# Reliability and Validity of the Portable KForce Plates for Measuring Countermovement Jump (CMJ)
## 3 Results  
### Reliability  
The results for testing reliability with the ICC between repeated measurements by the same examiner on the 1 st and 7 th days and between the two instruments for the dependent variables (jump height and flight time) were excellent for both variables (Table 1).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**CMJ** & \multicolumn{2}{c}{**1 st Day**} & \multicolumn{2}{c}{**7 th Day**} \\
**KForce Plates–My Jump 2 App** & & & \\ \hline
**N = 34** & **ICC** & **95\% CI** & **ICC** & **95\% CI** \\ \hline Jump height (cm) & 1.000 & 1.000–1.000 & 1.000 & 1.000–1.000 \\ \hline Flight time (s) & 0.999 & 0.998–1.000 & 0.999 & 0.998–1.000 \\ \hline \hline \end{tabular} ICC: intraclass correlation coefficient; CI: confidence interval.  
\end{table}
Table 1: ICC on the 1 st and 7 th days between the 2 instruments, the portable KForce Plates and the My Jump 2 application, for measuring CMJ variables.  
Figure 3: Take-off (a) and landing (b) captures on the portable KForce Plates with the concurrent use of the My Jump 2 app, (Explanations; A\(\kappa\)\(\theta\)\(\omega\)\(\omega\)\(\Omega\): Cancel).  
The ICCs and the mean difference for the CMJ variables (jump height and flight time) of the 1 st and 7 th days between the two instruments (the KForce Plates and the My Jump 2) are presented in Table 2.  
To compare the limits of agreement of CMJ measurements for the jump height variable between the 1 st and 7 th days and also between the two instruments (the portable KForce Plates and the My Jump 2 application), we used the Bland-Altman plots (Figure 4). The mean difference (MD = 0.004) was not statistically significant (t = 0.187, \(p\) = 0.853). The lower bound of the 95% confidence interval was \(-\)0.265, and the upper bound was 0.274 (\(\pm\)1.96 standard deviations). There was a strong relationship between the measurements of the two instruments (the portable KForce Plates and the My Jump 2 application) and an agreement of \(\approx\)1 cm.  
Furthermore, in order to compare the limits of agreement of CMJ measurements for the flight time variable between the 1 st and 7 th days and also between the two instruments (the portable KForce Plates and the My Jump 2 application), we used the Bland-Altman plots (Figure 5). The mean difference (MD = 0.00) was not statistically significant (t = \(-\)1.193, \(p\) = 0.242). The lower bound of the 95% confidence interval was \(-\)0.007, and the upper bound was 0.006 (\(\pm\)1.96 standard deviations).  
\begin{table}
\begin{tabular}{c c c} \hline
**CMJ** & **KForce Plates and My Jump 2 Measurements** \\ \hline
**N = 34** & **ICC (95\% CI)** & **MD (95\% CI)** \\ \hline Jump height (cm) & 0.872 (0.744; 0.936) & \(-\)4.06 (\(-\)6.20; \(-\)1.91) \\ \hline Flight time (s) & 0.877 (0.755; 0.939) & \(-\)0.04 (\(-\)0.06; \(-\)0.02) \\ \hline \end{tabular}
\end{table}
Table 2: ICCs of the mean difference for the CMJ measurements for the variables jump height and flight time between the 2 instruments of the KForce Plates and the My Jump 2 app.  
Figure 4: Bland–Altman plots illustrating the limits of agreement of the 2 measurements (the portable KForce Plates and the My Jump 2 application) for jump height between the 1 st and 7 th days.