# The Concurrent Validity of Mobile Application for Tracking Tennis Performance
## 2 Materials and Methods
### Measurement Procedure  
The testing procedure was conducted on an outdoor clay court. Weather conditions during testing were typical for the summer part of the tennis season on a clay surface. The test was conducted in the morning hours, and the temperature was 26 degrees, with no wind. Prior to the start of the measurement, the participants performed a standardized 20 min warm-up consisting of basic (off-court) and specific (on-court) tennis warm-up drills. After finishing the warm-up protocol, the players were instructed to serve 60 times (30 on the deuce side and 30 on the advantage side). Overall, 150 serves were observed on each serving side. The participants were instructed to serve the first serve anywhere in the service box, as they would serve during a competitive match. Upon the completion of the serving procedure, players were asked to play a training match. The first 100 shots (overall 500 shots) played were used for further analysis to determine SV validity. The observed shots in terms of the type of strokes consisted of forehand, backhand, serve, forehand volley, backhand volley, and overhead shot. Strokes in terms of spin type consisted of a kick serve, as well as slice, flat, and topspin shots. A kick serve is performed by generating a significant amount of topspin by hitting up on the ball and snapping the wrist when contacting the ball. Slice shots are characterized by adding sidespin with a low bounce of the ball after hitting the ground. Flat shots are hit without spin and are characterized by the high speed of the ball. Lastly, for topspin shots, it is necessary to swing up and brush the back of the ball when making contact. Before starting the SV application, attention should be paid to the correct placement of the mobile phone. It is recommended to place the mobile phone high enough that a few feet behind each baseline is visible. and that the top of the net does not overlap with the far service/baseline. Furthermore, for optimal accuracy, it is recommended to zoom in as close to the court while still keeping all corners of the singles court visible on screen. The phone was placed on the mount at 3 m height on the fence. The position was calibrated and aligned with the use of in-built software calibration, as presented in Figures 1 and 2.  
According to manufacturer instructions, radar settings were set at range 4, with low speed set to 70 km/h and high speed set at 250 km/h to avoid unnecessary movements around the court. Speed was measured on serve behind the player at the level of impact on the ball. The service box was divided into three equal zones from the \"T\" line to the wide line of the service box. Each zone was 1.37 m wide (Figure 3). A fourth zone represented missed shots (net or outside the service box). The server position from the deuce and advantage sides on the baseline was displayed (\(\Delta\)). A video camera (Panasonic Lumix DC-GH5 II) was placed next to the service box that recorded every contact of the ball with the surface after the service. The authors of this paper used this camera to record the location of the contact of the ball with the surface and compared it with the recorded video via the application (Figure 4). For the additional verification of the actual point of contact, the ball's bounce was assessed on the court after each serve, checking its trace, and this was possible because the test was conducted on a clay surface, and thus the ball left a clear mark on the surface upon contact.  
Figure 1: The illustration of instructions for calibration of phone position (1).  
Figure 3: Server position (\(\blacktriangle\)) and accuracy zones (1, 2, 3, 4).  
Figure 2: The illustration of instructions for calibration of phone position (2).