## 2 Materials and Methods
### Static Balance Assessment  
From a biomechanical perspective, the two-legged quiet stance is treated as the most conventional condition of balance assessment in order to gather evidence about a subject's ability to regulate one's center-of-mass oscillations within the limits of the base of support [31; 32]. This test is typically accompanied by single-legged stance trials, for they are considered the basic mode of postural challenge with regards to manipulating the base of support [31]. This method of static balance assessment has been previously used either in children or adolescents [29; 33; 34] as well as in athletes [35; 36]. Therefore, the assessment of static balance took place before (PRE) and after the 6-week period (POST) of the training intervention during single-legged and two-legged quiet stance trials in a quiet and spacious room of the sport club's facilities. During assessment of the single-legged stance trials, the athletes were instructed to assume a straight body posture with their arms hanging relaxed by their sides, to flex their hip joint and their knee joint at 90 degrees and to stand as motionless as possible on the force plate with either left or right leg. For the two-legged trials, the athletes assumed the same posture as described above, with their feet hip width apart. During the trials with eyes open, the athletes were instructed to fix their gaze on an imaginary point on the wall 2-3 m in front of them, while keeping their heads in a neutral position parallel to ground level, while during the trials with eyes closed, care was taken so that one researcher was always situated behind the athletes for safety reasons. Two successful trials per stance and visual condition were performed in a randomized order of 30 s duration each with 15 s of rest across trials and 1 min between testing conditions. Static balance performance was determined by the recording of center-of-pressure (CoP) data with a vertical force plate (Wii, A/D converter, 24-bit resolution, 1.000 Hz, Biovision, Wehrheim, Germany). Offline, the data were filtered using a 2 nd bi-directional-order digital low-pass Butterworth filter with a 15 Hz cut-off frequency and analyzed with MATLAB custom-made scripts (R2012a, 64 Bit; Mathworks, Natick, MA, USA) from the 5 th to the 25 th second (\(\Delta\)t = 20 s) of each 30 s trial duration. Based on the CoP displacement, whose derived values represent the geometrical location of the vertical ground reaction force vector on the platform during quiet standing [37], the following parameters were determined: (a) CoP path length, defined as the sum of Euclidean distances between adjacent measurement points, and (b) CoP sway range, defined as the range (i.e., from minimum to maximum) of the CoP values in the anteroposterior and mediolateral directions. To assess performance in the two-legged quiet stance trials, the average values of the two trials were used, whereas in the single-legged trials, the average value of the mean left and right leg trials was used for analysis, since no statistically significant differences were observed between sides (paired \(t\) test: \(p\) > 0.05) in any parameter.