Assessment of Maximum Oxygen Uptake in Elite Youth Soccer Players: A Comparative Analysis of Smartwatch Technology, Yoyo Intermittent Recovery Test 2, and Respiratory Gas Analysis.

The maximum oxygen uptake (VO2max) is a critical factor for endurance performance in soccer. Novel wearable technology may allow frequent assessment of V̇O2max during non-fatiguing warm-up runs of soccer players with minimal interference to soccer practice. The aim of this study was to assess the validity of VO2max provided by a consumer grade smartwatch (Garmin Forerunner 245, Garmin, Olathe, USA, Software:13.00) and the YoYo Intermittent Recovery Run 2 (YYIR2) by comparing it with respiratory gas analysis. 24 trained male youth soccer players performed different tests to assess VO2max: i) a treadmill test employing respiratory gas analysis, ii) YYIR2 and iii) during a non-fatiguing warm-up run of 10 min wearing a smartwatch as recommended by the device-manufacturer on 3 different days within 2 weeks. As the device-manufacturer indicates that validity of smartwatch-derived VO2max may differ with an increase in runs, 16 players performed a second run with the smartwatch to test this claim. The main evidence revealed that the smartwatch showed an ICC of 0.37 [95% CI: -0.25; 0.71] a mean absolute percentage error (MAPE) of 5.58% after one run, as well as an ICC of 0.54 [95% CI: -0.3; 8.4] and a MAPE of 1.06% after the second run with the smartwatch. The YYIR2 showed an ICC of 0.17 [95% CI: -5.7; 0.6]; and MAPE of 4.2%. When using the smartwatch for VO2max assessment in a non-fatiguing run as a warm-up, as suggested by the device manufacturer before soccer practice, the MAPE diminishes after two runs. Therefore, for more accurate VO2max assessment with the smartwatch, we recommend to perform at least two runs to reduce the MAPE and enhance the validity of the findings.

Comparison of Metabolic Power and Energy Cost of Submaximal and Sprint Running Efforts Using Different Methods in Elite Youth Soccer Players: A Novel Energetic Approach.

Sprinting is a decisive action in soccer that is considerably taxing from a neuromuscular and energetic perspective. This study compared different calculation methods for the metabolic power (MP) and energy cost (EC) of sprinting using global positioning system (GPS) metrics and electromyography (EMG), with the aim of identifying potential differences in performance markers. Sixteen elite U17 male soccer players (age: 16.4 ± 0.5 years; body mass: 64.6 ± 4.4 kg; and height: 177.4 ± 4.3 cm) participated in the study and completed four different submaximal constant running efforts followed by sprinting actions while using portable GPS-IMU units and surface EMG. GPS-derived MP was determined based on GPS velocity, and the EMG-MP and EC were calculated based on individual profiles plotting the MP of the GPS and all EMG signals acquired. The goodness of fit of the linear regressions was assessed by the coefficient of determination (R2), and a repeated measures ANOVA was used to detect changes. A linear trend was found in EMG activity during submaximal speed runs (R2 = 1), but when the sprint effort was considered, the trend became exponential (R2 = 0.89). The EMG/force ratio displayed two different trends: linear up to a 30 m sprint (R2 = 0.99) and polynomial up to a 50 m sprint (R2 = 0.96). Statistically significant differences between the GPS and EMG were observed for MP splits at 0-5 m, 5-10 m, 25-30 m, 30-35 m, and 35-40 m and for EC splits at 5-10 m, 25-30 m, 30-35 m, and 35-40 m (p ≤ 0.05). Therefore, the determination of the MP and EC based on GPS technology underestimated the neuromuscular and metabolic engagement during the sprinting efforts. Thus, the EMG-derived method seems to be more accurate for calculating the MP and EC in this type of action.

Energy expenditure and dietary intake in professional female football players in the Dutch Women's League: Implications for nutritional counselling.

In contrast to male football players, research on the nutritional requirements of female football players is limited. This study aimed to assess total daily energy expenditure (TDEE) in professional female football players, along with body composition, physical activity and dietary intake. This observational study included 15 professional football players playing in the highest Dutch Women's League. TDEE was assessed by doubly labelled water over 14 days, along with resting metabolic rate (RMR; ventilated hood), fat-free mass (FFM; dual-energy x-ray absorptiometry), and dietary intake (24-h recalls). Physical activity energy expenditure (PAEE) was derived from subtracting RMR and estimated diet-induced thermogenesis (10%) from TDEE. TDEE was 2882 ± 278 kcal/day (58 ± 5 kcal/kg FFM) and significantly (p < 0.05) correlated with FFM (r = 0.62). PAEE was 1207 ± 213 kcal/d. Weighted energy intake was 2344 kcal [2023-2589]. Carbohydrate intakes were 3.2 ± 0.7, 4.4 ± 1.1 and 5.3 ± 1.9 g/kg body mass for rest, training and match days, respectively, while weighted mean protein intake was 1.9 ± 0.4 g/kg body mass. In conclusion, the energy requirements of professional female football players are moderate to high and can be explained by the substantial PAEE. To fuel these requirements, sports nutritionists should consider shifting the players' focus towards prioritizing adequate carbohydrate intakes, rather than emphasizing high protein consumption.

The Sprint Mechanics Assessment Score: A Qualitative Screening Tool for the In-field Assessment of Sprint Running Mechanics.

BACKGROUND: Qualitative movement screening tools provide a practical method of assessing mechanical patterns associated with potential injury development. Biomechanics play a role in hamstring strain injury and are recommended as a consideration within injury screening and rehabilitation programs. However, no methods are available for the in-field assessment of sprint running mechanics associated with hamstring strain injuries. PURPOSE: To investigate the intra- and interrater reliability of a novel screening tool assessing in-field sprint running mechanics titled the Sprint Mechanics Assessment Score (S-MAS) and present normative S-MAS data to facilitate the interpretation of performance standards for future assessment uses. STUDY DESIGN: Cohort study (diagnosis); Level of evidence, 3. METHODS: Maximal sprint running trials (35 m) were recorded from 136 elite soccer players using a slow-motion camera. All videos were scored using the S-MAS by a single assessor. Videos from 36 players (18 men and 18 women) were rated by 2 independent assessors blinded to each other's results to establish interrater reliability. One assessor scored all videos in a randomized order 1 week later to establish intrarater reliability. Intraclass correlation coefficients (ICCs) based on single measures using a 2-way mixed-effects model, with absolute agreement with 95% CI and kappa coefficients with percentage agreements, were used to assess the reliability of the overall score and individual score items, respectively. T-scores were calculated from the means and standard deviations of the male and female groups to present normative data values. The Mann-Whitney U test and the Wilcoxon signed-rank test were used to assess between-sex differences and between-limb differences, respectively. RESULTS: The S-MAS showed good intrarater (ICC, 0.828 [95% CI, 0.688-0.908]) and interrater (ICC, 0.799 [95% CI, 0.642-0.892]) reliability, with a standard error of measurement of 1 point. Kappa coefficients for individual score items demonstrated moderate to substantial intra- and interrater agreement for most parameters, with percentage agreements ranging from 75% to 88.8% for intrarater and 66.6% to 88.8% for interrater reliability. No significant sex differences were observed for overall scores, with mean values of 4.2 and 3.8 for men and women, respectively (P = .27). CONCLUSION: The S-MAS is a new tool developed for assessing sprint running mechanics associated with lower limb injuries in male and female soccer players. The reliable and easy-to-use nature of the S-MAS means that this method can be integrated into practice, potentially aiding future injury screening and research looking to identify athletes who may demonstrate mechanical patterns potentially associated with hamstring strain injuries.

Exposure assessment for repeated sub-concussive head impacts in soccer: The HEalth and Ageing Data IN the Game of football (HEADING) study.

The purpose of this paper was to develop exposure estimates for repetitive sub-concussive head impacts (RSHI) for use in epidemiological analyses. We used a questionnaire to collect lifetime history of heading and other head contacts associated with training and playing football from 159 former footballers all members of the English professional football association. We used linear mixed effect regression with player as the random effect, to model the number of headers, blows to the head and head-to-head impacts as a function of potential exposure affecting factors, which were treated as the fixed effects. Exposure affecting factors included playing position, league, context of play (game vs training) and decade of play. Age at time of response to the questionnaire was also included in the models. In model results, playing position was important, with RSHIs being highest among defenders and lowest among goalkeepers. Players headed the ball more during games than in training, and when playing in amateur or youth leagues compared with semi-professional or professional leagues. The average number of reported head impacts declined linearly throughout the observation period (1949-2015). The derived final model for headers explained 43%, 9% and 36% of the between player, within player and total variance in exposure, respectively with good precision and predictive performance. These findings are generally in agreement with previously published results pointing towards the models forming a valid method for estimating exposure to RSHI among former footballers although some further external validation is still warranted.

Individualised reference ranges for markers of muscle recovery assessment in soccer.

Recently an individualisation algorithm has been developed and shown to significantly improve the diagnostic accuracy of creatine kinase (CK) and urea in endurance sports and Badminton. In this study, the applicability and benefit of this algorithm was evaluated using repeated measures data from 161 professional German soccer players monitored during the 2015-2017 seasons. Venous blood samples were collected after a day off (recovered state) and after a minimum of two strenuous training sessions within 48 h (non-recovered state) and analysed for CK and urea. Group-based reference ranges were derived from that same dataset to ensure the best possible reference for comparison. A z-test was conducted to analyse differences in error rates between individualised and group-based classifications. CK values for the individualised approach showed significantly lower error rates in the assessment of muscle recovery compared to both a population-based (p < .001; z-value: -17.01; test-pass error rate: 21 vs. 67%; test-fail: 19 vs. 64%) and a group-based cut-off (p < .001; z-value: -15.29; test-pass error rate: 65%; test-fail: 67%). It could be concluded that the assessment of muscle recovery in soccer using individualised interpretations of blood-borne markers may offer higher diagnostic accuracy than a population-based and a sample-specific group-based approach.HighlightsAssessing muscle recovery via CK using individualised ranges seems to offer a higher diagnostic accuracy than a sample-specific group-based analysis.Using an individualised algorithm seems to be a promising approach to overcome diagnostic problems arising from large inter- and intraindividual variability in blood parameters as it significantly improved the diagnostic accuracy of CK as a recovery marker.As recovery assessment in elite soccer ultimately aims at the accurate detection of differences in the individual player this algorithm seems to offer coaches and sport scientists a more sensitive approach compared to group-specific evaluations.

Extended Energy-Expenditure Model in Soccer: Evaluating Player Performance in the Context of the Game.

Every soccer game influences each player's performance differently. Many studies have tried to explain the influence of different parameters on the game; however, none went deeper into the core and examined it minute-by-minute. The goal of this study is to use data derived from GPS wearable devices to present a new framework for performance analysis. A player's energy expenditure is analyzed using data analytics and K-means clustering of low-, middle-, and high-intensity periods distributed in 1 min segments. Our framework exhibits a higher explanatory power compared to usual game metrics (e.g., high-speed running and sprinting), explaining 45.91% of the coefficient of variation vs. 21.32% for high-, 30.66% vs. 16.82% for middle-, and 24.41% vs. 19.12% for low-intensity periods. The proposed methods enable deeper game analysis, which can help strength and conditioning coaches and managers in gaining better insights into the players' responses to various game situations.

Anthropometric and Physiological Profiles of Hungarian Youth Male Soccer Players of Varying Ages and Playing Positions: A Multidimensional Assessment with a Critical Approach.

BACKGROUND: This study aimed to create preliminary anthropometric and physiological profiles of Hungarian male soccer players belonging to different age categories (14, 15, 16, and 17-18-years) and assigned to different playing positions (forward, defender, midfielder, goalkeeper). METHODS: Anthropometric and physiological profiles were created for four age groups: 14- (n = 20), 15- (n = 16), 16- (n = 22) and 17-18-year-olds (n = 23) representing the Hungarian soccer academy. Additionally, the variables were analyzed across the four player positions mentioned above. RESULTS: The mean values of body mass, fat mass and BMI were within normal limits, although in some cases the anthropometric and body composition values were too high, particularly among the 17-18-year-olds. The mean values of HRrest were lowest among the 15-year-olds. The highest mean and maximal values of rVO2max and rVO2/AT [57.6 ± 8.12 (43.8-68.3) and 51.2 ± 7.24 (38.9-60.8) mL/kg/min, respectively] were noted in 14-year-olds. Goalkeepers performed significantly better than the remaining soccer players in terms of the most anthropometric and physiological characteristics, except for the Yo-Yo test (p < 0.001). CONCLUSIONS: The values of anthropometric parameters increased with age. As expected, the oldest group achieved the best results in the performance tests. Goalkeepers outperformed the players representing other playing positions in the tests when assessing lower limb strength, sprint performance (5- and 10-m distance), and agility tests. From a practical point of view, the presented anthropometric and physiological profiles of players representing different age groups and playing positions can be useful for soccer coaches, strength and conditioning specialists, and athletic trainers of other soccer clubs in terms of the individualization and optimalization of soccer training.

Value of isokinetic strength testing for hamstring injury risk assessment: Should the 'strongest' mates stay ashore?

Although isokinetic strength testing is commonly used in hamstring strain injury (HSI) rehabilitation and prevention, research findings concerning its predictive value remain inconclusive. Existing research focuses on peak torque (PT) and angle of PT, not analysing the torque behaviour throughout the testing range of motion (ROM). This study intended to assess the value of isokinetic curve evaluation in association with HSI. A sample of 116 male football players with and without a recent HSI history was submitted to bilateral isokinetic assessment of the knee and hip muscles. Raw isokinetic data were filtered and normalized prior to curve analysis submission in MATLAB. Torque development of each muscle group throughout the entire testing ROM was assessed using HSI history as an independent variable. Curve analysis revealed significant differences in torque behaviour in function of injury history. Players with an HSI history demonstrated significantly stronger concentric knee flexion and extension, eccentric knee extension and concentric hip extension patterns compared to the controls and their uninjured limb. HSI history was also associated with lower concentric hip flexion torques and lower mixed H:Q ratios compared to the control group and their contralateral limb. HSI history was associated with altered knee and hip muscle strength profiles, potentially due to isolated focus on local strength training in rehabilitation or mechanisms of neuromuscular inhibition. Because the differences in torque amplitude were range-dependent and did not systematically concur with the point of PT achievement, isokinetic strength evaluation should most probably be conducted using curve analysis.

Assessment of muscle volume using magnetic resonance imaging (MRI) in football players after hamstring injuries.

Muscle injuries of the hamstrings are among the most frequent in football and a main cause for significant time away from training and competition. The purpose of this study was to prospectively evaluate the loss of muscle volume in recreational football players three and six weeks after initial trauma. We hypothesized that significant muscle volume loss occurs within 6 weeks after the initial injury event. Twenty recreational football players (mean-age=25 ± 4years; mean-height=181 ± 8cm; mean-weight=81 ± 10kg) with type3a (minor partial muscle tear) and type3b (moderate partial muscle tear) injuries were included. Muscle volume was determined using established methods for the hamstrings and the quadriceps femoris muscle within 3 days and after 3 and 6 weeks following the initial injury. The injured hamstrings lost 6.5% (mean=64 cm3(95%CI=31-98 cm3), p<0.001), the healthy hamstrings lost 2.1% (mean=21 cm3(3-44 cm3),p=0.096) of muscle volume after six weeks. The quadriceps in the injured leg lost 3.8% (mean=78 cm3(51-104 cm3), p<0.001) and 4.5% (83 cm3 (45-121 cm3), p<0.001) in the healthy leg. Muscle volume loss inversely correlated with activity levels in the healthy leg for the quadriceps (r=0.96 (0.90-0.98); R2=0.92; p<0.001) and the hamstrings (r=0.72 (0.40-0.88); R2=0.51; p<0.001), as well as the quadriceps in the injured leg (r=0.70 (0.37-0.87); R2=0.49; p<0.001), but not the injured hamstrings. Muscle volume ratio of hamstrings to quadriceps in the control limb was 0.52 ± 0.06 and 0.53 ± 0.06 in the injured leg. The rehabilitation period of six weeks did not have a relevant negative or a positive effect on ratios. Significant muscle volume loss in the upper thigh occurs in recreational soccer players within three, and within six weeks after a hamstring injury and lies between 2% and 7%.

Fitness assessment in talented football referees: an academy based longitudinal field-study.

BACKGROUND: This study aim was to profile physical fitness in talented football referees (FR) with performance relevant field-tests across time. METHODS: Thirty-eight male FR (age 28±1.5 years, height 178±5.1 cm, body mass 69.0±7.34 kg, body fat 17.2±2.87%) were observed for 15 months. Endurance was assessed with the yo-yo intermittent recovery test level 1 (YYIR1) and repeated sprint ability (RSA) with 5×30 m line-sprints with 30 s recovery (5×30 m). Long sprint endurance (LSA) and change of direction ability (COD) evaluated with novel field-tests. FR was tested six times (every three months) during the study. RESULTS: YYIR1 performance showed large increments across testing occasions. Sprint time in the COD largely and significantly decreased across the testing occasions. Very large associations were reported between 5×30 m and LSA tests grand mean (r=0.89, 0.78-0.94, P<0.0001). A nearly perfect (r=0.97, 0.94-0.99, P<0.0001) association was observed between 5×30 m best sprint and 5×30 m grand means. CONCLUSIONS: The results of this study revealed ability-related variations in performance across time. The effect of training and competitions on the determinism of physical fitness in refereeing seem plausible. Interestingly, sprint endurance tests shared a very large variance proposing tests interchangeability and mutual physiological demands. This study information provides useful information for the development of sound field-tests batteries in talented FR.

Modulators of Change-of-Direction Economy After Repeated Sprints in Elite Soccer Players.

PURPOSE: To investigate the acute effect of repeated-sprint activity (RSA) on change-of-direction economy (assessed using shuttle running economy [SRE]) in soccer players and explore neuromuscular and cardiorespiratory characteristics that may modulate this effect. METHODS: Eleven young elite male soccer players (18.5 [1.4] y old) were tested on 2 different days during a 2-week period in their preseason. On day 1, lower-body stiffness, power and force were assessed via countermovement jumps, followed by an incremental treadmill test to exhaustion to measure maximal aerobic capacity. On day 2, 2 SRE tests were performed before and after a repeated-sprint protocol with heart rate, minute ventilation, and blood lactate measured. RESULTS: Pooled group analysis indicated no significant changes for SRE following RSA due to variability in individual responses, with a potentiation or impairment effect of up to 4.5% evident across soccer players. The SRE responses to RSA were significantly and largely correlated to players' lower-body stiffness (r = .670; P = .024), and moderately (but not significantly) correlated to players' force production (r = -.455; P = .237) and blood lactate after RSA (r = .327; P = .326). CONCLUSIONS: In summary, SRE response to RSA in elite male soccer players appears to be highly individual. Higher lower-body stiffness appears as a relevant physical contributor to preserve or improve SRE following RSA.

Motor performance is not related to injury risk in growing elite-level male youth football players. A causal inference approach to injury risk assessment.

OBJECTIVE: To identify the causal relation between growth velocity and injury in elite-level youth football players, and to assess the mediating effects of motor performance in this causal pathway. DESIGN: Prospective cohort study. METHODS: We measured the body height of 378 male elite-level football players of the U13 to U15 age categories three to four months before and at the start of the competitive season. At the start of the season, players also performed a motor performance test battery, including motor coordination (Körperkoordinationstest für Kinder), muscular performance (standing broad jump, counter movement jump), flexibility (sit and reach), and endurance measures (YoYo intermittent recovery test). Injuries were continuously registered by the academies' medical staff during the first two months of the season. Based on the causal directed acyclic graph (DAG) that identified our assumptions about causal relations between growth velocity (standardized to cm/y), injuries, and motor performance, the causal effect of growth velocity on injury was obtained by conditioning on maturity offset. We determined the natural indirect effects of growth velocity on injury mediated through motor performance. RESULTS: In total, 105 players sustained an injury. Odds ratios (OR) showed a 15% increase in injury risk per centimetre/year of growth velocity (1.15, 95%CI: 1.05-1.26). There was no causal effect of growth on injury through the motor performance mediated pathways (all ORs were close to 1.0 with narrow 95%CIs). CONCLUSIONS: Growth velocity is causally related to injury risk in elite-level youth football players, but motor performance does not mediate this relation.

The Accuracy of a Low-Cost GPS System during Football-Specific Movements.

An affordable player monitoring solution could make the evaluation of external loading more accessible across multiple levels of football (soccer). The present study aimed to determine the accuracy of a newly designed and low-cost Global Positioning System (GPS) whilst performing match-specific movement patterns. Sixteen professional male football players (24 ± 3 years) were assigned a GPS device (TT01, Tracktics GmbH, Hofheim, Germany) and completed two experimental trials. In each trial, a continuous protocol including seven movements (sideways cornering, diagonal cornering, accelerating, decelerating, backwards jogging, shuttle running, and skipping) adding up to 500 m, was completed. Time-motion data was compared with criterion distance and velocity (photo-cell timing gates and radar). Validity was assessed through the standard error of the estimate (SEE) and reliability through the coefficient of variation (CV; both with 95% confidence limits). For the total distance covered during the protocol, the system was found to be valid (SEE = 3.1% [2.2; 5.8]) and reliable (intra-device CV = 2.0% [1.2; 7.6]). Similar results were found for velocity (SEE = 3.4% [2.6; 4.8], CV = 4.7% [3.2; 8.5]). In conclusion, the present GPS system, a low-cost solution, was found to be a valid and reliable tool for measuring physical loading during football-specific movements.

Reliability of Change-of-Direction Economy in Soccer Players.

PURPOSE: To evaluate the reliability of new change-of-direction-economy tests (assessing energetic efficiency when performing continuous shuttle runs) compared with common running-economy tests in soccer players Methods: Sixteen subelite, male soccer players were recruited to perform a testing battery involving running economy (RE), 10-m shuttle-running economy (SRE10), and 20-m shuttle-running economy (SRE20) at 8.4 km·h-1 mean speed on 2 different days within 48 hours. SRE10 and SRE20 consisted of continuous shuttle runs interspersed with 180° directional changes. During the RE, SRE20, and SRE10 tests, respiratory exchange ratio and oxygen uptake were collected and used to calculate the movement-economy values over any running condition as oxygen cost and energetic cost. The secondary variables (carbon dioxide production, heart rate, minute ventilation, and blood lactate) were also monitored during all tests. RESULTS: Depending on expression (oxygen cost or energetic cost), reliability was established for RE (CV: 5.5%-5.8%; ICC = .77-.88), SRE10 (CV: 3.5%-3.8%; ICC = .78-.96), and SRE20 (CV: 3.5%-3.8%; ICC = .66-.94). All secondary physiological variables reported good reliability (CV < 10%), except for blood lactate (CV < 35.8). The RE, SRE10, and SRE20 tests show good reliability in soccer players, whereas blood lactate has the highest variability among physiological variables during the economy tests. CONCLUSION: The assessment of change-of-direction economy through performing 20- and 10-m shuttle runs is reliable and can be applied to evaluate soccer players' energetic movement efficiency under more soccer-specific running conditions.

Assessment of energy availability and associated risk factors in professional female soccer players.

This study aimed to assess energy availability (EA), alongside possible risk factors of reduced or low EA of professional female soccer players during a competitive season. Thirteen players (age: 23.7 ± 3.4 y, stature: 1.69 ± 0.08 m, body mass: 63.7 ± 7.0 kg) engaged in a 5-day (two rest days, one light training, heavy training and match day) monitoring period. Energy intake (EI) and expenditure during exercise (EEE) were measured. EA was calculated and categorised as optimal, reduced or low (>45, 30-45, <30 kcal·kg FFM-1·day-1, respectively). Relationships between EA and bone mineral density, resting metabolic rate (RMR), plasma micronutrient status, biochemical markers and survey data were assessed. EA was optimal for 15%, reduced for 62% and low for 23% of players. Higher EA was observed on rest days compared to others (P<0.05). EA was higher for the light compared to the heavy training day (P<0.001). EEE differed significantly between days (P<0.05). EI (2124 ± 444 kcal), carbohydrate (3.31 ± 0.64 g·kg·day-1) and protein (1.83 ± 0.41 g·kg·day-1) intake remained similar (P>0.05). Survey data revealed 23% scored ≥8 on the Low Energy Availability in Females Questionnaire and met criteria for low RMR (ratio <0.90). Relationships between EA and risk factors were inconclusive. Most players displayed reduced EA and did not alter EI or carbohydrate intake according to training or match demands. Although cases of low EA were identified, further work is needed to investigate possible long-term effects and risk factors of low and reduced EA separately to inform player recommendations.

A multidisciplinary investigation into "playing-up" in academy football according to age phase.

In an attempt to facilitate more appropriate levels of challenge, a common practice in academy football is to play-up talented youth players with chronologically older peers. However, the context of playing-up in academy football is yet to be empirically explored. Thus, the purpose of this study was to examine the multidimensional factors that differentiated players who play-up from those who do not. Ninety-eight participants from a single football academy were examined within their age phase: Foundation Development Phase (FDP; under-9 to under-11; n = 40) and Youth Development Phase (YDP; under-12 to under-16; n = 58). Drawing upon the FA Four Corner Model, 27 factors relating to Technical/Tactical, Physical, Psychological, and Social development were assessed. Following MANOVA analysis within both the FDP and YDP, significant differences were observed for Technical/Tactical and Social sub-components (P < 0.05). Further differences were observed for Physical and Psychological sub-components (P < 0.05) within the YDP. In sum, Technical/Tactical and Social characteristics appeared to differentiate those who play-up compared to those who do not within the FDP. In the YDP however, there were measures representing all sub-components from the FA Four Corner Model. Subsequently, it is suggested coaches and practitioners consider these holistic factors when playing-up youth football players within relevant age-phases.

Intra- and interrater reliability of subjective assessment of the drop vertical jump and tuck jump in youth athletes.

OBJECTIVES: To investigate intra- and interrater reliability of the subjective assessments of filmed DVJ and TJA in youth male and female soccer players and to compare subjective assessment of the DVJ with two-dimensional movement analysis. DESIGN: Cross-sectional study. PARTICIPANTS: 115 soccer players (66 boys, 49 girls) mean age 14 ± 1 (range 13-16) years. MAIN OUTCOME MEASURES: Knee control during landing phase of DVJ was assessed using a 3-graded scoring scale. TJA was assessed according to ten criteria using a dichotomous grading scale. Subjective assessment of DVJ was compared with calculation of normalized knee separation distance (NKSD). RESULTS: Intrarater reliability for DVJ was substantial to almost perfect (kappa 0.72 rater 1; 0.85 rater 2). Interrater reliability was substantial to almost perfect (kappa from 0.68 to 0.83). The TJA total score intrarater reliability was ICC 0.59 for rater 1 and 0.90 for rater 2. Interrater reliability ranged from ICC 0.51 to 0.60. There were between-group differences in mean NKSD during DVJ for players rated as 0, 1 and 2, but within-group variability was large. CONCLUSION: Assessment of DVJ and TJA in youth athletes was rater dependent. Players with subjectively assessed reduced or poor knee control had smaller normalized knee separation distance in DVJ.

High-intensity Interval Training Shock Microcycle Improves Running Performance but not Economy in Female Soccer Players.

This study aimed to evaluate the effect of high-intensity interval training shock microcycles (HIITSM) on endurance, running economy and change of direction economy in female soccer players. Nineteen sub-elite female soccer players were randomised to two groups: HIITSM (10 HIIT sessions over 13 days) or HIITTRAD (4 HIIT sessions over 13 days) interventions. Endurance performance was evaluated through the 30-15 intermittent fitness test (30-15IFT); running economy over a 5-min treadmill run; and change of direction economy over two conditions: (1) 5-min 20m shuttle run, and (2) 5-min 10m shuttle run. HIITSM significantly improved 30-15IFT scores compared to baseline (+4.4%, p=0.009; d=0.96) and 30-15IFT scores relative to HIITTRAD (p=0.002; d=2.01). There was no significant interaction (group×time) for running economy and change of direction economy. Pre- to post- intervention there was a significant main time effect for blood lactate over 20m and 10m shuttle runs (p<0.001 and p=0.037, respectively), with large (d=0.93) and moderate (d=0.53) changes observed for the HIITSM over the two distances, respectively. HIITSM may be more effective than HIITTRAD to improve 30-15IFT over shorter training periods but may not affect running economy and change of direction economy.

Indirect Assessment of Skeletal Muscle Glycogen Content in Professional Soccer Players before and after a Match through a Non-Invasive Ultrasound Technology.

Skeletal muscle glycogen (SMG) stores in highly glycolytic activities regulate muscle contraction by controlling calcium release and uptake from sarcoplasmic reticulum, which could affect muscle contraction. Historically, the assessment of SMG was performed through invasive and non-practical muscle biopsies. In this study we have utilized a novel methodology to assess SMG through a non-invasive high-frequency ultrasound. Nine MLS professional soccer players (180.4 ± 5.9 cm; 72.4 ± 9.3 kg; 10.4% ± 0.7% body fat) participated. All followed the nutritional protocol 24 h before the official match as well as performing the same practice program the entire week leading to the match. The SMG decreased from 80 ± 8.6 to 63.9 ± 10.2; p = 0.005 on MuscleSound® score (0-100) representing a 20% ± 10.4% decrease in muscle glycogen after match. Inter-individual differences in both starting glycogen content (65-90) and in percentage decrease in glycogen after the match (between 6.2% and 44.5%). Some players may not start the match with adequate SMG while others' SMG decreased significantly throughout the game. Adequate pre-match SMG should be achieved during half-time and game-play in order to mitigate the decrease in glycogen. Further and more ample studies are needed before the application of this technology.