Effects of Combined Training on Physical Performance in Young Soccer Players

Primary outcomes

1. Height

   Time frame: After 6 weeks of training intervention

   Height (in cm) was measured using a stadiometer (SECA, Hamburg, Germany). These measurement was taken in the morning, before breakfast, while the players wore light clothing and no shoes. Measurement was performed twice to reduce the margin of error.

2. Body weights

   Time frame: After 6 weeks of training intervention

   Body weights (in kg) were measured using a body mass measuring device (Fakir, Germany). These measurement was taken in the morning, before breakfast, while the players wore light clothing and no shoes. Measurement was performed twice to reduce the margin of error.

3. Body mass index

   Time frame: After 6 weeks of training intervention

   Body mass index (BMI) (kilograms/m2) was calculated from height and body weight values.

4. 20-m sprint

   Time frame: After 6 weeks of training intervention

   The sprinting performance was assessed using the 20-m linear sprint test. The test was conducted using two pairs of photocells (Smartspeed, Fusion Sport Pty Queensland, Australia). The starting point was placed 70 cm behind the first pair of photocells. First, a familiarization trial was conducted at low intensity. The test was then performed twice, with two-minute passive recovery intervals. The best result was used for analysis.

5. Agility

   Time frame: After 6 weeks of training intervention

   To assess agility performance, the Illinois Agility Test (IAT), a valid and reliable test for male team athletes, was used. Prior to starting the test, the player lies in a prone position, with the chin touching the surface of the starting line. After the researcher's command started, the player quickly stood up and ran in the indicated direction, avoiding contact with the placed cones. First, a familiarization trial was conducted at low intensity. Two trials were then performed, with two-minute passive recovery intervals. The best time was used for analysis.

6. Repeated sprint ability (RSA)

   Time frame: After 6 weeks of training intervention

   RSA was assessed using the 6x20-m RSA test. A 20-second rest interval was provided between sprints. A photocell system (Smartspeed, Fusion Sport Pty, Queensland, Australia) was placed at the start and at the 20-m line to measure sprint time. The players started the test from the line marked 70 cm behind the first pair of photocells. The mean sprint time (mean sprint time over the six sprints) (RSAmean) and the total sprint time (total time over the six sprints) (RSAtotal) were used for the analysis.

7. Vertical jump

   Time frame: After 6 weeks of training intervention

   Vertical jump performance was evaluated using the countermovement jump (CMJ) test. The test was performed using the My Jump Lab smartphone application (Spain, Madrid, version 1.2.2), which has proven validity and reliability for vertical jump measurement. Players were instructed to begin the jump with their feet shoulder-width apart, in a 90° knee flexion position, with their hands on their hips. During the test, players were instructed to jump as high as possible while keeping their hands on their hips. The mobile phone was positioned 3 meters away from the player, and the camera height was set to 0.90 meters using a tripod. The players first performed a familiarization trial at low intensity. Each player then performed three jumps, with two-minute passive recovery intervals. The highest jump was used for analysis

8. Aerobic fitness

   Time frame: After 6 weeks of training intervention

   Aerobic fitness was assessed using the Yo-Yo Intermittent Recovery Test Level 1 (YYIRTL-1). The test was conducted following previously described instructions (Bangsbo et al., 2008). The players' estimated maximum oxygen consumption (VO2max) after the test was calculated using the formula ''VO2max = 36.4 + (0.0084 x covered distance in YYIRTL-1)''.