## 3 Results  
### Validity  
The descriptive statistics for countermovement jump (CMJ) and squat jump (SJ) performances assessed via Chronojump and MyJump are presented in Table 1. The mean CMJ height recorded using Chronojump was 36.1 \(\pm\) 4.93 cm, whereas MyJump produced slightly higher values (37.1 \(\pm\) 4.94 cm). A similar pattern was observed in the SJ test, where Chronojump measured a mean height of 34.6 \(\pm\) 2.90 cm, while MyJump yielded 35.6 \(\pm\) 3.01 cm. To evaluate the agreement between MyJump and Chronojump in measuring CMJ height, a Bland-Altman analysis was conducted. The results (Table 2, Figure 1) indicated a mean bias of +0.943 cm (95% CI: 0.409 to 1.476 cm), suggesting that MyJump systematically overestimates CMJ height compared to Chronojump. The 95% limits of agreement (LoA) were \(-\)1.354 cm to +3.240 cm, with confidence intervals ranging from \(-\)2.281 to \(-\)0.428 cm for the lower limit and 2.313 to 4.167 cm for the upper limit. These results suggest that despite minor individual deviations, MyJump and Chronojump demonstrated strong agreement in jump height estimation within an acceptable range. Regarding the squat jump (SJ) height, the agreement between MyJump and Chronojump was assessed using Bland-Altman analysis (Table 3, Figure 2). The results indicated a mean bias of +1.042 cm (95% CI: 0.636 to 1.449 cm), suggesting that MyJump systematically also overestimates SJ height compared to Chronojump (Table 3, Figure 2). The 95% limits of agreement (LoA) ranged from \(-\)0.709 cm to +2.794 cm, with confidence intervals spanning \(-\)1.415 to \(-\)0.002 cm for the lower limit and 2.087 to 3.499 cm for the upper limit. These findings could suggest that while individual measurements exhibited some variation, the two methods demonstrated strong overall agreement, with differences mostly within an acceptable range. \begin{table}
\begin{tabular}{c c c} \hline  & \multicolumn{2}{c}{**M \(\pm\) SD**} \\ \hline CMJ MyJump (cm) & 37.1 \(\pm\) 4.94 \\ CMJ Chronojump (cm) & 36.1 \(\pm\) 4.93 \\ SJ MyJump (cm) & 35.6 \(\pm\) 3.01 \\ SJ Chronojump (cm) & 34.6 \(\pm\) 2.90 \\ \hline \end{tabular} Note: M \(\pm\) SD—mean \(\pm\) standard deviation.  
\end{table} 
Table 1: Descriptive statistics for CMJ and SJ measurements using MyJump and Chronojump. \begin{table}
\begin{tabular}{c c c c} \hline  & \multicolumn{2}{c}{**95\% Confidence Interval**} \\  & **Estimate** & **Lower** & **Upper** \\ \hline Bias (n = 21) & 0.943 & 0.409 & 1.476 \\ Lower limit of agreement & \(-\)1.354 & \(-\)2.281 & \(-\)0.428 \\ Upper limit of agreement & 3.24 & 2.313 & 4.167 \\ \hline \end{tabular}
\end{table} 
Table 2: Bland–Altman analysis for CMJ measurements: MyJump vs. Chronojump. To further evaluate the association between MyJump and Chronojump for CMJ height measurements, a Pearson correlation analysis was conducted (Table 4 and Figure 3). The results indicated a strong positive correlation (r = 0.972, \(p\) < 0.001), suggesting that MyJump measurements closely align with those obtained from Chronojump, where 95% confidence interval (CI) ranged from 0.930 to 0.989. Additionally, [PERSON]'s rho (\(\rho\) = 0.960, \(p\) < 0.001) confirmed this strong association, indicating that the rank order of jump heights is preserved across both measurement methods. The correlation plot (Figure 3) illustrated a near-perfect linear trend between the two devices. The narrow confidence band around the regression line further suggests high measurement consistency between MyJump and Chronojump. Figure 1: Bland–Altman plot: agreement between MyJump and Chronojump for CMJ measurements. The x-axis represents the mean jump height between the two methods, while the y-axis shows the difference between MyJump and Chronojump for each participant. The blue line represents the mean bias (+0.943 cm), indicating a slight overestimation by MyJump. The dotted lines denote the 95% limits of agreement (LoA), ranging from \(-\)1.354 cm to +3.240 cm, defining the expected range within which most individual differences lie.