Latest research in football - week 21 - 2021

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 Influence of Fatigue on the Rapid Hamstring/Quadriceps Force Capacity in Soccer Players 

Reference: Front Physiol. 2021 Feb 5;12:627674. doi: 10.3389/fphys.2021.627674. eCollection 2021. 

Authors: Qingshan Zhang, Baptiste Morel, Robin Trama, Christophe A Hautier

Download link:

Summary: The objective of this study was to examine the effect of fatigue on maximal and rapid force capacities and muscular activation of the knee extensors and flexors. Seventeen professional soccer players volunteered to participate in this study. Peak torque (Tpeak) and rate of torque development (RTD) of knee flexor (90°. s-1, -30°. s-1) and extensor (90°. s-1) muscles were measured before and after fatigue (i.e., 30 maximal knee extension and flexion repetitions at 180°s-1) performed on an isokinetic dynamometer. Hamstring to quadriceps peak strength and RTD ratios were calculated. Besides, using surface EMG, the mean level of activation (RMSmean), Rate of EMG Rise (RER), and EMG Frequency-Time maps were measured on quadriceps and hamstring muscles. Following fatigue, Tpeak, RTD, RER declined significantly in the two muscle groups (all p < 0.05) without modification of RMSmean. No decrease in conventional and functional H/Q ratios was observed after fatigue except for a significant increase in the H ecc30/Q con180 ratios (1.03 ± 0.19 vs. 1.36 ± 0.33, p < 0.001). Besides, the RTD H/Q ratios decreased significantly after fatigue, and the statistical parametric mapping analysis (SPM) performed on the EMG/angle curves, and EMG Frequency-Time maps showed that fatigue strongly influenced the muscle activation during the first 100 ms of the movement, following the higher EMG frequency component shift toward the lower frequency component. Our results show that the reduction of RTD and RER during the first 100 ms of the contraction after fatigue exercise makes more sense than any H/Q ratio modification in understanding injury risk in soccer players. 



#2 Analysis of Fitness Status Variations of Under-16 Soccer Players Over a Season and Their Relationships With Maturational Status and Training Load 

Reference: Front Physiol. 2021 Feb 5;11:597697. doi: 10.3389/fphys.2020.597697. eCollection 2020. 

Authors: Hadi Nobari, Ana Filipa Silva, Filipe Manuel Clemente, Marefat Siahkouhian, Miguel Ángel García-Gordillo, José Carmelo Adsuar, Jorge Pérez-Gómez

Download link:

Summary: The purposes of this study were (i) to analyze the variations in maximal oxygen consumption (VO2m ax), maximal heart rate (HRmax), heart rate at rest, acceleration, maximal speed, agility, anaerobic sprint test (RAST) of peak power (RPP), RAST of minimum power, RAST of average power (RAP), and RAST of fatigue index (RFI) during the competitive season, using maturation status and accumulated training load as covariates, and (ii) to describe the differences between responders and non-responders in relation to baseline levels. Twenty-three elite players from the same team competing in the national under-16 competitions were evaluated for 20 weeks in period 1 (before league), middle (mid league), and period 2 (after league). The VO2m ax (p = 0.009), maximal speed (p = 0.001), RPP (p < 0.001), RAP (p < 0.001), and RFI (p < 0.001) significantly changed across the assessment periods. Interestingly, using accumulated training load and maturation status as covariates revealed no statistical significance (p > 0.05). When analyzing responders and non-responders, only HRmax (between periods 1 and 2) showed no differences between the groups. As a conclusion, it can be seen that accumulated training load and maturation status play an important role in the differences observed across the season. Thus, coaches should consider the importance of these two factors to carefully interpret fitness changes in their players and possibly adjust training decisions according to the maturation level of the players. 



#3 Physical Demands and Internal Response in Football Sessions According to Tactical Periodization 

Reference: Int J Sports Physiol Perform. 2021 Feb 24;1-7. doi: 10.1123/ijspp.2019-0829. 

Authors: Ibai Guridi Lopategui, Julen Castellano Paulis, Ibon Echeazarra Escudero 

Summary: The objectives of the present study were (1) to analyze the internal and external load profile of training and competition carried out by semiprofessional football players during a 27-week period and (2) to examine the possible link between this type of periodization and players' fitness status and their readiness to compete. Training and match data were obtained from 26 semiprofessional football players belonging to the reserve squad of a Spanish La Liga club during the 2018/19 season. For the purpose of this study, the distribution of external and internal load during a typical training microcycle, with 6 or 7 days between matches, was analyzed. Five types of sessions were considered: strength, duration, velocity, preofficial match, and official match. 

Results: The results showed a different internal and external load profile for each type of session, with the load being consistently higher during matches when compared with training sessions (28.9%-94% higher), showing significant differences in all the variables. There was a clear tapering strategy in the last days of the week to arrive with enough freshness to compete, shown by the decrease of the values in the 2 days before the match (15%-83% reduction, depending on the variable). Furthermore, the horizontal alternation of the load allowed the players to maintain their fitness level during the 27-week period. Our findings suggest that this weekly periodization approach could help achieve a double conditional target, allowing a short tapering strategy to face the match with enough freshness and serving as a strategy for maintaining or optimizing players' physical performance during the season. 



#4 Coaches' Corrective Feedback, Psychological Needs, and Subjective Vitality in Mexican Soccer Players 

Reference: Front Psychol. 2021 Feb 4;11:631586. doi: 10.3389/fpsyg.2020.631586. eCollection 2020. 

Authors: José Tristán, Rosa María Ríos-Escobedo, Jeanette M López-Walle, Jorge Zamarripa, Miguel A Narváez, Octavio Alvarez

Download link:

Summary: In the sport context, an essential aspect of an athlete's development and performance happens during the interaction with the coach while receiving information on the aspects of performance that need to be modified (corrective feedback). Grounded in the Self-Determination Theory and particularly on the basic psychological needs theory, a structural equation model (SEM) was tested with the following sequence: perception of the amount of corrective feedback generated by the coach, perceived legitimacy of corrective feedback, satisfaction of basic psychological needs, and vitality in soccer players. Additionally, simple mediation and serial (double) mediation models were also tested. Participants were 377 Mexican soccer players (Maged = 16.46, SD = 1.08), who completed the instruments that evaluated the study variables. SEM results reported positive and significant variables' interrelations in the sequence. The analysis of serial mediation model showed that the perceived legitimacy of feedback and the satisfaction of basic psychological needs fully mediated the relationship between the perception of the amount of corrective feedback generated by the coach and the perception of the subjective vitality of Mexican soccer players. Results suggest that coaches have to ensure that athletes accept the corrective feedback provided and meet their basic psychological needs. Based on SDT tenets, this research highlights the importance for coaches to be aware of the athlete's perceptions when they are providing corrective feedback and their implications for athlete's technical development and well-being. It is suggested to incorporate those aspects to training programs for coaches. 



#5 Open Interphalangeal Dislocation of the Great Toe in an Amateur Football Player: A Rare Case Report 

Reference: J Orthop Case Rep. 2020 Jul;10(4):78-81. doi: 10.13107/jocr.2020.v10.i04.1812. 

Authors: Michael-Alexander Malahias, Dimitrios Oikonomou, Vikram V Kadu, Michail Kotsapas, Nikolaos K Paschos, Dimitrios Giotis

Download link:

Summary: Open interphalangeal (IP) dislocations are completely uncommon. Up to now, different patterns of dislocation have been described. The combination of axial loading and hyperdorsiflexion forces, leading to plantar dislocation of the distal phalanx, is a rare type of injury, which has yet not been reported. A rare case of traumatic open dislocation of the left great toe IP joint in a highly active, overweighted, male, amateur football player is presented. The possible underlying mechanism was impact of the left great toe against the ground and subsequent hyperdorsiflexion. The distal phalanx was dislocated plantarly, whereas the proximal phalanx was protruding out the dorsal skin of the toe. Open exploration and reduction led to excellent clinical results 6 months after surgery. Open traumatic IP dislocation of the great toe due to low force activity is a very rare mode of injury, which requires adequate treatment including immediate purification of the exposed joint, control of the sesamoids' position, exclusion of intra-articular fractures, joint's reduction, soft-tissue repair, and proper stabilization. 



#6 The Relationship between Body Mass Index, Body Fat Percentage, and Dietary Intake with Muscle Fatigue in Adolescent Football Players 

Reference: J Nutr Sci Vitaminol (Tokyo). 2020;66(Supplement):S134-S136. doi: 10.3177/jnsv.66.S134. 

Authors: Mochammad Rizal, Calista Segalita, Trias Mahmudiono

Download link:

Summary: It is important for football players to maintain muscle strength through the entire match. The aim of this study was to investigate body mass index (BMI), body fat percentage (BF%), dietary intake (energy, carbohydrate, and protein) and its relationship with muscle fatigue among adolescent football players. This was a cross-sectional study involving 26 football players aged 15-17 y. BMI was determined using WHO Anhtro Plus, BF% was analyzed using Bioelectrical Impedance Analysis (BIA) and categorized using bodyfat curves for children, and dietary intake was assessed using 3×24 h dietary recall. Running-Based Anaerobic Sprint Test (RAST) was conducted twice and averaged to identify muscle fatigue. Pearson correlation and multiple-regression analysis were performed to determine the relationship between variables. The results showed that overall participants had healthy weight (17.61±1.82 kg/m2), good diet pattern (energy 99.08±14.34%, carbohydrate 92.88±9.54% and protein 95.96±23.41%), but low body fat (6.76±2.12%). In pearson test, negative correlations were found in muscle fatigue and BMI (r=-0.393, p=0.047), as well as BF% (r=-0.458, p=0.019), but positive between muscle fatigue and energy intake (r=0.538, p=0.005). Furthermore, multiple-regression analysis only confirmed statistically significant relationship between energy intake and muscle fatigue (p=0.028). We conclude that the higher BMI and BF% may lead to greater muscle fatigue, while higher energy intake has significant improvement to reduce muscle fatigue. Hence, it is essential for football players to consume adequate energy, and consider to maintain BMI and BF% at optimal range. 



#7 Jejunal rupture resulting from a collision in soccer 

Reference: J Surg Case Rep. 2021 Feb 15;2021(2):rjab005. doi: 10.1093/jscr/rjab005. eCollection 2021 Feb. 

Authors: Ho-Cing Victor Yau

Summary: Small bowel rupture in blunt force trauma usually results from high-velocity and high-energy forces. The occurrence of this following collision in sport is a rare event that requires urgent surgical intervention. This is the case of a 27-year-old male who sustained a jejunal rupture following a collision during a game of soccer. 



#8 Recovery Kinetics Following Small-Sided Games in Competitive Soccer Players: Does Player Density Size Matter? 

Reference: Int J Sports Physiol Perform. 2021 Feb 24;1-11. doi: 10.1123/ijspp.2020-0380. Online ahead of print. 

Authors: Konstantinos Papanikolaou, Panagiotis Tsimeas, Angeliki Anagnostou, Alexandros Varypatis, Christos Mourikis, Theofanis Tzatzakis, Dimitrios Draganidis, Dimitrios Batsilas, Theodoros Mersinias, Georgios Loules, Athanasios Poulios, Chariklia K Deli, Alexios Batrakoulis, Athanasios Chatzinikolaou, Magni Mohr, Athanasios Z Jamurtas, Ioannis G Fatouros 

Summary: The purpose was to examine the recovery kinetics of exercise-induced muscle damage (EIMD), neuromuscular fatigue, and performance following small-sided games (SSGs) of different densities in soccer. Ten male players randomly completed 3 trials: a control trial (no SSGs), 4v4 SSGs (62.5 m2/player), and 8v8 SSGs (284.4 m2/player). External and internal load were monitored using GPS technology, heart-rate monitors, and rating of perceived exertion. Delayed-onset muscle soreness (DOMS), creatine kinase (CK), isokinetic strength, countermovement jump (CMJ), and sprint were determined at baseline, as well as at 24, 48, and 72 hours post-SSGs. Neuromuscular fatigue was assessed at baseline and at 1, 2, and 3 hours post-SSGs. DOMS increased (P < .05) in 4v4 for 72 hours and in 8v8 for 24 hours with that of knee flexors being more pronounced than that of extensors. CK increased (P < .05) in 4v4 for 72 hours and in 8v8 for 24 hours. Neuromuscular fatigue increased (P < .05) in 4v4 for 2 hours and in 8v8 for 3 hours. Strength declined (P < .05) in 4v4 for 48 hours and in 8v8 for 72 hours. CMJ decreased (P < .05) in 4v4 for 24 hours and in 8v8 for 48 hours. Sprint decreased (P < .05) for 48 hours in 4v4 and for 72 hours in 8v8. SSGs are associated with a prolonged rise of EIMD and induce short-term neuromuscular fatigue and slow recovery kinetics of strength, jump, and sprinting performance. The time for complete recovery is longer for SSGs of lower density. 



#9 Reviewing the role of the environment in the talent development of a professional soccer club 

Reference: PLoS One. 2021 Feb 25;16(2):e0246823. doi: 10.1371/journal.pone.0246823. eCollection 2021. 

Authors: Vincent Gesbert, Fabienne Crettaz von Roten, Denis Hauw

Download link:

Summary: This two-part study examined the perceptions of talented Swiss soccer players about their talent development environment. The first study presented the translation and validation of the Talent Development Environment Questionnaire (TDEQ) into French using a recommended methodology for translating and culturally adapting questionnaires. Two hundred and three Swiss athletes (M = 16.99 years old) responded to the 25 items of the TDEQ-5. One item was excluded due to low factor loadings, and the descriptive statistics showed that the re-specified TDEQ-5 instrument had acceptable global model fit according to the thresholds in the literature (χ2 (df = 17) = 484.62, p<0.001, CFI = 0.91, TLI = 0.90, RMSEA = 0.07, SRMR = 0.06). This adaptation is thus valid for assessing the effectiveness of talent development processes. For the second study, a holistic design was used to examine the perceptions of a set of players embedded in a top-level Swiss soccer academy (i.e., 64 elite soccer players from 14 to 18 years old) by using the TDEQ-5. The results showed some relative strengths (i.e., F1-Long-Term Focus for the M15 and M16 age-groups) and weaknesses (i.e., F2-Alignment of Expectations for the M17 and M18 age -groups and F3-Communication for M17). They also highlighted that the talent pathways of these Swiss soccer players could not be summarized by a single type of transition toward a professional team. Rather, there were context-specific requirements, such as the critical period between the M15-M16 and M17-M18 age-groups, suggesting that when the players first entered their TDE they experienced a set of affordances to develop and flourish, which thereafter were perceived as less rich and/or abundant. These results offer a starting point for optimizing talent pathways. 



#10 Effects of Caffeine Supplementation on Physical Performance of Soccer Players: Systematic Review and Meta-Analysis 

Reference: Sports Health. 2021 Mar 5;1941738121998712. doi: 10.1177/1941738121998712. 

Authors: Raphael Einsfeld Simões Ferreira, Rafael Leite Pacheco, Carolina de Oliveira Cruz Latorraca, Rachel Riera, Ricardo Guilherme Eid, Ana Luiza Cabrera Martimbianco

Summary: Caffeine is 1 of the most popular supplements consumed by athletes, and the evidence for improving soccer performance remains limited.  The aim was to investigate and update the effects (benefits and harms) of caffeine to improve performance on soccer players. Electronic search in Medline (via PubMed), CENTRAL, Embase, SPORTDiscus, and LILACS, from inception to March 28, 2020 was performed. Randomized clinical trials (RCTs) assessing the effects of caffeine on the performance of soccer players. Data extraction was conducted independently by 2 authors using a piloted form. We assessed methodological quality (Cochrane risk-of-bias [RoB] table) and the certainty of the evidence (GRADE [Grading of Recommendations Assessment, Development and Evaluation] approach).  Sixteen RCTs were included. Overall methodological quality was classified as unclear to low risk of bias. When assessing aerobic endurance, meta-analyses did not demonstrate the differences between caffeine and placebo (mean difference [MD], 44.9 m; 95% confidence interval [CI], -77.7 to 167.6). Similarly, no difference was observed during time to fatigue test (MD, 169.8 seconds; 95% CI, -71.8 to 411.6). Considering anaerobic power, meta-analyses also did not find differences for vertical jump (MD, 1.01 cm; 95% CI, -0.68 to 2.69) and repeated sprint tests (MD, -0.02 seconds; 95% CI, -0.09 to 0.04), as well as reaction time agility test (MD, 0.02 seconds; 95% CI, -0.01 to 0.04) and rating of perceived exertion (MD, 0.16 points; 95% CI, -0.55 to 0.87). Regarding safety, a few minor adverse events were reported. Based on the GRADE approach, the certainty of this evidence was classified as very low to low. We found no significant improvement in soccer-related performance with caffeine compared with placebo or no intervention. However, caffeine appears to be safe. 



#11 Relationships Between Aerobic Performance, Hemoglobin Levels, and Training Load During Small-Sided Games: A Study in Professional Soccer Players 

Reference: Front Physiol. 2021 Feb 16;12:649870. doi: 10.3389/fphys.2021.649870. eCollection 2021. 

Authors: Saeid Younesi, Alireza Rabbani, Filipe Manuel Clemente, Rui Silva, Hugo Sarmento, António José Figueiredo

Download link:

Summary: The purposes of this study were (1) to analyze between-session variations of external and internal load measures during small-sided games (SSGs) and (2) to test the relationships between the maximum speed reached (VIFT) during the last stage of the 30-15 Intermittent Fitness Test, hemoglobin levels, and training load measures during SSG intervals among professional soccer players. Sixteen professional soccer players (mean ± SD; age 27.2 ± 3.4 years, height 174.2 ± 3.6 cm, body mass 69.1 ± 6.4 kg, and body fat 10.4 ± 4.1%) participated in this study. Hemoglobin and aerobic performance were first tested, and then a 3-week SSG program was applied using a 3 vs. 3 format. During those 3 weeks, internal and external load of entire sessions were also monitored for all training sessions. Trivial-to-small, standardized differences were observed between sessions for external and internal measures during SSGs. Total distance (TD) and mechanical work (MW) were the only variables that indicated small changes. Large-to-very-large relationships were found between VIFT and external loads: TD (r range: 0.69; 0.87), high-intensity running (HIR; r range: 0.66; 0.75), and MW (r range: 0.56; 0.68). Moderate-to-large negative relationships were found between hemoglobin levels and internal loads: Edwards' TRIMP (r range: -0.36; -0.63), %HRmax (r range: -0.50; -0.61), and red zone (r range: -0.50; -0.61). VIFT had unclear relationships with overall internal loads, while hemoglobin levels presented unclear relationships with overall external loads. In conclusion, no meaningful changes were found between sessions considering the format of play used. Additionally, the detected relationships indicate that VIFT and hemoglobin levels are good indicators of the performance capacity and physiological profile of players during SSGs. Also, the use of SSGs protocols as a monitoring complement of the 30-15IFT is suggested. 



#12 The effects of 14-week betaine supplementation on endocrine markers, body composition and anthropometrics in professional youth soccer players: a double blind, randomized, placebo-controlled trial 

Reference: J Int Soc Sports Nutr. 2021 Mar 4;18(1):20. doi: 10.1186/s12970-021-00417-5. 

Authors: Hadi Nobari, Mehdi Kargarfard, Vazgen Minasian, Jason M Cholewa, Jorge Pérez-Gómez

Download link:

Summary: Betaine supplementation may enhance body composition outcomes when supplemented chronically during an exercise program. The purpose of this study was to evaluate the effect of betaine supplementation on development-related hormones, body composition, and anthropometrics in professional youth soccer players during a competitive season. Twenty-nine players (age, 15.45 ± 0.25 years) were matched based upon position and then randomly assigned to a betaine group (2 g/day; n = 14, BG) or placebo group (PG, n = 15). All subjects participated in team practices, conditioning, and games. If a subject did not participate in a game, a conditioning protocol was used to ensure workload was standardized throughout the 14-week season. Growth hormone (GH), insulin-like growth factor-1 (IGF-1), testosterone, cortisol, height, weight, and body composition were assessed at pre-season (P1), mid-season (P2) and post-season (P3). Anthropometric variables were also measured following a one-year follow-up (F). Significant (p < 0.05) group x time interactions were found for testosterone and testosterone to cortisol ratio (T/C). Both variables were greater in BG at P2 and P3 compared to P1, however, the testosterone was less in the PG at P3 compared to P2. There was no significant group by time interactions for GH, IGF-1, lean body mass, or body fat. There was a significant (p < 0.05) group x time interaction in height and weight at F, with the greater increases in BG compared to PG. Betaine supplementation increased testosterone levels and T/C ratio in youth professional soccer players during a competitive season. Betaine supplementation had no negative effects on growth (height and weight) and may attenuate reductions in testosterone due to intense training during puberty. 



#13 External and internal loads during the competitive season in professional female soccer players according to their playing position: differences between training and competition 

Reference: Res Sports Med. 2021 Mar 4;1-13. doi:0.1080/15438627.2021.1895781. Online ahead of print. 

Authors: Blanca Romero Moraleda, Niels J Nedergaard, Esther Morencos, David Casamichana, Rodrigo Ramirez-Campillo, Jos Vanrenterghem

Summary: The aim of this study was to compare external (EL) and internal loads (IL) during training sessions compared to official matches between elite female soccer players according to their playing position. Training and match data were obtained during the 2017/18 season from eighteen players (age: 26.5±5.7 years; height: 164.4±5.3 cm; body mass: 58.56±5.58 kg) from a first Division Spanish team. The EL (total distance covered; high-speed running distance; number of accelerations and decelerations) was assessed with a Global Positioning System (GPS) and triaxial accelerometer. The IL was assessed with ratings of perceived exertion (RPE; and session-RPE). The EL and the IL from official matches were higher compared to training sessions (p<0.05; effect size [ES]:0.6-5.4). In matches, the EL was greater in Attackers (AT) and Central Midfielders (CM) versus Central Backs (p<0.05; ES:0.21-1.74). During training sessions, the EL was similar between playing positions (p>0.05; ES:0.03-0.87). The EL and the IL are greater in matches compared to training sessions, with greater match-related EL in AT and CM players. Current results may help practitioners to better understand and modulate training session's loads according to playing position, potentially contributing to their performance readiness and injury risk reduction. 



#14 A Sleep Analysis of Elite Female Soccer Players During a Competition Week 

Reference: Int J Sports Physiol Perform. 2021 Mar 3;1-7. doi: 10.1123/ijspp.2020-0706. 

Authors: Craig Thomas, Helen Jones, Craig Whitworth-Turner, Julien Louis 

Summary: The aim was to compare the sleep of female players from a professional soccer team to nonathlete controls across an in-season week and (2) to compare the sleep of core and fringe players from the same team on the night after a match to training nights. Using an observational design, 18 professional female soccer players and 18 female nonathlete controls were monitored for their sleep via wristwatch actigraphy across 1 week. Independent-sample t tests and Mann-Whitney U tests were performed to compare sleep between groups, while an analysis of variance compared sleep on training nights to the night after a match. Soccer players had significantly greater sleep duration than nonathlete controls (+38 min; P = .009; d: 0.92), which may have resulted from an earlier bedtime (-00:31 h:min; P = .047; d: 0.70). The soccer players also had less intraindividual variation in bedtime than nonathletes (-00:08 h:min; P = .023; r: .38). Despite this, sleep-onset latency was significantly longer among soccer players (+8 min; P = .032; d: 0.78). On the night after a match, sleep duration of core players was significantly lower than on training nights (-49 min; P = .010; d: 0.77). In fringe players, there was no significant difference between nights for any sleep characteristic. During the in-season period, sleep duration of professional female soccer players is greater than nonathlete controls. However, the night after a match challenges the sleep of players with more match involvement and warrants priority of sleep hygiene strategies. 



#15 Factors Influencing Creatine Kinase Response in Youth National Team Soccer Players 

Reference: Sports Health. 2021 Mar 4;1941738121999387. doi: 10.1177/1941738121999387.

Authors: Gabor Schuth, Gyorgy Szigeti, Gergely Dobreff, Peter Revisnyei, Alija Pasic, Laszlo Toka, Tim Gabbett, Gabor Pavlik

Summary: Previous studies have examined the relationship between external training load and creatine kinase (CK) response after soccer matches in adults. This study aimed to build training- and match-specific CK prediction models for elite youth national team soccer players. Training and match load will have different effects on the CK response of elite youth soccer players, and there will be position-specific differences in the most influential external and internal load parameters on the CK response. Forty-one U16-U17 youth national team soccer players were measured over an 18-month period. Training and match load were monitored with global positioning system devices. Individual CK values were measured from whole blood every morning in training camps. The dataset consisted of 1563 data points. Clustered prediction models were used to examine the relationship between external/internal load and consecutive CK changes. Clusters were built based on the playing position and activity type. The performance of the linear regression models was described by the R2 and the root-mean-square error (RMSE, U/L for CK values). The prediction models fitted similarly during games and training sessions (R2 = 0.38-0.88 vs 0.6-0.77), but there were large differences based on playing positions. In contrast, the accuracy of the models was better during training sessions (RMSE = 81-135 vs 79-209 U/L). Position-specific differences were also found in the external and internal load parameters, which best explained the CK changes. The relationship between external/internal load parameters and CK changes are position specific and might depend on the type of session (training or match). Morning CK values also contributed to the next day's CK values. The relationship between position-specific external/internal load and CK changes can be used to individualize postmatch recovery strategies and weekly training periodization with a view to optimize match performance. 


The Training Manager -