay-off time for indirect injuries amounted to 18.5 days and differed significantly from direct injuries which accounted for 7 days (p<0.001). 60% of indirect injuries and 76% of direct injuries occurred in match situations. Foul play was involved in 7% of all thigh muscle injuries, as well as in 2% of indirect injuries and 42% of direct injuries. Muscle anterior and posterior thigh injuries in elite football are more frequent than have been previously described. Direct injuries causing time loss are less frequent than indirect ones, and players can usually return to full activity in under half the average time for an indirect injury. Foul play is involved in 7.5% of all thigh muscle injuries.
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