Experimental approach to the study
Participants completed 3 testing sessions, 7 days apart. During each session, we collected data from a cross-over hop test, 505 and the 90° COD test. Participants were familiar with all the tests as they were part of their regular fitness testing battery. Both testing sessions were completed at the same time of the day and during participants’ regular training times and always before training. This study was conducted in May of 2020/2021 season and consisted in a weekly resistance training session on day-4 (Wednesday), allowing a rest of 72 h prior to match and within the usual training hours (15:30–18:00 h). The assessments were carried under weather conditions (29 °C and 50% humidity). The type of grass was synthetic and of the 3rd generation basically characterized by being composed of the following elements: a support fabric and fibers woven to the support with a height between 35 and 70 mm and with a distance between them that allows the incorporation of a filling.
A total of 94 young highly trained male soccer players from high-performance academy were part of the present study. The soccer players were assigned into two categories: under 16 (U16) [(n = 27); age: 14.81 ± 0.40 years; height: 170.61 ± 5.61 cm; weight 64.74 ± 8.44 kg)] or under 19 (U18) [(n = 67); age: 16.64 ± 1.25 years; height: 173.73 ± 7.19 cm; weight 66.70 ± 7.95 kg)]. All participants were familiar with the evaluations carried out and they had experience in the different test carried out. These players trained 9 h of soccer training and played one competitive match per week.
Each soccer players and respective parents were informed about the objective of present study and giving the signed consent before the beginning of the study. The inclusion criteria were as follows: (1) being an active player with federation license with previous experience of ≥ 5 years (2) not presenting any injuries during the previous two month; (3) belong in the academy a full season; (4) participating in 3 tests proposed (100%) and (5) giving consent.
The participants were treated according to American Psychological Association (APA) guidelines, which ensured the anonymity of participants’ responses. In addition, the study was conducted in accordance with the ethical principles of Helsinki declaration16 for human research and was approved by the Research Ethics Committee of the Pontifical University of Comillas (internal project n°: 2021/65).
Prior to conducting any tests, participants conducted a standardized warm up consisting of low aerobic activity, dynamic stretching, progressive sprinting, and submaximal pre-planned changes of direction, lasting 10 min. Following the standardized warm up, participants received verbal instruction and demonstrations from the research team immediately prior to conducting 2 familiarization attempts for each test. Recovery intervals between attempts were standardized at three minutes for each test. For the selection of the dominant leg, the players were asked which leg they preferably use to control, pass and throw the ball regardless of playing position20. All the evaluations (see Fig. 1) were performed at the same time and space, with the usual clothing for the soccer player, the specific footwear and supervised by the same technical specialists.
Cross-over hop test
For the cross-over hop test (CHT), the participant performed three consecutive jumps over a 15 cm line that had been marked on the floor. The test consists of doing three jumps starting from a monopodal support and landing with the same leg as the impulse21. Subjects were instructed to place their hands on their hips and to maintain the landing position for 3 s (sec), without loss of balance or performing additional movements involving the free limb. The distance reached was measured in centimeters from the take-off line to the heel in the final position21. They made two attempts and the best was selected. See Fig. 2.
505 COD test
The methodology for the 505-COD test was as per originally established methods22, see Fig. 3. Therefore, this involved a 10 m linear sprint from a static start, a 180° turn using the right or left leg to stop the run (ensuring contact with a designated line), and a 5 m return sprint through an identified finish line. The time taken to complete the final 5 m of the 10-m linear sprint, turn, and 5 m return sprint was recorded20. For speed evaluation, 2 attempts were performed with a recovery time of 2 min between repetitions and an average of the two repetitions for subsequent analysis24,25. Times were measured in sec. The evaluation system was carried out through FitLight Trainer® sensors (Ontario, Canada). This system is used to calculate agility, reaction time, speed and coordination in football26. The optoelectronic devices were adjusted to an appropriate hip height as per the mean stature of the sample group. The recorded time for each of the players was stored in a portable tablet with an Android system and its subsequent analysis in the Microsoft Windows® Excel program (Redmond, Washington, USA).
90° COD test
Finally, they performed three 90° COD test (right/left) (10 m) with 90° (COD). See Fig. 4. The times of each repetition were taken in seconds. For data analysis, the average of the two attempts made in each series with 2 min of recovery among them was chosen24,25. For the evaluation, 2 Led FitLight Trainer® sensors were placed, one at the beginning and the other at the end of the route. Timing gates were adjusted to an appropriate hip height as per the mean stature of the sample group.
Data were analyzed using software IBM SPSS Statistics (version 26). For data processing, mean and standard deviation were used. The variability of the results in each test [(percentage of variability (CV)] was also computed having into account the totality of the test performed per subject for each test (two to the dominant side and two to the non-dominant side). To evaluate the asymmetries at the intragroup level in each of the COD’s tests, a t-test of paired samples was performed, comparing the time of the test depending on different criteria (to facilitate the understanding of the results).
In the test CHT the criterion was the leg used to perform the jumps (dominant or non-dominant).
In the test 505 COD test the criterion was the legs that performs the last support before changing direction (dominant or non-dominant).
In the 90° COD test the criteria was the turning direction (dominant or non-dominant).
To study the asymmetries between U16 and U18 in the CHT, the symmetry index (SI) in percentage was calculated27 using different formulas, based on the mentioned criteria’s: the formula (1) was used for the cross-over hop test (SI-CHT), the formula (2) was used for the 505 COD test (SI-505) and formula (3) was used for the 90° COD test (SI-90). Then a t-test between U16 and U18 players was performed for independent samples using the symmetry index as the dependent variable
$$SI-CHT \left(\%\right)=\frac(H_d-H_nd)0.5\times (H_d+H_nd)\times 100;$$
where Hd is the time of the COD test performing the hop with the dominant leg and Hnd the time performing the hop with the non-dominant leg
$$SI-505 \left(\%\right)=\frac(S_d-S_nd)0.5\times (S_d+S_nd)\times 100;$$
where Sd is the time of the COD test performing the stop with the dominant leg and Snd the time of the COD test performing the stop with the non-dominant leg
$$SI-90 \left(\%\right)=\frac(H_d-H_nd)0.5\times (H_d+H_nd)\times 100;$$
where Td is the time of the COD test performing the turn toward the dominant side and Tnd the time of the COD test performing the turn towards the non-dominant side.
Finally, an independent sample t-test was used to compare the variability (CV [%]) between the two age categories in each of the three tests performed. Significant p value was established at 0.01. The effect size was evaluated using the Cohen scale28: (1) 0–0.20, negligible effect; (2) 0.20–0.50, small effect; (3) 0.50–0.80, medium effect; (4) 0.80–1, large effect.
Ethics approval and consent to participate
Players and their parents or guardian were then invited to sign an informed consent document before any of the tests were performed. The study has conducted according to the Declaration of Helsinki, and the study was conducted in accordance with the ethical principles of the 1964 Helsinki declaration for human research and was approved by the Research Ethics Committee of the Pontifical University of Comillas (2021/65).
Consent for publication
Authors have a formal written consent of each participant, in order to publish data after the manuscript may be accepted.