## 2 Materials and Methods
### Experimental Procedures  
The CMJ tests were performed in one day from 11 a.m. to 3 p.m. The air temperature in the laboratory was 24 \(\pm\) 1 degrees Celsius. The tests were preceded by a 10 min warm-up involving treadmill running as well as static and dynamic stretching exercises. After a few trials, each jumper performed three successful CMJs from an upright standing position, with hands on their hips and a flexion angle in both knee joints of approximately 90\({}^{\circ}\) for the lowest CoM position during the braking phase. Half-minute rest periods between these jumps were assumed. In turn, the rest periods between the warm-up and CMJ trials, as well as between the CMJ trials and three successful CMJs, were approximately one minute. Moreover, the students were instructed to jump as high as possible and land on both feet. All CMJs were performed on the force platform while being recorded with the use of a smartphone camera and mo-cap system. Before the tests, a special spherical marker (diameter of 20 mm) reflecting infrared rays was placed on the sacrum between the posterior superior iliac spines of each jumper [26]. Force platform. The stationary force platform 800 Hz (BP400600, AMTI, Watertown, MA, USA) was used to assess the CMJ height. The AMTI force platform was adopted as the gold standard for comparisons with other measurement tools. The vertical ground reaction force (VGRF) was collected using the BTS Smart Capture software (BTS Bioengineering, Milan, Italy). The VGRF data vs. time for the 63 trials were exported as files with the.xls extension. Then, in Microsoft Excel 2016 software, the values of displacement of the jumper's CoM (s\({}_{\mathrm{z}}\)) were calculated based on the numerical integration of the acceleration (a\({}_{\mathrm{z}}\)) and velocity data (v\({}_{\mathrm{z}}\)), according to the method of least trapezoids. The following formulas were used:  
\[\mathrm{a}_{\mathrm{z}}=\frac{1}{m}(\mathrm{R}_{\mathrm{z}}-\mathrm{g}), \tag{1}\]where m--body mass, R\({}_{x}\)--vertical ground reaction force, and g--gravity acceleration (9.81 m\(\cdot\)s\({}^{-2}\)),  
\[\mathrm{v}_{x}(\mathrm{t})=\mathrm{v}_{\mathrm{oz}}+\int\mathrm{a}_{x}(\mathrm{t })\mathrm{dt}, \tag{2}\]  
where v\({}_{x}\)(t)--vertical velocity vs. time, v\({}_{\mathrm{oz}}\)--initial vertical velocity, and a\({}_{x}\)(t)--vertical acceleration vs. time,  
\[\mathrm{s}_{x}(\mathrm{t})=\mathrm{s}_{\mathrm{oz}}+\int\mathrm{v}_{x}( \mathrm{t})\mathrm{dt}, \tag{3}\]  
where s\({}_{x}\)(t)--vertical displacement of the jumper's CoM vs. time, and s\({}_{\mathrm{oz}}\)--initial vertical displacement. For the s\({}_{x}\)(t) graph, the highest value of s\({}_{x}\) as the MH of the CMJ was determined. The iPhone 13 smartphone (Apple Inc., Cupertino, CA, USA) with iOS 17 was used for testing. All CMJs were recorded using the camera (frame rate of 240 Hz, HD 1080p) of the smartphone, which was placed on a stand to ensure its stabilization and thus reduce the recording error. The smartphone was located approximately 1.5 m behind the participant (in the frontal plane), at the height of approximately 0.1 m from the ground, according to the guidelines of [PERSON] et al. [27]. The estimation of the MH of the CMJ was performed in the My Jump 2 app (version 2.2.6), following its instructions [2]. My Jump 2-FT. The flight time (t\({}_{\mathrm{f}}\)) of the jumper, i.e., the time between point C and point D (Figure 1) when VGRF = 0, was analyzed. Using the My Jump 2 app, the t\({}_{\mathrm{f}}\) between the first frame of the recording when both feet of the jumper were lifted off the ground and the first frame of the recording when the first foot touched the ground during the landing was determined. The MH was calculated based on the t\({}_{\mathrm{f}}\) of the jumper according to [PERSON]'s formula [10]:  
\[\mathrm{MH}=1.22625\cdot\mathrm{t}_{\mathrm{f}}{}^{2} \tag{4}\]  
This method of assessing the MH using t\({}_{\mathrm{f}}\) was called My Jump 2-FT.  
Figure 1: The VGRF(t) and s\({}_{x}\)(t) graphs for the representative trial of CMJ.