## 2 Materials and Methods
### Study Design and Intervention Protocol  
A one-group repeated-measures design was applied with pre-training (PRE) and post-training (POST) measurements. Soccer's demanding nature with regards to physical and neuromuscular requirements necessitates multimodal training approaches. To that aim, based on previous work that implemented solely glymetric [9; 10; 11; 12] or balance training [16; 18; 21] or a combination of both [22; 23; 27], we designed a 6-week training intervention (2 sessions/week, 20-25 min/session), comprising static balance tasks, dynamic stabilization tasks and multidirectional glymetric exercises. The intervention was divided into two phases of 3 weeks duration each, which were characterized by a progressive increase in task/exercise difficulty with regards to the base of support (two-legged vs. single-legged base of support), visual information (eyes open vs. eyes closed), movement direction and execution velocity [28; 29]. Table 1 presents, in detail, the implemented training intervention. Prior to the main part of their afternoon typical soccer training, the athletes followed a standardized warm-up of 5-7 min in duration, involving jogging and 2-3 running drilis at light intensity followed by dynamic stretching of the major muscle groups of the upper and lower body, and then carried on with the training intervention. Caution was taken so that a 48 h break period apart from training sessions occurred to avoid fatigue and to provide full recovery. Athletes participated in two familiarizations sessions before the start of the training intervention. In one of those sessions, anthropometric measurements of body mass, height, sitting height and left and right lower limb length (measured distance from the top iliac crest to medial malleolus in the supine position) were carried out. Biological maturation was assessed based on the equations, as previously proposed [30]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Phase A**} & \multirow{2}{*}{**Balance Tasks**} & **Set \(\times\) Exercise** & **Dynamic Stabilization** & **Set \(\times\) Exercise** & **Multidirectional** & **Set \(\times\) Exercise** \\  & & **Duration/Rest** & **Tasks** & **Duration/Rest** & **Exercises** & **Duration/Rest** \\ \hline \multirow{4}{*}{\begin{tabular}{c} Two-legged stance-EC * \\ \end{tabular} } & \multirow{4}{*}{\(1\times 30\) s/ 15 s} & Forward & \multirow{4}{*}{\
\begin{tabular}{c} Standing \\ Walking Lunge - \\ Execution rate: \\ Slow \\ Distance: short \\ \end{tabular} } & \multirow{4}{*}{\(1\times 15\) s per leg/ } & Standing & \multirow{4}{*}{\(1\times 30\) s/ 15 s} \\  & & & Walking Lunge - & & vertical jump \\ \cline{1-1}  & & 15 s & & with alternating & 15 s \\ \cline{1-1}  & & & Slove: short & & landing \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Single-legged stance-EC * \\ \end{tabular} } & \multirow{4}{*}{\(1\times 30\) s/ 15 s} & Diagonal & \multirow{4}{*}{\(1\times 15\) s per leg/ } & \multirow{4}{*}{Side jumps} & \multirow{4}{*}{\(1\times 30\) s/ 15 s} \\  & & Forward Lunge & & & \\ \cline{1-1}  & & – & & & \\ \cline{1-1}  & & – & & & \\ \cline{1-1}  & & 15 s & & & \\ \cline{1-1}  & & Distance: short & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Combined balance and multidirectional glymetric training intervention of the study.