## 3 Results
\begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Device: Method** & **ABJ: Displacement** & **QMS: Flight-Time** & **FP: Flight-Time** & **EJ: Flight-Time** & **MP: Flight-Time** & **FP: Double Integration** & **MP: Double Integration** \\ \hline Accuracy (mm) & 0.8 & 6.1 & \(-\)45.3 & \(-\)72.9 & \(-\)52.8 & 4.0 & \(-\)31.4 \\ \hline Precision (mm) & \(\pm\)14.7 & \(\pm\)17.1 & \(\pm\)18.8 & \(\pm\)21.3 & \(\pm\)28.4 & \(\pm\)26.1 & \(\pm\)122.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The accuracy and precision of jump height when comparing different approaches to a QMS marker displacement method. Figure 4: The Bland-Altman plots of the jump height, obtained from different devices and methods. double integration had a proportional bias with a high positive slope, corresponding to a tendency to strongly overestimate the jump height. The true flight-time is identified by considering the take-off and landing instants represented in Figure 5. This shows the contactless interval (flight-time) identified from the vertical acceleration is shorter than the elapsed time determined from the maximum and minimum velocity approach. Flight-time can be directly measured using contact with the EJ mat or estimated from elapsed time between the maximum and minimum velocity (Figure 5) or by time integration of the vertical acceleration from FP or MP data. There appears to be an underestimation of the flight-time and subsequent jump height values obtained from EJ, which presented lower accuracy than those obtained from FP and MP devices. However, when the flight-time is defined using the method of QMS, jump height can be closely approximated to the true value. Concerning the results obtained from the double integration method, MP presents much lower accuracy and precision than FP. To investigate the reason for these differences, a comparison between the accelerations measured by the two devices was made. The vertical acceleration measured by MP and determined from ground reaction force measured by FP are presented in Figure 6.  
For FP acceleration, it was possible to identify a stable negative baseline equal to the gravitational acceleration, which corresponded to the situation where there is an absence of contact with the platform. The same was not observed for MP acceleration that showed some fluctuations over the same interval and less distinct data events at initiation and ending of the flight phase (i.e., a slower slew rate at the transitions). By applying the double integration method to the two accelerations the respective mass centre vertical flight trajectory could be compared, as in Figure 7, with the vertical trajectory of the pelvis markers tracked by QMS. Figure 5: The vertical position, velocity, and acceleration were recorded during the jump, with the indication of flight-time. The FP path follows the QMS vertical trajectory closely, but MP presents a very different path. A possible explanation for this difference is the lower slew rate and the deviation in vertical orientation of the acceleration measured by the MP device. This last deduction is supported by fluctuations in the vertical acceleration observed during the flight phase, where these would be expected to be constant and equal to the gravitational acceleration. Figure 6: The vertical acceleration time, measured using the MP and determined from the vertical reaction force measured by FP. Figure 7: The vertical trajectories of the body centre mass, registered by QMS marker and determined by double integration method of the acceleration measured with FP and MP.