# Kinematic Analysis of Short and Long Services in Table Tennis
## 2 Materials and Methods
### Experimental Design and Procedures  
Each player performed two tasks, presenting a forehand serve with back-sidespin rotation:  
1. A short serve (ShS) was performed in such a way that the second reflection of the ball on the opponent's half was placed in the rectangle designated at the right corner of the table (Figure 1). 2. A long serve (LoS) was performed so that the first reflection of the ball on the opponent's half was placed in the right rectangle designated at the right corner of the table, same as above (Figure 1). Each type of serve was performed until the player made ten correct serves in the hitting zone (40 \(\times\) 65 cm) on the other side of the table (Figure 2). Only accurate serves were used for further analysis. The participants were asked to hit a forehand serve stroke with the maximum effort per serve type. This was performed to respect the individual players' technique and style. In the vast majority of cases, the players performed up to 30 serves of a given type to complete the task. The time of execution (including breaks between individual serves) was not limited. All measurements of the Italian players were taken in the Laboratorio Record of the Department of Biomedical and Neuromotor Science, Bologna University (Bologna, Italy) by a single skilled technician. The same employee,accompanied by the authors of the work, carried out tests on ten athletes in a sports hall in Poland (MOSiR Brzeg Dolny - Brzeg Dolny, Poland). Both study sessions were performed on Donic Persson 25 table tennis tables (Donic, Saarbrucken, Germany). The players performed services after a standard warm up: general (15 min) and specific, at tables (20 min). The Noraxon Ultium Motion system (myoMOTION(tm), Noraxon, Scottsdale, AZ, USA) was used to assess the course of movement in individual body segments during the performed tasks. The system, which is referred to in the literature as an Inertial Motion Unit (IMU), allows for the high-precision assessment of variables in both slow clinical and fast sports movements [21; 22]. The results presented in the literature obtained from IMU systems are consistent with those obtained from systems based on optical motion capture [21] and are characterized by a high accuracy [22]. Inertial sensors (wearable devices including triaxial accelerometers, magnetometers, and gyroscopes) allow for the determination of the 3D rotation angles of each sensor in a subject-oriented space [23]. In the present research, 16 sensors were used, which were attached to the player's body with elastic straps and self-adhesive tape. The sensors were placed symmetrically so that the positive x-coordinate on the sensor label corresponded to a superior orientation for the segment [23]. Sensor fusion algorithms incorporating Kalman and particle filters  
Figure 1: The scheme of two types of serves. 1, 2, 3 - subsequent ball bounces on the table. Figure 2: The study design. enhanced the accuracy of angle estimation. A biomechanical model was developed using a flexible framework for defining standardized protocols, as outlined by [PERSON] et al. [23]. Following the International Society of Biomechanics (ISB) recommendations on definitions of the joint coordinate system for the shoulder, elbow, wrist, and hand [24] and for the ankle, hip, and spine [25], the following angles were chosen for both sides and sampled every 0.01% of the task time. * Shoulder total flexion: A compound angular movement of the humerus relative to the thorax that includes contributions from the sagittal plane (flexion-extension) and the transverse plane (arm movement inward or outward). Positive values indicate the humerus moving outward (elevation with external rotation), while negative values indicate the humerus moving inward (elevation with internal rotation). * Shoulder flexion-extension: Angular rotation of the humerus relative to the thorax in the sagittal plane; a negative sign denotes extension while a positive sign denotes flexion.