# The Concurrent Validity of Mobile Application for Tracking Tennis Performance
## 4 Discussion  
The most important findings of this research are the results that support the purpose of using the application among tennis players. In the conducted research on the tested variables in situational conditions, the application shows good results in the metric characteristics of validity. For validity analysis, the number and characteristics of the shots collected with SV were compared with the number of shots collected from the video camera during the tennis match and comfort serve drill. When analyzing the serve shot directed from the deuce side of the court, it can be concluded that the detection of placement had good validity (ICC = 0.83). Likewise, placement_AD also had good validity (ICC = 0.87). When analyzing the placement zones individually, it can be noted that the most similar results were between VA and SV in Zones 1, 2, and 3 on the AD side, while on the DE side, in Zone 4, there was a greater match in the results. The good validity coefficients of placement variables prove that the SV app has a quality algorithm for identifying shot placement on the service box. It is not clear why the results on the advantage side are more related than those on the deuce side of the court. In the instructions for the use of the application, when mounting a mobile phone on the court fence and when calibrating the application, it is not clearly indicated whether there are differences if the mobile phone is placed on the left or right side from the central longitudinal line. When comparing the speed calculated from the application and detected with the speed radar, the results had good validity. The highest correlation of the results was obtained when the serve shot was performed in Speed_AD (ICC = 0.80). This result presents good validity for serves that are performed in the mean range of 136.00 to 145.60 km/h. The assumption is that the lowest serve speed would have greater validity, whereas serves over 200 km/h would have lower validity results. As presented in Table 1, the results of the minimum speed did not differ more than 6 km/h, but the results of the maximum speed in Speed_AD had an 11 km/h difference in speed detection. Accordingly, the measured results showed good validity in the Speed_AD and Speed_DE variables. Monitoring the speed of service in major competitions takes place during each point. With SwingVision, which validly measures service speed, this is also possible at each point in real time. In previous research [21], a correlation was observed between strength and serve speed when participants had a serve speed of 170.22 \(\pm\) 15.18. In relation to this study, the presented results of serving speed are, on average, 30 km/h slower than others in the same age category. Since it is proved that serve speed is highly related to sport level [22] determined by the national team score, the presented results showed a medium sport level in the performance of serve speed. Moreover, when analyzing the stroke calculated from the application, very good validity was observed (ICC = 0.97). The above finding indicates the high accuracy of the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **VA** & & **SV** \\ \hline  & **Count** & **\%** & **Count** & **\%** \\ \hline
**Slice** & 55 & 11.00 & 62 & 12.40 \\ \hline
**Topspin** & 234 & 46.80 & 213 & 42.60 \\ \hline
**Flat** & 174 & 34.80 & 191 & 38.20 \\ \hline
**Kick** & 37 & 7.40 & 34 & 6.80 \\ \hline
**ICC** & & 0.76 & & \\ \hline \hline \end{tabular}  
\(\overline{\text{SV}}\)—software-gained data; VA—data gained with analysis of recorded video; Count—number of serve shots per zone; %—the percentage of serve shots in a zone; ICC—correlation between measurements.  
\end{table} 
Table 4: Shot frequency and validity measures for observed spin shots. application in recognizing the type of stroke shot in the changing conditions of tennis play. For example, in terms of the forehand shots, VA recognized a total of 224, compared with SV's total of 223.