## 4 Discussion  
The present study evaluated the validity and reliability of the My Jump 2 app for MH estimation during the CMJ in recreationally active students. The MH data obtained from the s\({}_{\text{z}}\)(t) graphs after measuring the VGRF using the AMTI force platform (gold standard) were used for comparisons. Using the iPhone 13 smartphone with the My Jump 2 app, the MH was calculated based on the flight time (VGRF = 0) and the displacement time of the reflective marker placed on the jumper's sacrum according to the new concept. Moreover, the MH values were estimated based on the displacement of this marker using the BTS mo-cap system. The data analysis revealed poor agreement (ICC = 0.362) and a large systematic bias in the MH difference (12.4%) between the My Jump 2-FT and force platform, as well as a significantly lower MH (by 27.7%) for the My Jump 2-FT compared to the force platform. This significant difference results from the use of different methods of MH estimation, i.e., based on the flight time for the My Jump 2-FT and based on the s\({}_{\text{z}}\)(t) graph for the force platform. Importantly, the CoM of the jumper performing the CMJ is at a certain height (relative to the vertical zero position) at the end of the take-off phase (plantar flexion) and at the beginning of the landing phase (dorsiflexion). Therefore, the flight time is shorter than the time of the CoM displacement between its vertical zero positions during take-off and landing. The analysis of the s\({}_{\text{z}}\)(t) and VGRF(t) graphs showed that the CoM was at a height of 13.2 \(\pm\) 1.9 cm at the ending point of the take-off phase (point E, Figure 1) and at a height of 11.3 \(\pm\) 5.8 cm at the starting point of the landing phase (point F, Figure 1). In this study, using the My Jump 2 app, the displacement time of the marker placed on the jumper's sacrum between its vertical zero positions during the take-off and landing phases was determined. The comparisons showed good absolute agreement (ICC = 0.858) and a small systematic bias in the MH difference (\(-\)0.2 cm) between the My Jump 2-DT and force platform, as well as a non-significantly higher MH value (only by 0.7%) for this method than for the force platform. Thus, the use of the My Jump 2-DT in the CMJ test provided similar MH results to those obtained based on the displacement of the jumper's CoM from the registered VGRF, commonly considered the most accurate method [33, 34, 35]. Previous studies have also assessed the validity of using My Jump for determing the MH during vertical jumps [2, 3, 6, 9, 12, 13, 14, 15, 27]. Considering the comparison of this app with the force platform, [PERSON] et al. [2], [PERSON] et al. [6], [PERSON] et al. [14] and [PERSON] et al. [15] reported very close MH results and an almost perfect agreement (ICC greater than 0.970) between both methods, recommending the My Jump app as a valid tool for evaluating the CMJ height. However, these researchers estimated the MH based on the flight time according to [PERSON]'s formula [10] for both this application and the gold standard. Thus, they took the incorrect approach because the force platform is not used to assess the jump height from flight time. Using the force platform and software analysis system (e.g., BioWare), the MH is estimated according to the displacement of the jumper's CoM (s\({}_{\text{z}}\)(t) graph). Compared to this best method, the present study showed the high accuracy of the new MH estimation method for the CMJ, i.e., My Jump 2-DT.