## 2 Materials and Methods
### Measures and Instrumentation  
To record the accuracy in the forehand approach shots to the net, a video camera was used, filming at a frequency of 150 Hz (Casio, model Exilim FH 100, Tokyo, Japan). Each bounce of the ball on the court was recorded using this video camera, fixed at a distance of 8 m from the baseline of the tennis court and at a height of 4 m, placed on top of the rear lighting and directed to the sending area marked as a target, with dimensions of \(0.7\times 0.7\) m, located 1.0 m away from the singles sideline and 1.0 m away from the baseline. This location is considered ideal by other authors [18] for accuracy tasks of these characteristics and using this type of stroke. In addition to taking the spatial coordinates X (longitudinal) and Y (transversal) of the center of the target as a point of maximum accuracy, the backcourt area of a singles game was established as a reference system, from the back line of the service boxes to the baseline, thus having a total of 5 coordinates to establish the reference system. The recordings were played back using Kinovea, version 0.9.5. [19]. This software is a video annotation tool, which is open source, free, and designed for sports analysis. It allows users to capture, slow down, compare, annotate, and measure motion in videos. Each X and Y coordinate of the reference system and the target was digitized and transformed into real coordinates, with an error of less than 1.0 cm. The radial error (RE) was calculated as a measure of accuracy in the strokes using the formulae proposed by other authors [20]. Formula (1) was applied to the digitized coordinates of each bounce of the ball on the court. Finally, the obtained data were processed for subsequent statistical treatment. \[\text{RE}=\sqrt{\left(\text{X}-\text{X}^{\prime}\right)^{2}+\left(\text{Y}- \text{ Y}^{\prime}\right)^{2}} \tag{1}\]  
X and Y = longitudinal and transversal coordinate of the ball bounce on the court. X' and Y'' = real coordinates of maximum accuracy according to the location of the target. Formula (1). Formula for obtaining accuracy in strokes through the calculation of radial error (RE). The data collection was carried out on outdoor tennis courts, taking into account the wind speed, which can interfere with the speed and effectiveness of certain tennis strokes [5]. To control for this potential interference, an anemometer (Technoline, EA 3000, NeXtime, Haarlem, The Netherlands) was used, and it was observed that during the measurement, the wind speed remained below 3.33 m/s. In this context, a study artificially inducing wind during tennis serve execution demonstrated that intensities below 4.3 m/s did not negatively impact the achieved accuracy [5]. In the context of maintaining consistency in ball delivery, a ball-throwing machine (Playmate Volley, Morrisville, North Carolina, United States) was employed to launch balls to the players. Prior to data collection, a reliability test was conducted, yielding the following results (Table 1). The ball-throwing machine was calibrated before each trial, and the reliability of the device was assessed to observe the launch frequency and velocity based on the calculation of the coefficient of variation (CV). For launch velocity measurements, a mobile radar system (Stalker ATS 4.02; Radar Sales, Plymouth, MN, USA) was employed. The results of the reliability tests obtained in the frequency (CV = 4.2%) and in the throwing speed (CV = 1.2%) indicate a high degree of consistency, in line with other studies in which the same methodology was used [21]. To analyze the effect of the training sessions, a pre-test was applied before starting the intervention phase, as well as a post-test after the training sessions and a re-test one week after the post-test. Each of them consisted of 60 repetitions, 4 sets of 15 repetitions with 60 s of rest between sets and 4.6 s between strokes. All three groups performed all the tests under the same conditions, using standard intermediate bounce balls (type 2) and clay courts.