# Reliability and Validity of the Portable KForce Plates for Measuring Countermovement Jump (CMJ)
## 4 Discussion  
The present study evaluated the test-retest reliability and the concurrent validity of the portable KForce Plates compared to the validated My Jump 2 application installed on an iPhone 13 for measuring CMJ in collegiate athletes. The KForce Plates system was found to be highly reliable and valid for measuring jump height and flight time variables in CMJ in comparison to the My Jump 2 application. Furthermore, the data illustrated in the Bland-Altman plots (Figures 3 and 4) demonstrate that most of the values are close to the mean of differences between the two tools, therefore depicting a high level of agreement [29]. The CMJ is a simple method to identify athletes' strengths and weaknesses and also to monitor training progress [26; 30]. There are a variety of methods and devices for evaluating jump height and flight time variables; however, most of them are restricted to a clinical laboratory environment. In our study, test-retest reliability revealed that jump height and flight time for measuring CMJ appeared to be reliable assessment outcomes (jump height: ICC = 1.000; CI = 1.000;  
Figure 5: Bland–Altman plots illustrating the limits of agreement of the 2 measurements (the portable KForce Plates and the My Jump 2 application) for flight time between the 1 st and 7 th days. \begin{table}
\begin{tabular}{c c c} \hline \hline
**CMJ** & **1 st Day** & **7 th Day** \\
**KForce Plates–My Jump 2 App** & & \\ \hline
**N = 34** & **PCC** & **PCC** \\ \hline Jump height (cm) & 1.000 & 1.000 \\ \hline Flight time (s) & 0.999 & 0.999 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Pearson correlation coefficients (PCC) of the 2 measurements (the portable KForce Plates and the My Jump 2 application) for CMJ variables (jump height and flight time) between the 1 st and 7 th days. 1.000, \(p\) value \(<\) 0.001; and flight time: ICC = 0.999; CI = 0.998; 1.000, \(p\) value \(<\) 0.001). A number of reliability studies for the My Jump application used these guidelines for interpreting ICC values: less than 0.5 = poor, between 0.5 and 0.75 = moderate, between 0.75 and 0.9 = good, and greater than 0.90 = excellent ICC [21; 22; 29; 31; 32; 33]. Thus, the results of the current study indicate excellent reliability. The portable KForce Plates measurements showed that there was a mean difference of 4.06 cm for jump height and 0.04 ms for flight time. These findings are not in agreement with the study of [PERSON] et al. [34], who conducted a reliability study with recreational athletes using the My Jump application. In that case, the mean jump height was 0.43 cm, and flight time was not evaluated. However, our study was conducted among collegiate athletes using the My Jump 2 application on an iPhone 13. We observed a high correlation between the portable KForce Plates and the My Jump 2 application in both jump height (r = 1.000; \(p\) value \(<\) 0.001) and flight time (r = 1.000; \(p\) value \(<\) 0.001), which indicated the concurrent validity of the KForce Plates system for assessing CMJ in collegiate athletes. Previous studies compared various technologies for measuring CMJ with force platforms [21; 22]. The My Jump and My Jump 2 applications have been compared to force platforms in a number of different jumps [22; 25; 31] and have shown a high correlation (r = 0.97-0.99) for CMJ and SJ in athletes [25; 31] and for drop jumps (r = 0.94-0.97) in students [22]. The rapid evolution of new technologies suggests that a contemporary smartphone includes a high-speed camera with high-speed frequency, thus reducing the measurement error of the My Jump 2 application. Several research papers have compared portable instruments with unportable force plates, with a mean difference between \(-\)1.06 and 11.7 cm for CMJ performance (jump height) [18; 19; 35], which is in agreement with our study. Differences between portable instruments and force plates could be attributed to the fact that the sampling rate and athletic performance vary in many study designs.