# Age-Related Changes in Landing Mechanics in Elite Male Youth Soccer Players: A Longitudinal Study
## 2 Materials and Methods  
### Participants  
The present mixed-longitudinal study involved a group of 89 male youth soccer players who were recruited from a professional soccer club in the Czech Republic. The players pertained to two chronological competition age groups at the beginning of the study: U14y (\(n=44\)) and U16y (\(n=45\)). The players played in the highest national soccer league in their respective age categories and had at least 6 years of organized training experience. The inclusion criterion was the absence of a severe thigh and knee injury during the period six months prior to the measurement. The reasons for exclusion were non-participation in training sessions as a result of injury exceeding 4 weeks during the 3-year period of observation, absence in the measurement session and, predominantly, a change in club or release from the club. This resulted in the final sample for data analysis consisting of 25 players, including 15 players from the U14 age group (training age 6-7 years) who were students of the sports elementary school, and 10 players from the U16 age group (training age 8-9 years) who were students of different types of secondary school. The description of the players is shown in Table 1.  
All of the participants trained five to seven times a week (total of 7-12 h) and usually played one competitive match in a week. Training typically consisted of age-related training aimed at youth athletes on an elite pathway: physical fitness (especially strength and power, speed and agility with and without a ball, repeated sprint ability with and without a ball) and skill-oriented training (technical-tactical training, game-like training,  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Year** & **U14 (\(n=15\))** & **U16 (\(n=10\))** & \(p\) \\ \hline Chronological age (years) & 1 st & 13.3 \(\pm\) 0.4 & 15.4 \(\pm\) 0.4 & 0.31 \\  & 2 nd & 14.3 \(\pm\) 0.4 & 16.4 \(\pm\) 0.4 & \\  & 3 rd & 15.2 \(\pm\) 0.3 & 17.4 \(\pm\) 0.3 & \\ Maturity offset (years) & 1 st & \(-\)0.3 \(\pm\) 0.7 & 1.8 \(\pm\) 0.5 & 0.023 \\  & 2 nd & 0.6 \(\pm\) 0.7 & 2.8 \(\pm\) 0.6 & \\  & 3 rd & 1.2 \(\pm\) 0.7 & 3.3 \(\pm\) 0.6 & \\ Stature (cm) & 1 st & 161.2 \(\pm\) 8.7 & 178.7 \(\pm\) 3.7 & 0.023 \\  & 2 nd & 168.9 \(\pm\) 9.4 & 181.9 \(\pm\) 4.5 & \\  & 3 rd & 171.9 \(\pm\) 10.2 & 182.5 \(\pm\) 4.2 & \\ Body mass (kg) & 1 st & 47.8 \(\pm\) 9.6 & 67.2 \(\pm\) 5.2 & 0.12 \\  & 2 nd & 57.8 \(\pm\) 10.5 & 72.5 \(\pm\) 4.6 & \\  & 3 rd & 63.7 \(\pm\) 12.0 & 75.7 \(\pm\) 4.3 & \\ \hline \hline \end{tabular} \(n\)—sample size; \(p\)—significance of the comparison between the U14 group in the 3 rd year and the U16 group in the 1 st year (Mann–Whitney U-test).  
\end{table}
Table 1: Anthropometric characteristics of players.  
recovery training). Neither age group in the current study was involved in systematic landing training, although pyometric training was included in the conditioning program of both groups.  
The study was approved by the Ethics Committee of the Faculty of Physical Culture, Palacky University (approval no. 14/2015) and conformed to the Declaration of Helsinki regarding the use of human subjects. All of the players were informed about the objective of the research and the applicable testing procedures. The players' parents and the children submitted a written informed consent expressing their agreement with the testing procedure and the use of the data for further research purposes. The testing was performed over three competitive seasons, 2016/17, 2017/18 and 2018/19, and approximately three weeks into each season (end of August), always in the club's sports hall during the training units of the observed groups. The day prior to the testing, the participants were not exposed to any high-intensity exercises.