Latest research in football - week 49 - 2020

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 Capturing and Quantifying Tactical Behaviors in Small-Sided and Conditioned Games in Soccer: A Systematic Review
Reference: Res Q Exerc Sport. 2020 Oct 8;1-15. doi: 10.1080/02701367.2020.1823307. Online ahead of print.
Authors: Nuno Coito , Keith Davids, Hugo Folgado , Teresa Bento, Bruno Travassos
Summary: The aim was to systematically describe and analyze the tracking systems, the variables, and the statistical methods used to evaluate the players and teams' tactical behavior in small-sided and conditioned games (SSCGs). A search was done in Web of Science, PubMed, Science Direct, and Scielo databases to identify manuscripts published between 2008 and 2019 that manipulated small-sided and conditioned games (SSCGs) and analyzed tactical behaviors of players and teams. rom 349 articles identified, 31 were selected for review. To collect positional data, the global positioning system (GPS), the local position measurement (LPM) system, and TACTO were identified as reliable tracking systems. Twenty-one positional variables were identified to evaluate tactical behaviors, grouped into five main categories: team balance, playing space, width and length of playing space, and interpersonal distance. Tactical behavior patterns were analyzed using approximate entropy, sample entropy, Shannon entropy, and patterns of coordination between players and teams were analyzed using relative phase and running correlation. The tracking systems analyzed were reliable but revealed different advantages and disadvantages of their use. Authors should define the use of each tracking system based on their purpose and level of precision required for analysis. A great duplication was observed on the variables used with similar purposes of tactical analysis. The identification of the variables according to their purpose of analysis will allow a better understanding of their use in the future.

#2 Quantifying the Peak Physical Match-Play Demands of Professional Soccer Substitutes Following Pitch-Entry: Assessing Contextual Influences
Reference: Res Q Exerc Sport. 2020 Oct 8;1-12.
Authors: Samuel P Hills, Stephen Barrett, Bradley Thoseby , Liam P Kilduff , Martin J Barwood, Jon N Radcliffe, Carlton B Cooke, Mark Russell
Summary: The purpose was to quantify the peak post-pitch-entry physical responses of soccer substitutes while assessing contextual influences. Peak responses may be important performance indicators for substitutes introduced to provide a physical impact. Thirty-three professional substitutes wore Microelectromechanical Systems during 44 matches (4 ± 3 observations·player-1). Post-pitch-entry relative peak values for total and high-speed (> 5.5 m·s-1) distances, average acceleration, and PlayerLoad™ were calculated using rolling averages over 60-s to 600-s. Linear mixed models assessed contextual influences (position, substitution timing, scoreline, and location). Substitutes introduced during the final ~15 min of match-play covered less high-speed distance than first-half substitutes (~2.8-3.1 m·min-1) over 480-s to 600-s epochs, and less than 60:00-74:59 min substitutes (~1.7-1.8 m·min-1) during 540-s and 600-s epochs. Average acceleration during all except 180-s epochs was lower for 75:00+ min substitutes compared with first-half replacements (~0.27-0.43 m·s-2), and lower than 60:00-74:59 min substitutes during 60-s (~0.13 m·s-2). Substitutes introduced when their team was winning recorded greater distances over 120-s to 600-s (~6.2-7.7 m·min-1), and higher PlayerLoad™ values during 120-s, 180-s, 300-s, and 480-s epochs (~2.7-3.6 arbitrary units·min-1), compared with when scores were level at pitch-entry. Irrespective of substitution timing, substitute midfielders exceeded the total distance of substitute attackers (~5.9-16.2 m·min-1) for all except 360-s and 600-s epochs, and defenders (~13.3-26.7 m·min-1) during epochs < 300-s. This study provides benchmark data for practitioners tailoring training and recovery protocols, particularly "top-up" conditioning, to the competitive demands of soccer substitutes. Knowing how contextual factors influence substitutes' peak match-play responses may help managers/coaches assess the efficacy of substitution strategies.

#3 Varying Demands and Quality of Play Between In-Conference and Out-of-Conference Games in Division I Collegiate Women's Soccer
Reference: J Strength Cond Res. 2020 Oct 5. doi: 10.1519/JSC.0000000000003841. Online ahead of print.
Authors: Brittany N Bozzini, Bridget A McFadden, Alan J Walker , Shawn M Arent
Summary: The purpose of this study was to assess differences in physical workloads, physiological responses, and performance variables between in-conference (IC) and out-of-conference (OC) games during a collegiate women's soccer season. Female field players (N = 11), who played a minimum of 45 minutes for >50% of games, were evaluated using an integrative GPS and HR monitoring system to determine training load (TL), exercise energy expenditure (EEE), total distance covered (DIS), sprints, time spent in HR zones 4 and 5 (HRZ4 = 80-89% HRmax; HRZ5 = 90-100% HRmax), and distance covered in speed zones 4 and 5 (DISZ4 = 15.0-19.9 km·h; DISZ5 = ≥20 km·h). In addition, percent passing accuracy (PA%), dribbling success (DS%), tackling success (TS%), and challenges won (CW%) were generated for all games. Workload data were analyzed as a rate per minute playing time (PT) per game to account for differences in game duration and PT between OC (n = 7) and IC games (n = 11). Repeated-measures multivariate analyses of variance with univariate follow-ups and effect sizes (Hedges' g) were conducted to compare conditions (OC vs. CON) (p < 0.05). There were significantly greater TL, DIS, EEE, and HRZ5 per minute PT in OC versus IC games (Hedges' g: TL = 0.48; DIS = 0.20, EEE = 0.55; HRZ5 = 0.83; p < 0.05). Further analysis found significant differences in first half play favoring OC games (p < 0.05), but not second half play (p > 0.05). Based on these findings, OC games seem to be more demanding compared to IC, particularly during first half play. Emphasis should be placed on tailoring TL to the accumulating in-season demands through athlete-monitoring technology to prevent declines in performance in the latter half of the season.

#4 Effects of Moderate-to-Heavy Sled Training Using Different Magnitudes of Velocity Loss in Professional Soccer Players
Reference: J Strength Cond Res. 2020 Oct 1. doi: 10.1519/JSC.0000000000003813. Online ahead of print.
Authors: Rafael Grazioli, Irineu Loturco, Pedro Lopez, Igor Setuain, Jean Goulart, Filipe Veeck, Martinho Inácio, Mikel Izquierdo, Ronei S Pinto, Eduardo L Cadore 
Summary: This study investigated the effects of a 11-week moderate-to-heavy sled training intervention with different magnitudes of velocity loss on sprint and jump performance, mechanical muscle function, and body composition in professional soccer players. Seventeen players (age 25.8 ± 4.3 years; height 180.0 ± 8.6 cm; weight 77.7 ± 9.7 kg) were randomly allocated into 2 groups, based on different magnitudes of velocity loss: 10% of velocity decrease (G10, n = 8) and 20% of velocity decrease (G20, n = 9). The velocity-based sled training consisted of 20-m resisted sprints with a progressive loading increase from 45 to 65% of body-mass throughout the intervention. Pre-intervention and postintervention sprint and jump performance, hamstring and quadriceps peak torque and isometric rate of torque development, and lower-limb lean mass measured by dual X-ray absorptiometry were assessed and compared. Two-way repeated measures analysis of variance revealed a significant time-effect for decreases in 10- and 20-m sprint times (p = 0.018 and p = 0.033, respectively), but without a time-group interaction. The G10 showed greater beneficial effects than G20 for both 10-m (-5.5 ± 3.3%, magnitude-based inference [MBI]: possibly vs. -1.7 ± 5.9%, MBI: possibly trivial) and 20-m (-2.5 ± 2.1%, MBI: possibly vs. -1.4 ± 3.7%, MBI: likely trivial) sprint times. Moreover, there was a significant time effect for countermovement jump height and quadriceps isometric peak torque, which decreased significantly after training (p = 0.019 and p = 0.010, respectively), with no within-group effect of time vs. group interaction for these respective outcomes. The novel velocity-based sled model proposed here, especially under lower magnitudes of velocity loss, was able to significantly improve linear sprint performance in professional soccer players.

#5 Performance on sprint, agility and jump tests have moderate to strong correlations in youth football players but performance tests are weakly correlated to neuromuscular control tests
Reference: Knee Surg Sports Traumatol Arthrosc. 2020 Oct 8. doi: 10.1007/s00167-020-06302-z.
Authors: Sofi Sonesson, Hanna Lindblom, Martin Hägglund
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Summary: This study aimed at evaluating the correlation between seven different performance tests and two neuromuscular control tests in youth football players and to evaluate the influence of sex and age groups on test results. One-hundred and fifteen football players (66 boys, 49 girls) mean age 14 ± 0.7 (range 13-16) years from youth teams were tested at the start of the second half of the competitive season. A test battery including agility t-test, 505 agility test, single-leg hop for distance test, side-hop test, countermovement jump test, 10-m sprint test, 20-m sprint test, tuck jump assessment (TJA) and drop vertical jump (DVJ) was completed. Correlations between the seven different performance tests of agility, jump and sprint ability were generally moderate to strong (r = 0.534-0.971). DVJ did not correlate with the performance tests (rho = 0.004 to - 0.101) or with TJA total score (rho = 0.127). There were weak to moderate correlations between TJA total score and the performance tests (r = - 0.323-0.523). Boys performed better than girls in all performance tests (p < 0.001) and in TJA total score (p = 0.002). In boys, older players performed better than younger players in the majority of the tests, while there was no clear age influence among girls. Sprint performance was moderately to strongly correlated with agility and jump performance, and performance tests were weakly to moderately correlated to TJA, while DVJ did not correlate with the other tests. Boys performed better than girls on performance tests and TJA. An age effect on performance was evident in boys but not in girls.

#6 Football - Novel Approaches to Tackle Diabetes
Reference: Exp Clin Endocrinol Diabetes. 2020 Oct 7. doi: 10.1055/a-1262-6352. Online ahead of print.
Authors: Karsten Müssig, Henning E Adamek
Summary: Balanced diet and regular physical activity are of key importance to the prevention of the development and progression of non-communicable diseases. In 2050, 50% of the European population is expected to be obese. Cardiovascular diseases, diabetes mellitus, cancer, as well as joint impairments, will further increase. Therefore, programmes are critical to the improvement of the population's health status in the long run. New ways have to be found that allow addressing more people than with the current approaches. Football has a great potential to attract people at risk for or with already existing non-communicable diseases, such as type 2 diabetes, to participate in health-promoting programmes. The novel football version, walking football was developed for elderly players, aiming at avoiding injuries and physical overload. The present article gives a brief overview on the metabolic effects of recreational football, particularly walking football, as well as health-promoting programmes offered by professional football clubs in humans at risk for or with already existing non-communicable diseases.

#7 Flywheel squats versus free weight high load squats for improving high velocity movements in football. A randomized controlled trial
Reference: BMC Sports Sci Med Rehabil. 2020 Oct 2;12:61. doi: 10.1186/s13102-020-00210-y. eCollection 2020.
Authors: Edvard H Sagelv, Sigurd Pedersen, Lars Petter R Nilsen, Andrea Casolo, Boye Welde, Morten B Randers, Svein Arne Pettersen 
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Summary: High load (HL: > 85% of one repetition maximum (1RM)) squats with maximal intended velocity contractions (MIVC) combined with football sessions can be considered a relevant and time-efficient practice for maintaining and improving high velocity movements in football. Flywheel (FW) resistance exercise (RE) have recently emerged with promising results on physical parameters associated with football performance. In this randomized controlled trial over 6 weeks, 38 recreationally active male football players randomly performed RE with MIVCs two times per week as either 1) FW squats (n = 13) or 2) barbell free weight (BFW) HL squats (n = 13), where a third group served as controls (n = 12). All three groups conducted 2-3 football sessions and one friendly match a week during the intervention period. Pre- to post changes in 10-m sprint, countermovement jump (CMJ) and 1RM partial squat were assessed with univariate analyses of variance. The FW and BFW group equally improved their 10-m sprint time (2 and 2%, respectively, within group: both p < 0.001) and jump height (9 and 8%, respectively, within group: both p < 0.001), which was superior to the control group's change (between groups: both p < 0.001). The BFW group experienced a larger increase (46%) in maximal squat strength than the FW group (17%, between groups: p < 0.001), which both were higher than the control group's change (both p < 0.001). Squats carried out with FWs or BFWs where both are performed with MIVCs and combined with football sessions, were equally effective in improving sprint time and jump height in football players. The BFW group experienced a more than two-fold larger increase in maximal partial squat strength than the FW group in maximal partial squat strength. This presents FW RE as an alternative to BFW HL RE for improving high velocity movements in football.

#8 Mass Gathering Emergency Medicine Organization for the Union of European Football Associations' Under-21 Championship 2019 in Bologna, Italy
Reference: Disaster Med Public Health Prep. 2020 Oct 7;1-4. doi: 10.1017/dmp.2020.291. Online ahead of print.
Authors: Guglielmo Imbriaco, Alfonso Flauto, Tiziano Bussolari, Fiorella Cordenons, Giovanni Gordini 
Summary: Football events represent a type of Mass Gathering Events (MGE) where crowd behavior, temperature and Heat Index, absence of free water, and alcohol consumption can lead to an increased need for medical assistance in participants. This report describes the environmental issues, organization, and healthcare assistance provided during the four matches of the Union of European Football Associations' (UEFA) Under-21 tournament held in Bologna in June, 2019. The four matches had a total of 72655 spectators; 31 patients required medical assistance with a mean Patient Presentation Rate (PPR) of 0.41; Mean Transport To Hospital Rate (TTHR) of 0.04; with PPR and TTHR comparable with literature findings. Majority of patients suffered from minor injuries and illnesses, and were treated directly in stadium medical sites. Medical assistance involved volunteer rescuers, emergency nurses, and physicians; resources were efficiently allocated and provided effective care to every patient.Climate factors, heat and humidity, the absence of free water, and increased alcohol consumption appear to be associated with increased requests for medical assistance. The retrospective analysis of a wider range of environmental factors, and the historical experience developed during similar MGEs suggest the need for a more comprehensive, improved approach for adequately assessing risk and planning the necessary healthcare resources.

#9 Does Distance Produce Beauty? The Influence of COVID-19 Lockdown on the Coach-Athlete Relationship in a Chinese Football School
Reference: Front Psychol. 2020 Sep 11;11:560638. doi: 10.3389/fpsyg.2020.560638. eCollection 2020.
Authors: Juan Li, Hongyan Gao, Pan Liu, Caixia Zhong
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Summary:  This paper examined the relationship between coaches and youth athletes in China by comparing data collected before and after the lockdown. A total of 221 youth athletes aged 13-19 years in one professional football school completed coach-athlete relationship questionnaires. The rank-sum test was used to verify the differences in the data. The results of the Mann-Whitney U test showed that mean value of the three dimensions of the coach-athlete relationship (closeness, commitment, and complementarity) increased after the COVID-19 lockdown. The results also showed that athletes of different age categories showed different changes in the coach-athlete relationship after the lockdown, and the changes were not significantly related to the severity of the COVID-19 epidemic. The theoretical and practical implications are discussed.

#10 The Influence of Emotion in the Management of Amateur Football Organizations
Reference: Front Psychol. 2020 Sep 4;11:2218. doi: 10.3389/fpsyg.2020.02218. eCollection 2020.
Authors: Melany Hebles , Vicente Javier Prado-Gascó, Orlando Llanos-Contreras, Mario Alguacil
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Summary: This article is oriented to the analysis of organizational and emotional variables in amateur sporting organizations. The general objective is to analyze the influence of organizational variables such as service quality, transactional leadership, and transformational leadership and emotional variables such as affective commitment, emotional attachment investment, and emotional attachment dividend to predict the credibility that members of amateur sporting organizations perceive, as well as their degree of identification and loyalty. The opinions of 203 members of Chilean amateur football teams [169 men and 34 women, with ages between 18 and 68 years (mean = 32.75 years, DT = 9.92)] have been analyzed through a self-completed questionnaire. To reach the objectives, two types of differential but complementary analyses, in the form of hierarchical regression models (from hereon, HRMs) and qualitative comparative analysis (from hereon, QCA), were performed. The results obtained suggest that the organizational variables are better predictors than the emotional variables in all of the cases. In the same way, the inclusion of the emotional variables improves the predictive capacity of the proposed models to explain identification and loyalty, but not in the case of credibility. In general, the variables considered seem to explain 37% of the credibility, 56% of loyalty, and 65% of identification. On the other hand, considering the results of the QCA, no variable turned out to be necessary. However, different combinations of variables (conditions) were observed that were able to explain between 47 and 91% of the cases of the variables analyzed. In general, based on these results, it was observed that the emotional variables were important in interaction with other organizational ones since they are present in the three combinations that most explain identification and loyalty and are also present in the three combinations that most explain credibility. This study contributes to the literature by supporting the importance of managing emotions in order for sporting organizations to be more successful.

#11 Differentiation Between Tendinous, Myotendinous and Myofascial Injuries by L-BIA in Professional Football Players
Reference: Front Physiol. 2020 Sep 4;11:574124. doi: 10.3389/fphys.2020.574124. eCollection 2020.
Authors: Lexa Nescolarde, Joaquim Terricabras, Sandra Mechó, Gil Rodas, Javier Yanguas
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Summary: The aim was to differentiate by localized bioimpedance (L-BIA) measurements 24 h after injury, between tendinous, myotendinous junction (MTJ), and myofascial junction (MFJ) injuries, previously diagnosed by MRI exam. To evaluate by L-BIA, the severity of MTJ injuries graded from 1 to 3, and to determine the relationship between days to return to play (RTP) and L-BIA measurements. 3T MRI and tetra polar L-BIA was used to analyzed 37 muscle injuries 24 h after injury in 32 male professional football players, (23.5 ± 1.5 kg m-2; 1.8 ± 0.1 m; 20-30 year.) between the 2016-2017 and 2017-2018 seasons. Muscle injuries were classified by The British Athletics Muscle Injury Classification (BAMIC). Percentage difference of L-BIA parameters [resistance (R), reactance (Xc), and phase angle (PA)] of the injured side were calculated considering contralateral non-injured side as the reference value. According to BAMIC classification and by MRI exam, we found tendinous (n = 4), MTJ (n = 26), and MFJ (n = 7) muscle injuries. In addition, MTJ injuries were grouped according to the severity of injury in grade 1 (n = 11), grade 2 (n = 8), and grade 3 (n = 7). Significant decrease (P < 0.01) was found in the L-BIA parameters R, Xc, and PA, in both MTJ and MFJ as well as in the different grades of MTJ injuries. In particular, in Xc (P < 0.001), which is related to muscle cell disruption. Regarding days to RTP, there was statistical significance among the three different grades of MTJ injuries (P < 0.001), especially when grade 1 was compared to grade 3 and grade 2 compared to 3. L-BIA is a complementary method to imaging diagnostic techniques, such as MRI and US, to quantify MTJ and MFJ injuries. In addition, the increase in the severity of the MTJ injury resulted in higher changes of the Xc parameter and longer time to RTP.

#12 Short and long lever adductor squeeze strength values in 100 elite youth soccer players: Does age and previous groin pain matter?
Reference: Phys Ther Sport. 2020 Oct 6;46:243-248. doi: 10.1016/j.ptsp.2020.10.001. Online ahead of print.
Authors: Matthew D DeLang, J Craig Garrison, Joseph P Hannon, Ryan P McGovern, John Christoforetti, Kristian Thorborg
Summary: The purpose was to examine adductor squeeze strength in elite youth soccer players by investigating the relationship of age and previous one-year groin pain on adductor squeeze force outputs, and to provide reference values for youth players. Elite youth soccer players (n = 100; age 14.5 ± 1.9 years; height 168.0 ± 10.7 cm; mass 60.7 ± 13.0 kg) participated. Adductor squeeze tests were captured in short and long lever positions, and groin pain assessed via subjective retrospective questionnaire. Multiple linear regressions were computed to compare the effects of age and previous one-year groin pain on adductor squeeze strength. Raw adductor squeeze force values (N) had a moderate positive relationship with age (short r = 0.517, p < 0.001; long r = 0.457, p < 0.001), but not when force is normalized to body mass (N/kg; short r = 0.014, p = 0.444; long r = -0.173, p = 0.043). Previous groin pain did not have an effect on short or long lever squeeze strength. Reference values for long lever adductor squeeze strength (3.59 ± 0.77 Nm/kg) are provided. Age and previous groin pain do not have an effect on adductor squeeze strength values in elite youth soccer players, so comparing values to the present adolescent cohort can be quickly interpreted without adjustment for age or previous injury.

#13 The role of alcohol in the link between national football (soccer) tournaments and domestic abuse - Evidence from England
Reference: Soc Sci Med. 2020 Oct 22;268:113457. doi: 10.1016/j.socscimed.2020.113457. Online ahead of print.
Authors: Anna Trendl, Neil Stewart, Timothy L Mullett
Summary: Domestic abuse is increasingly recognised as a serious public health concern worldwide. Previous research has suggested a link between national football (soccer) tournaments and domestic abuse. While hypothesized to be a significant factor, the role alcohol plays in this relationship has not yet been explored quantitatively. In this study, using 10 years' worth of crime data (from 2010 to 2019) from the second largest police force in England (West Midlands Police), we explored the effect of England draws, losses, and wins in national football tournaments on the number of alcohol and non-alcohol-related domestic abuse cases reported to the police. Results from a series of negative binomial regression analyses show that the number of reported alcohol-related domestic abuse cases increases by 47%, 95% confidence interval [26%-71%], following an England football victory. This effect is limited to alcohol-related cases. The estimate translates into a 0.53, 95% CI [0.3-0.8], increase in the daily rate of alcohol-related cases per 100,000 individuals. The England win effect survives various robustness checks (including the re-analysis of a dataset from another geographical area in England), and its time course is strongly consistent with a causal link between England's football victories and an increase in alcohol-related domestic abuse. We also found a comparable increase in the number of other (not classified as domestic abuse) alcohol-related violent crimes on England win days. Further research is required to understand the exact causal pathway between national football tournaments, alcohol consumption, and violent behaviours in domestic settings.

#14 A New Strategy to Integrate Heath-Carter Somatotype Assessment with Bioelectrical Impedance Analysis in Elite Soccer Player
Reference: Sports (Basel). 2020 Oct 27;8(11):E142. doi: 10.3390/sports8110142.
Authors: Francesco Campa, Tindaro Bongiovanni, Catarina N Matias, Federico Genovesi, Athos Trecroci, Alessio Rossi, F Marcello Iaia, Giampietro Alberti, Giulio Pasta, Stefania Toselli
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Summary: Easy-to-apply and quick methods for evaluate body composition are often preferred when assessing soccer teams. This study aimed to develop new equations for the somatotype quantification that would reduce the anthropometric measurements required by the Heath and Carter method, integrating the somatotype assessment to the bioelectrical impedance analysis (BIA). One hundred and seventy-six male elite soccer players (age 26.9 ± 4.5 years), registered in the Italian first division (Serie A), underwent anthropometric measurements and BIA. Endomorphy, mesomorphy, and ectomorphy were obtained according to the Heath and Carter method, while fat mass (FM) and fat free mass (FFM) estimated using a BIA-derived equation specific for athletes. The participants were randomly split into development (n = 117) and validation groups (n = 59, 1/3 of sample). The developed models including resistance2/stature, FM%, FFM, contracted arm and calf circumference, triceps, and supraspinal skinfolds had high predictive ability for endomorphy (R2 = 0.83, Standard Error of Estimate (SEE) = 0.16) mesomorphy (R2 = 0.80, SEE = 0.36), and ectomorphy (endomorphy (R2 = 0.87, SEE = 0.22). Cross validation revealed R2 of 0.80, 0.84, 0.87 for endomorphy, mesomorphy, and ectomorphy, respectively. The proposed strategy allows the integration of somatotype assessment to BIA in soccer players, reducing the number of instruments and measurements required by the Heath and Carter approach.

#15 Effects of Unloaded vs. Ankle-Loaded Plyometric Training on the Physical Fitness of U-17 Male Soccer Players
Reference: Int J Environ Res Public Health. 2020 Oct 27;17(21):E7877. doi: 10.3390/ijerph17217877.
Authors: Mehrez Hammami, Nawel Gaamouri, Katsuhiko Suzuki, Ridha Aouadi, Roy J Shephard, Mohamed Souhaiel Chelly
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Summary: The aim of this study was to compare the impact of two differing plyometric training programs (loaded plyometrics (with 2.5% of body mass placed above the ankle joint) vs. unloaded plyometrics), performed biweekly for 10 weeks, on the physical fitness of elite junior male soccer players. Participants aged 16.0 ± 0.5 years were randomly assigned between unloaded plyometrics (UP; n = 12), loaded plyometrics (LP; n = 14) and control (C; n = 12) groups. Two-way analyses of performance (group x time) were assessed by 40-m sprint times; 9-3-6-3-9 m sprints with 180° turns (S180°); 9-3-6-3-9 m sprints with backward and forward running (SBF); and 4 × 5 m sprints (S4 × 5 m); four jump tests; measures of static and dynamic balance; repeated change of direction tests and the Yo-Yo intermittent recovery test. Both LP and UP enhanced sprinting performance relative to C (p < 0.05) but performance increased more in LP relative to UP (p < 0.05) in all sprints except 40 m. Change of direction times were also significantly shortened by LP relative to UP (p < 0.05) and C (p < 0.01) in all tests, with no significant differences between UP and C. Jumps heights increased similarly in both LP and UP relative to C (p < 0.05), with no significance between LP and UP. LP and UP also enhanced repeated change of direction scores relative to C (p < 0.01) with greater changes in LP than in UP (p < 0.01). Finally, LP enhanced some balance scores relative to UP (p < 0.05) and C (p < 0.05). We conclude that the introduction of 10 weeks of in-season loaded plyometrics into the regimen of U17 male soccer players yields gains in several physical performance scores relative to either unloaded plyometrics or the control training regimen.

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