## 4 Discussion  
The primary aim of this study was to examine the validity and intra-session reliability of the MyJump 2 smartphone application for assessing countermovement jump (CMJ) and squat jump (SJ) performance in a sample of youth soccer players using the Chronojump system as the reference. Overall, the findings support the use of MyJump 2 as a valid and reliable alternative for measuring vertical jump height in both CMJ and SJ protocols across a mixed-age athletic population, including adolescent and junior players. The Bland-Altman analyses revealed small systematic biases, with MyJump 2 slightly overestimating jump height in both CMJ (+0.943 cm) and SJ (+1.042 cm) compared to Chronojump. Despite these differences, the limit of agreement remained within a practically acceptable range, aligning with prior validation research [7,11], where small positive biases were also reported. The narrow confidence intervals around the bias estimates further reinforce the consistency and robustness of these differences, indicating that MyJump 2 systematically overestimates performance by approximately 1 cm. These findings are further contextualized by the calculation of the MDC, which provides a critical threshold for distinguishing genuine performance changes from measurement error. For CMJ, the MDC was calculated at 2.26 cm, and for SJ, it was 1.31 cm, suggesting that any observed changes in jump performance exceeding these thresholds can be interpreted as real and not attributable to inherent measurement variability. This has important implications for practitioners seeking to monitor  
Figure 12: Bland–Altman analysis of SJ MyJump test–retest agreement. Bland–Altman plot illustrating the agreement between squat jump (SJ) measurements obtained using MyJump during the test and retest sessions. The central blue line represents the mean bias (0.186 cm), while the shaded gray area indicates the confidence interval for the mean difference. The dotted black lines mark the limits of agreement (LoA), which range from \(-\)1.122 cm to 1.495 cm. The green and red shaded regions highlight the upper and lower LoA ranges, respectively. Individual data points represent the difference between test and retest values plotted against their mean. neuromuscular adaptations in youth athletes, as it delineates the minimum change required for meaningful interpretation when using the MyJump 2 application in field settings. Coaches and practitioners should, nonetheless, be aware of this systematic difference when interpreting absolute values or comparing data across different measurement systems. Correlation analyses further supported the validity of MyJump 2, with Pearson's r values of 0.972 for CMJ and 0.955 for SJ, both statistically significant and accompanied by similarly high [PERSON]'s rho values. These results are in line with previous studies validating MyJump 2 against gold-standard systems such as force platforms and infrared devices [8; 9], confirming the strong linear association and rank-order consistency of MyJump-derived measurements. Moreover, regression analyses demonstrated excellent predictive accuracy, with MyJump accounting for 94.4% (CMJ) and 91.2% (SJ) of the variance in Chronojump-derived values. These findings are particularly relevant for practitioners seeking reliable field-based tools without the cost or logistical demands of traditional force platforms. Several studies have reinforced the validity and reliability of the MyJump 2 application across various athletic populations and conditions. For example, in a cohort of judo athletes, [PERSON] et al. [40] reported a strong correlation (\(r\) = 0.97) and minimal bias between MyJump 2 and Chronojump, indicating its utility beyond soccer contexts. Similarly, [PERSON] et al. [12] found almost perfect agreement between MyJump and force platforms for CMJ assessment (ICC = 0.997).