Design and procedures
A prospective cohort study design was used in this study. An under-15 youth elite football team was analyzed throughout 33 weeks. Anthropometric and body composition measures were conducted. Also, the maturity status of each player was assessed using anthropometric data. Physical assessments were carried out in August 2021, and in April 2022. The participants were assessed during three days. Anthropometry, body compositions, flexibility and change of direction (COD) assessments were carried out in the first day. The assessment of anaerobic performance was made during the second day, using the Wingate test. In the third day of assessments, the 30-15 intermittent fitness test (30-15 IFT) was conducted to assess the participant’s aerobic performance. Only the COD and the 30-15 IFT assessments were conducted outdoors, on a synthetic turf soccer field.
Participants
Twenty-seven youth elite football players (age: 15.0 ± 0.4 years old; height: 175 ± 0.6 cm; body mass: 62.1 ± 7.0 kg) from the same team participated in this study. The inclusion criteria were: (i) all players had to participate in at least 90% of training sessions throughout the season; (ii) for each week, the players had to participate in all training sessions; and (iii) not be injured during the observations and assessments. The goalkeepers were excluded from the sample. Before the beginning of this study, all participants and their parents or their legal representants signed a written informed consent form. Thus, all the advantages and disadvantages of the study procedures were well explained to all the involved. The present study followed the ethical recommendations for the study in humans as suggested by the Declaration of Helsinki (updated version from 2013).
Internal intensity quantification
Internal intensity was collected using the rate of perceived exertion (RPE) based on the CR-10 Borg scale [27]. Thus, the RPE values were collected approximately 10–30 min after each training session, as recommended in previous research [28]. Based on the CR-10 scale, 1 means “very light activity” and 10 means “maximal exertion”. All players answered to the question “How intense was your session?”. Their responses were given in an individual way, and without the influence of their colleagues. Additionally, the duration of the training sessions, in minutes, was recorded. After obtaining each player RPE value, the session-rate of perceived exertion (s-RPE) was used [29]. To obtain the s-RPE values, the duration of each training session was recorded and multiplied by the RPE value attributed by each player, and was presented as an arbitrary unit (A.U.).
Moreover, from the s-RPE values, the weekly training intensity (wTI) (sum of the intensity of all sessions and match), mean training intensity (mTI) (mean of the intensity of all sessions and match), 5-day average (5d-AVG) (mean of the intensity of five training sessions without match), the training monotony (TM) (mean of training intensity of 7 days divided by the standard deviation), and the training strain (TS) (sum of the intensity of all sessions and match multiplied by training monotony per week) were calculated.
Physical Fitness Assessments
Anthropometry and body composition
Each participant height was measured using a stadiometer (Seca model 213, Germany) with an accuracy of ±5 mm. While, the participants weight was measured using a balance (Seca model 813, UK) with a precision of 0.1 per kilogram. All players were assessed without shoes and with their lower back as close to the stadiometer as possible. For measuring body composition, three-point skinfolds (chest, abdominal and thigh) were conducted to measure the the body fat percentage (BF%). All skinfold measures were assessed using a Lafayette caliper (Lafayette, IN, USA) with an accuracy of 0.1 mm. The skinfold measurements were applied twice on the right side of the athlete’s body, and the final score recorded were the mean of two measurements. If the measurement error was high (>5%), the measurements had to be performed again and the median of the three repetitions were used for analysis. All measurements were performed by an ISAK credited person. Thus, the calculations of body density and body fat% were calculated based on the Jackson and Pollock formula [30].
Sit and Reach test
For conducting the Sit and Reach test, all participants had to sit on the floor with their bare feet against the sit-and-reach equipment and with their middle fingers stacked on top of one another. Participants were informed to stretch as far as possible without bending their knees. The final outcome to be used was the distance between the tip of the middle fingers and the toe line, as previously recommended [31].
Modified 5-0-5 COD test
For measuring the participant's ability to change directions, the modified 505 test was used as in elsewhere [32]. Three cones were placed at 5-meters apart from each other, and a pair of photocells with a digital timer connected to it was placed at cone B. The photocell system used was the Newtest Power timer 300-series, that was adjusted to each player’s hip height. Each participant started the test 70 cm before the cone A (starting line). After a beep sound, each participant had to run as quickly as possible until reaching cone C, turn on the cone C line and return as quickly as possible through the photocells (cone B). Test time was measured to the nearest 0.01 s with the fastest value obtained from 2 maximal trials. After each trial, the players had a 3-minute recovery.
Wingate test
For measuring the anaerobic performance of each participant, the Wingate Anaerobic Test (WAnT) was performed on a cycle ergometer (Monark model 894-E, Vansbro, Sweden). During 5 seconds, the participants had to pedal at maximum speed to determine the repetition per minute (RPM) in the ergometer monitor. After that, a braking force was determined by the product of body mass in kg by 0.075. The participants had to pedal at their maximum effort during 30 seconds with verbal encouragement from the coach and/or colleagues. The peak power (PP) and fatigue index (FI) measures were used for further analysis [33].
30-15 Intermittent Fitness test
For measuring the aerobic performance, the 30-15 Intermittent Fitness test (30-15 IFT) was applied. The test initial velocity was set at 8 km.h−1 during the first run, and was increased by 0.5 km/h-1 after each running sequence. All participants had to run back and forth within a 40-meter straight line. Each shuttle consists of 30-second runs interspersed with 15-seconds of walking. 3-meter zones were delineated in both extremities and at the middle of the test setup. Each participant had to complete as many stages as possible, and the test ended when the players could not maintain the running speed demanded, or could not reach the 3-meter zone before the beep during three times. As a final outcome to be analyzed, the velocity of intermittent fitness test (VIFT) score of each participant was recorded. The VIFT consists of the final velocity recorded during the last stage [34, 35]. Also, the Vo2max was estimated by the following equation for each player [36]: Estimated Vo2max = 28.3 −(2.15 × 1) −(0.741 × age) −(0.0357 × mass) + (0.0586 × age × VIFT) + (1.03 × VIFT).
Maturity Status
Maturity Offset and Age at PHV
To determine the age at peak heigh velocity (PHV) of each player, the maturity offset was calculated using the chronological age, standing height, sitting height, leg length and body weight measures, according to the following equation [37]: Maturity Offset = −9.236 + 0.0002708 (leg length × sitting height) −0.001663 (chronological age × leg length) + 0.007216 (chronological age × sitting height) + 0.02292 (mass by height ratio). For measuring the sitting height, the athletes were asked to sit on the 50 cm height box, facing forwards. Then the height between the highest point of the head and the bottom of the box the player was sitting in, was measured. For measuring the leg length, the standing height minus the sitting height was calculated for each athlete. Finally, to obtain the age at PHV, the chronological age was subtracted by the maturity offset score of each player.
2D:4D Ratio
The digit two (2D) and digit four (4D) length fingers were measured [38]. Each player placed the right and left-hand palm on a scanner with the fingers kept 2 cm apart from each other. The image of player’s palms in scanner was transferred to a computer and the Kinovea software was used to analyse fingers’ length. The 2D and 4D fingers length were measured from the proximal phalanx to the distal phalanx. The ratio of both fingers was calculated as follows: 2D/4D. The model of the scanner was the Scanjet (5590 HP Scanjet, USA) with an accuracy of 0.01 cm measurement of second and fourth finger to the tip of the finger. The difference of right-fingers 2D:4D ratio (RF2D:4D) minus left-fingers 2D:4D ratio (LF2D:4D) was calculated [39]. The intra-observer reliability was assessed by the same observer two times a week apart. The intra-class correlation (ICC) for 2D:4D ratio was 0.93 and 0.95, respectively.
Statistical Analysis
Tests of normal distribution and homogeneity (Kolmogorov–Smirnov and Levene’s, respectively) were conducted on all data before analysis. A repeated-measures ANOVA was used to analyse TI measures from W1 to W38. Effect size was indicated with partial eta squared for Fs. A paired sample t-test was used to determine differences as a repeated measures analysis in two conditions (Pre – Post) for physical fitness variables. Cohen d was used as the effect size indicator. To interpret the magnitude of the effect size, we adopted the following criteria [40]: d = 0.20, small; d = 0.50, medium; and d = 0.80, large. Posteriorly, the percentage of change of internal training intensity measures were calculated [100-(Pre*100)/Post] considering Pre as the mean values from W1 to W4 and Post as the mean values from W35 to W38. In addition, percentage of changes were calculated for physical fitness assessments [100-(Pre*100)/Post].
Also, the Pearson’s correlation coefficient r was used to examine the relationship between the maturity status [i) maturity-offset and age at PHV; and ii) 2D:4D finger length], and training intensity and physical fitness variations. To interpret the magnitude of these correlations, the following criteria was adopted [41]: r ≤ 0.1, trivial; 0.1 < r ≤ 0.3, small; 0.3 < r ≤ 0.5, moderate; 0.5 < r ≤ 0.7, large; 0.7 < r ≤ 0.9, very large; and r > 0.9, almost perfect. A regression analysis was used to model the prediction of percentage of change of maturity status assessment from the remaining variables with significant correlations. All data were analysed using the software Statistica (version 13.1; Statsoft, Inc., Tulsa, OK, USA) and the significance level was set at p<0.05.