Normative Reference Centiles for Sprint Performance in High-Level Youth Soccer Players: The Need to Consider Biological Maturity.

PURPOSE: To compute reference centiles for 5- and 30-m sprint times relative to chronological and skeletal age in youth soccer players. Subsequently, to compare individual's sprint performance scores derived from the chronological and skeletal age reference centiles. METHODS: Sprint times were collected for a sample of male U11 to U19 soccer players (n = 1745 data points). Skeletal age data were available for a subsample (n = 776 data points). Reference centiles were fitted using generalized additive models for location, scale, and shape. Individual z scores relative to chronological and skeletal age reference centiles were computed and compared for each maturity group (late, on-time, early, and very early) using standardized mean differences (SMD). RESULTS: Reference centiles for chronological age increased more rapidly between 10.5 and 15.5 years, while reference centiles for skeletal age increased more rapidly between 13.0 and 16.5 years. Differences in chronological and skeletal z scores for very early (SMD: -0.73 to -0.43) and late (SMD: 0.58 to 1.29) maturing players were small to large, while differences for early (SMD: -0.30 to -0.19) and on-time (SMD: 0.16 to 0.28) were trivial to small. CONCLUSION: Reference centiles provide a valuable tool to assist the evaluation of sprint performance in relation to chronological and skeletal age for talent identification purposes in youth soccer players.

Physiological and Locomotor Profiling Enables to Differentiate Between Sprinters, 400-m Runners, and Middle-Distance Runners.

ABSTRACT: Thron, M, Woll, A, Doller, L, Quittmann, OJ, Härtel, S, Ruf, L, and Altmann, S. Physiological and locomotor profiling enables to differentiate between sprinters, 400-m runners, and middle-distance runners. J Strength Cond Res 38(8): 1419-1427, 2024-Different approaches exist for characterizing athletes, e.g., physiological and locomotor profiling. The aims of this study were to generate and compare physiological and locomotor profiles of male and female runners and to evaluate relationships between the different approaches. Thirty-four highly trained adolescent and young adult female and male athletes ( n = 11 sprinters; n = 11,400-m runners; n = 12 middle-distance runners) performed two 100-m sprints on a running track to determine maximal sprinting speed (MSS) and maximal lactate accumulation rate (cLa max ). A cardiopulmonary exercise test was performed on a treadmill to determine maximal aerobic speed (MAS) and maximal oxygen uptake (V̇ o2 max). Anaerobic speed reserve (ASR) was calculated as the difference between MSS and MAS. Group comparisons were conducted with a 2-way ANOVA (discipline × sex; p < 0.05) and Bonferroni post hoc tests and Cohen's d as effect size. Parameters were correlated by Pearson's correlation coefficients. Maximal aerobic speed and V̇ o2 max were higher in 400-m and middle-distance runners compared with sprinters ( p ≤ 0.02; -2.24 ≤ d ≤ -1.29). Maximal sprinting speed and cLa max were higher in sprinters and 400-m runners compared with middle-distance runners (0.03 ≤ p ≤ 0.28; 0.73 ≤ d ≤ 1.23). Anaerobic speed reserve was highest in sprinters and lowest in middle-distance runners ( p ≤ 0.03; 1.24 ≤ d ≤ 2.79). High correlations were found between ASR and MAS, MSS, and cLa max ( p < 0.01; -0.55 ≤ r ≤ 0.91) and between cLa max and MSS ( p < 0.01; r = 0.74). Our results indicate that athletes of different sprinting and running disciplines show differing physiological and locomotor profiles, and that the parameters of these approaches are related to each other. This can be of interest for assessing strengths and weaknesses (e.g., for talent identification) or training prescription in these disciplines.

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.

Poor Reliability of Measurement Instruments to Assess Acute Responses to Load in Soccer Players Irrespective of Biological Maturity Status.

PURPOSE: To assess the short-term reliability of measurement instruments to quantify the acute psychophysiological response to load in adolescent soccer players in relation to biological maturity. METHODS: Data were collected from 108 U12 to U17 soccer players on 2 consecutive weeks (pre, n = 32; at, n = 34; and post, n = 42 estimated peak height velocity). Measurements consisted of the Short Recovery and Stress Scale, a countermovement jump, assessment of leg stiffness, and a submaximal run to assess exercise heart rate and heart rate recovery. Test-retest reliability was assessed with the coefficient of variation (CV) and intraclass correlation coefficient (ICC). RESULTS: Items of the Short Recovery and Stress Scale showed poor reliability across maturity groups (CV = 7.0%-53.5%; ICC = .28 to .79). Only few countermovement jump variables (jump height, concentric impulse, and concentric velocity) possessed good reliability. For most variables of the countermovement jump, reliability was better for the post peak height velocity group followed by at-peak height velocity and prepeak height velocity. Very high levels of reliability across maturity groups were observed for exercise heart rate (CV < 1.8%; ICC > .94), while heart rate recovery was more variable (CV < 16.5%; ICC > .48). CONCLUSION: Results suggest that the majority of investigated variables have poor reliability, questioning their ability to detect small, yet meaningful changes in acute responses to load in adolescent soccer players.

Construct validity of age at predicted adult height and BAUS skeletal age to assess biological maturity in academy soccer.

BACKGROUND: The assessment of biological maturity status plays an important role in talent identification and development programs. AIM: To compare age at predicted adult height and BAUS skeletal age as indicators of biological maturity status in youth soccer players using a construct-validity approach. SUBJECTS AND METHODS: Participants were 114 players from the U12 to U17 age groups of a professional youth soccer academy. Maturity status was determined via percentage of predicted adult height based upon the Khamis-Roche method (somatic maturity) and assessed via the SonicBone BAUSTM system (skeletal maturity). Convergent and known-groups validity were evaluated between maturity assessment methods and by comparing maturity-related selection biases across age groups. RESULTS: Although maturity status indicators were largely interrelated (r = .94, 95%CL 0.91-0.96), concordance (κ = 0.31 to 0.39) and Spearman's rank-order correlations (ρ = 0.45-0.52) of classification methods were moderate. A selection bias towards early maturing players emerged in the U14 age group which remained relatively consistent through to the U17 age group. CONCLUSIONS: Results confirm the construct-validity of both methods to assess biological maturity status although further validation relative to established indicators of biological maturity is needed. Furthermore, caution is also warranted when interpreting maturity status classification methods interchangeably given the poor concordance between classification methods.

Reliability of the Load-Velocity Relationship and Validity of the PUSH to Measure Velocity in the Deadlift.

Chéry, C and Ruf, L. Reliability of the load-velocity relationship and validity of the PUSH to measure velocity in the deadlift. J Strength Cond Res 33(9): 2370-2380, 2019-This study investigated the reliability between load and mean velocity, mean propulsive velocity, peak velocity, mean power, and peak power in the deadlift. Also, we looked at the validity of an inertial sensor (PUSH) and a linear-position transducer (Tendo) to measure velocity variables. Ten strength-trained men (23.4 ± 1.3 years) were involved in three 1 repetition maximum (1RM) testing sessions, separated by at least 72 hours. The protocol used 6 different lifting intensities, comprising 20, 40, 60, 80, 90, and 100% of 1RM. Reliability of the load-velocity and load-power relationship was assessed by comparing velocity and power measurements from session 2 and 3 for each relative %1RM. The validity of tested devices was analyzed regarding to each relative intensity by comparing results from each instrument to a reference instrument (GymAware). The findings revealed that intermediate intensities (ranging from 60 to 90% of 1RM) seem to be reliable. Furthermore, extreme points of the load-velocity curve (20, 40, and 100% of 1RM) were found to be less reliable and should therefore be used with caution when implemented as part of autoregulating strategies. Tendo produced measurements that were highly correlated with GymAware and thus, constitutes a valid and cheaper alternative. By contrast, measurements from the PUSH presented a low level of precision and accuracy. Therefore, PUSH cannot be considered as a valid tool to measure velocity variables in the deadlift.

Validity and Reliability of the Load-Velocity Relationship to Predict the One-Repetition Maximum in Deadlift.

Ruf, L, Chéry, C, Taylor, KL. Validity and reliability of the load-velocity relationship to predict the 1RM in deadlift. J Strength Cond Res 32(3): 681-689, 2018-The aim of this study was to verify the reliability and validity of using submaximal loads from the load-velocity relationship to predict the actual 1 repetition maximum (1RM) in the deadlift. Data from 11 resistance-trained athletes were analyzed performing three 1RM assessments separated by at least 3 days. Reliability was assessed by comparing predicted 1RMs of sessions 2 and 3, whereas for validity purposes, predicted 1RMs of session 3 were compared with actual 1RMs of session 2. Mean concentric velocity at 1RM (v at 1RM) was entered in individualized linear regression equations, derived from the load-velocity relationship for 3 (20-60%, 40-80%, and 60-90% of 1RM), 4 (20-80% and 40-90% of 1RM), and 5 (20-90% of 1RM) incremental loads to predict 1RMs. There were trivial changes for all predicted 1RMs between sessions with 20-90% of 1RM being the most reliable model. Similarly, the actual 1RM was very stable (effect size [ES] = 0.04, 90% confidence limit [CL] [-0.03 to 0.12], typical error of measurement [TE] = 3.4 kg [2.5-5.4], intraclass coefficient [ICC] = 0.99 [0.96-0.996], and coefficient of variation [CV] = 1.9% [1.4-3.0]), whereas the v at 1RM was unreliable between trials (ES = -0.30, 90% CL [-0.78 to 0.17], TE = 0.029 m·s [0.022-0.047], ICC = 0.63 [0.19-0.86], and CV = 15.7% [11.7-26.1]). However, predicted 1RMs computed from all submaximal load ranges substantially overestimated the actual 1RM with considerable differences between athletes. Although 1RM predictions showed high reliability, they all overestimated the actual 1RM, which was stable between sessions. Therefore, it is not recommended to apply the prediction models used in this study to compute daily 1RMs.

Assessing anaerobic speed reserve: A systematic review on the validity and reliability of methods to determine maximal aerobic speed and maximal sprinting speed in running-based sports.

PURPOSE: Locomotor profiling using anaerobic speed reserve (ASR) enables insights into athletes' physiological and neuromuscular contributing factors and prescription of high-intensity training beyond maximal aerobic speed (MAS). This systematic review aimed to determine the validity and reliability of different methods to assess the characteristics of ASR, i.e., MAS and maximal sprinting speed (MSS). METHODS: A comprehensive search of the PubMed and Web of Science databases was conducted according to the PRISMA guidelines. Studies were included if they reported data on validity and/or reliability for methods to assess MAS or MSS. RESULTS: 58 studies were included with 28 studies referring to MAS and 30 studies to MSS. Regarding MAS, different methods for cardiopulmonary exercise testing yielded different values (four out of seven studies) of MAS (Cohen's d (ES) = 0.83-2.8; Pearson's r/intraclass correlation coefficient (r/ICC) = 0.46-0.85). Criterion validity of different field tests showed heterogeneous results (ES = 0-3.57; r/ICC = 0.40-0.96). Intraday and interday reliability was mostly acceptable for the investigated methods (ICC/r>0.76; CV<16.9%). Regarding MSS, radar and laser measurements (one out of one studies), timing gates (two out of two studies), and video analysis showed mostly good criterion validity (two out of two studies) (ES = 0.02-0.53; r/ICC = 0.93-0.98) and reliability (r/ICC>0.83; CV<2.43%). Criterion validity (ES = 0.02-7.11) and reliability (r/ICC = 0.14-0.97; CV = 0.7-9.77%) for global or local positioning systems (seven out of nine studies) and treadmill sprinting (one out of one studies) was not acceptable in most studies. CONCLUSION: The criterion validity of incremental field tests or shuttle runs to examine MAS cannot be confirmed. Results on time trials indicate that distances adapted to the participants' sporting background, fitness, or sex might be suitable to estimate MAS. Regarding MSS, only sprints with radar or laser measures, timing gates, or video analysis provide valid and reliable results for linear sprints of 20 to 70 m.

Concurrent validity of countermovement and squat jump height assessed with a contact mat and force platform in professional soccer players.

Purpose: The aim of this study was to assess the concurrent validity of a contact mat against force plates to measure jump height in countermovement jump (CMJ) and squat jump (SJ) in professional soccer players. Methods: 23 male professional soccer players performed the CMJ and SJ, which were concurrently recorded using a portable contact mat (SmartJump) and a portable dual force plate system (ForceDecks). Equivalence testing between both systems (contact mat vs. force plate) and the two methods (impulse-momentum vs. flight-time and flight-time vs. flight-time) was performed compared to equivalence bounds of ±1.1 cm for the CMJ and ±1.6 cm for the SJ. Additionally, 95% Limits of Agreement (LoA) and intraclass correlation coefficients (ICC) were computed. Results: Mean differences for the impulse-momentum vs. flight-time comparison for CMJ [3.2 cm, 95% CI (2.3-4.1)] and SJ [2.7 cm, (1.8-3.6)] were non-equivalent between both systems. LoA were larger than the equivalence bunds for CMJ and SJ, while ICCs were good [CMJ, 0.89, (0.76-0.95)] and excellent [SJ, 0.91, (0.79-0.96)]. As for the flight-time vs. flight-time comparison, mean differences were non-equivalent for the CMJ [1.0 cm (0.8 to 1.2 cm)] and equivalent for the SJ [0.9 cm (0.7-1.1 cm)]. LoA were narrower than the equivalence bounds for CMJ and SJ, while ICCs were excellent [CMJ, 0.995, 95% CI (0.989-0.998); SJ, 0.997, 95% CI (0.993-0.997)]. Conclusion: Our findings indicate that the SmartJump contact mat cannot be used interchangeably with the ForceDecks force platform to measure jump height for the CMJ and SJ.

Soccer coaches vs. sport science and medicine staff: who can more accurately predict the skeletal age of high-level youth soccer players?

Biological maturity is an important aspect in the context of talent identification and development processes within elite youth soccer players. The aim of this study was to investigate the accuracy of soccer coaches (SC) as well as sports science and medicine staff (SSMS) to predict the skeletal age of high-level youth soccer players. We also aimed to evaluate the inter-rater reliability of the skeletal age predictions among the SC and SSMS. Skeletal ages were collected for 89 male academy soccer players registered for the U12 to U16 age groups at a professional German Bundesliga club. In addition, 12 SC and five SSMS provided their skeletal age predictions for each player of their respective age group. Standardised mean differences and equivalence testing were performed between actual and predicted skeletal ages. Intra-class correlations (ICC) were calculated to assess the inter-rater reliability. For the SC, differences between predicted and actual skeletal ages were trivial and equivalent to zero for the U12, U14, and entire sample, while for the SSMS, standardised mean differences ranged from trivial to small for all age groups and the entire sample. ICC for skeletal age predictions for the entire sample was good among the SC and excellent among the SSMS, but was somewhat lower when age groups were analysed separately. While, on average, predictions were close to the actual skeletal age, SC were slightly more accurate than the SSMS. However, variability among the SSMS was large on an individual level.

Advancing and critical appraisal of an integrative load monitoring approach in microcycles in professional soccer.

Despite load monitoring being considered an integral part of targeted performance management, including injury and illness prevention, there is currently no consensus of an effective monitoring system in professional soccer. Thus, the aims were to apply an integrative load monitoring approach, previously established in rink-hockey, in professional soccer; extend this approach with further data (Short Recovery and Stress Scale); assess this (extended) approach, thereby further evaluating the relationship between the used external load (EL) measures (total distance, distance above 55% and 70% of individual maximal speed, number of accelerations and decelerations > 4 m/s2, total loading) and the internal load (IL) measure session rate of perceived exertion training load (sRPE-TL) as well as between the used EL measures and sRPE. This retrospective observational cohort study analyzed data from a Swiss team collected over a 14 week-period during the 2021/22 season. Based on our findings, the integrative approach tested proved to be an applicable load monitoring tool in professional soccer, placing players on a fitness-fatigue continuum throughout the different microcycle sessions without using tests, thus providing relevant information to individually tailor training programs. sRPE-TL (ρ [95% CI] = .55 [.51 to .59] to .87 [.85 to .88]; all p < .001) better reflected the EL experienced by players than sRPE (ρ [95% CI] = .45 [.40 to .50] to .71 [.69 to .75]; all p < .001) supporting the definition of sRPE-TL as a measure of IL. However, for even stronger relevance of the tested tool, further research is warranted, especially to ascertain its sensitivity and determine an optimal selection of EL and IL measures. In sum, the present data clearly demonstrate the importance of load management taking place at an individual level, even within team structures, thereby analyzing a set of both EL and IL measures.

Assessing the usefulness of submaximal exercise heart rates for monitoring cardiorespiratory fitness changes in elite youth soccer players.

AIM: This study aimed to assess the value of monitoring fitness in elite youth soccer players (U15 to U19 age groups) by analyzing the concomitant changes in heart rate at submaximal intensity (HR12km/h) and the velocity at a lactate concentration of 4 mmol/l (v4mmol/l). METHODS: Players were tested by means of an incremental treadmill test on two occasions during the summer pre-season in two consecutive seasons. Based on data from a total of 170 test comparisons from the U15 (n = 48 test comparisons), U16 (n = 40 test comparisons), U17 (n = 46 test comparisons), and U19 (n = 36 test comparisons) age groups, the agreement between substantial changes in HR12km/h and v4mmol/l was analyzed using the threshold combination of HR12km/h = 4.5% and v4mmol/l = 6.0%. RESULTS: Results revealed 2% full mismatches, 36% partial agreements, and 62% full agreements for the whole sample in terms of fitness change interpretation between both variables. The respective values for the U15 to U19 age groups ranged between 0% and 5% full mismatches, 28-44% partial agreements, and 56-68% full agreements with no meaningful differences between age groups. CONCLUSION: In conclusion, our findings confirm the practical value of using HR12km/h to monitor fitness changes in elite youth soccer players when lactate sampling during incremental tests is not possible.

Psychophysiological Responses to a Preseason Training Camp in High-Level Youth Soccer Players.

PURPOSE: This study aimed to examine the responsiveness of commonly used measurement instruments to a short training camp by examining the time course of psychophysiological responses in high-level youth soccer players. METHODS: Monitoring was carried out in 14 U15 male soccer players of 1 professional youth academy. Players provided data 3 days prior to (D - 3), during (D2-D4), and 1 (D + 1) and 4 days (D + 4) after the camp: 4 items for the Short Recovery and Stress Scale (SRSS), a countermovement jump (CMJ), and a submaximal run to assess exercise heart rate and heart-rate recovery. Training load during the camp followed an alternating low-high pattern, with lower training loads on D1 and D3 and higher training loads on D2 and D4. RESULTS: Changes in SRSS physical performance capability, emotional balance, overall recovery, muscular stress, and overall stress were small to moderate on D3 and moderate to large on D + 1, while changes were trivial on D + 4. Some CMJ parameters related to the eccentric phase were slightly improved on D3, and these parameters were slightly impaired on D4. Changes in CMJ parameters were trivial on D + 1 and D + 4. After a moderate decrease in exercise heart rate on D3, there was a small decrease on D + 4 and a moderate increase in heart-rate recovery. CONCLUSION: Measurement instruments such as the SRSS and submaximal runs can be used to monitor acute psychophysiological responses to load, while the CMJ may provide little insight during periods of intensified training load.

Sprint and jump performances of female athletes are enhanced in the evening but not associated with individual chronotype.

Sprint and jump performances represent performance-determining parameters in individual and team sports. Fluctuations in performance depending on the daytime raise the question of the best time to train and compete. Given the scarce research on females, this study aimed to analyze the influence of daytime on sprint and jump performances and to investigate whether the performance difference is related to the chronotype. Thus, 23 female sports students completed a questionnaire to assess their chronotype followed by performing two 30 m sprints, 5 Repeated Jump Tests (5RJT), and countermovement jumps (CMJ) in the morning (7:00-9:00 h) and evening (17:00-19:00 h). Time after 5 m, 10 m, and 30 m during the sprints, reactive strength index (RSI) during the 5RJTs, and jump height during the CMJs were examined. The performance during the 30 m sprint (t(22) = 5.28, p < 0.01 moderate effect size: 0.50) and the two jump tests (5RJT: t(22) = 8.27, p < 0.01 large effect size: 0.95; CMJ: t(22) = 5.46, p < 0.01 moderate effect size: 0.79) were significantly better in the evening than in the morning. There was no significant correlation between chronotype and the time-of-day effect. The results should be considered when planning training and competition.

Seasonal analysis of match load in professional soccer players: An observational cohort study of a Swiss U18, U21 and first team.

The aim of this study was to quantify and compare various external match load measures in three age groups and leagues in male soccer (U18 in highest league of their age group vs U21 in fourth highest league vs first team in highest league). In this retrospective observational cohort study accelerations, decelerations, absolute and relative high-speed running as well as sprint distance, dynamic stress load, explosive distance, high intensity bursts total distance, high metabolic load (HML) distance, speed intensity, total distance, total time, and total loading were assessed in 416 individual player matches of 59 players. All these external load measures showed large inter-individual variability. At a group level, one-way ANOVAs or Kruskal-Wallis tests revealed statistically significant differences between the three teams for all measures analyzed (all p < 0.05), except accelerations. The first team displayed statistically significant higher dynamic stress load, explosive distance, HML distance, speed intensity, total distance and total loading compared to the two youth teams (all p < 0.05). The U18 featured statistically significant higher number of decelerations, absolute and relative high-speed running distance, high metabolic load distance, speed intensity, relative sprint distance, total distance, and total time than the U21, while for U21 higher dynamic stress load was observed than for U18 (all p < 0.05). Based on our data we conclude a routinely monitoring of match loads of different age groups and competitive settings to be required to 1) provide an indication of what players need to be prepared for, 2) track the athletic and match evolution, and 3) individually tailor training programs allowing players to fulfill the short- and long-term sport-specific requirements.

Are Measurement Instruments Responsive to Assess Acute Responses to Load in High-Level Youth Soccer Players?

Purpose: The aim of this study was to assess the short-term responsiveness of measurement instruments aiming at quantifying the acute psycho-physiological response to load in high-level adolescent soccer players. Methods: Data were collected from 16 high-level male youth soccer players from the Under 15 age group. Players were assessed on two occasions during the week: after 2 days of load accumulation ("high load") and after at least 48 h of rest. Measurements consisted of the Short Recovery and Stress Scale (SRSS), a countermovement jump (CMJ) and a sub-maximal run to assess exercise heart-rate (HRex) and heart-rate recovery (HRR60s). Training load was quantified using total distance and high-speed running distance to express external and sRPE training load to express internal load. It was expected that good instruments can distinguish reliably between high load and rest. Results: Odd ratios (0.74-1.73) of rating one unit higher or lower were very low for athlete-reported ratings of stress and recovery of the SRSS. Standardized mean high load vs. rest differences for CMJ parameters were trivial to small (-0.31 to 0.34). The degree of evidence against the null hypothesis that changes are interchangeable ranged from p = 0.04 to p = 0.83. Moderate changes were observed for HRex (-0.62; 90% Cl -0.78 to -0.47; p = 3.24 × 10-9), while small changes were evident for HRR60s (0.45; 90% Cl 0.08-0.80; p = 0.04). Only small to moderate repeated-measures correlations were found between the accumulation of load and acute responses across all measurement instruments. The strongest relationships were observed between HRex and total distance (rm-r = -0.48; 90% Cl -0.76 to -0.25). Conclusion: Results suggest that most of the investigated measurement instruments to assess acute psycho-physiological responses in adolescent soccer players have limited short-term responsiveness. This questions their potential usefulness to detect meaningful changes and manage subsequent training load and program adequate recovery.

Shortcomings of applying data science to improve professional football performance: Takeaways from a pilot intervention study.

Positional tracking data allows football practitioners to derive features that describe patterns of player behavior and quantify performance. Existing research using tracking data has mostly focused on what occurred on the pitch, such as the determinants of effective passing. There have yet to be studies attempting to use findings from data science to improve performance. Therefore, 24 professional players (mean age = 21.6 years, SD = 5.7) were divided into a control team and an intervention team which competed against each other in a pre-test match. Metrics were gathered via notational analysis (number of passes, penalty box entries, shots on goal), and positional tracking data including pass length, pass velocity, defensive disruption (D-Def), and the number of outplayed opponents (NOO). D-Def and NOO were used to extract video clips from the pre-test that were shown to the intervention team as a teaching tool for 2 weeks prior to the post-test match. The results in the post-test showed no significant improvements from the pre-test between the Intervention Team and the Control Team for D-Def (F = 1.100, p = 0.308, η2 = 0.058) or NOO (F = 0.347, p = 0.563, η2 = 0.019). However, the Intervention Team made greater numerical increases for number of passes, penalty box entries, and shots on goal in the post-test match. Despite a positive tendency from the intervention, results indicate the transfer of knowledge from data science to performance was lacking. Future studies should aim to include coaches' input and use the metrics to design training exercises that encourage the desired behavior.

Injury incidence, severity, and burden in elite youth soccer players - A 3-year prospective study.

OBJECTIVES: This study aimed to investigate age-related injury incidence, severity, and burden over a 3-year period in U14 to U19 elite male soccer players in a German youth academy. DESIGN: Descriptive epidemiology study. METHODS: Time-loss injuries of 166 unique players were prospectively documented and injury incidence, severity, and burden were calculated for each age group, body part, and type of injuries. RESULTS: A total of 562 time-loss injuries were recorded. Across all age groups (U14 to U19), injury incidence was 42.2 per squad season (ranging from 30.5 in the U19 to 67.7 in the U15), severity was 11.0 days lost (ranging from 8.0 in the U15 to 17.0 in the U17), and burden was 464.1 days lost per squad season (ranging from 304.9 in the U19 to 756.7 in the U14). The thigh, knee and ankle were the body parts with the highest injury incidence (9.2; 5.9; 5.9), severity (11.0; 16.0; 13.0), and burden (101.6; 94.9; 76.1). Cruciate ligament injuries had both the highest burden (164.4) and severity (137.0), and muscle injuries the highest incidence (11.0) of all types of injuries. U15 had the highest injury incidence (67.7) of all age groups. U14 and U17 had both the highest injury severity (15.0; 17.0) and burden (756.7; 618.8) of all age groups. Age-related patterns were observed for selected types of injuries such as muscle injuries, tendinopathy, bone-stress injury, and fractures. CONCLUSIONS: Injury incidence, severity, and burden substantially differed descriptively between age groups and clear age-related patterns for selected types of injuries were evident.

Overestimation of maximal aerobic speed by the Université de Montréal track test and a 1500-m-time trial in soccer.

Introduction: Maximal aerobic speed (MAS), usually measured by cardiopulmonary exercise testing (CPET) on a treadmill, is gaining popularity in soccer to determine aerobic performance. Several field tests are used to estimate MAS, although, gold standard methods are still not clarified. Therefore, this work aims 1) to compare two different CPET based methods to assess MAS and 2) to investigate the convergent validity of two common field tests to estimate MAS in soccer. Methods: Thirteen trained male soccer players completed an CPET on a treadmill to determine two VO2-kinetic based definitions of MAS (MASPlateau = speed at onset of VO2-plateau = gold standard; MAS30s = first speed of 30-s-interval of VO2max), the Université de Montreal Track Test (UMTT; VUMTT = speed of the last stage), and a 1500-m-time trial (1500-m-TT; V1500m = average speed). MASPlateau, MAS30s, VUMTT, and V1500m were compared using ANOVA. Additionally, limits of agreement analysis (LoA), Pearson's r, and ICC were calculated between tests. Results: MAS30s, VUMTT, and V1500m significantly overestimated MASPlateau by 0.99 km/h (ES = 1.61; p < 0.01), 1.61 km/h (ES = 2.03; p < 0.01) and 1.68 km/h (ES = 1.77; p < 0.01), respectively, with large LoA (-0.21 ≤ LoA≤3.55), however with large-to-very large correlations (0.65 ≤ r ≤ 0.87; p ≤ 0.02; 0.51 ≤ ICC≤ 0.85; p ≤ 0.03). Discussion: The overestimation and large LoA of MASPlateau by all estimates indicate that 1) a uniform definition of MAS is needed and 2) the UMTT and a 1500-m-TT seem questionable for estimating MAS for trained soccer players on an individual basis, while regression equations might be suitable on a team level. The results of the present work contribute to the clarification of acquisition of MAS in soccer.