# Kinematic Analysis of Short and Long Services in Table Tennis
## 4 Discussion  
The aim of this study was to assess the kinematics and determine the coordination of movements of most body segments during the execution of two types of serves (short and long) in table tennis. The aim was also to indicate the main differences between these serves when performed by high-level athletes. In the present study, an IMU system was used to collect the data. Despite the described great importance of serves for the effectiveness of the game, this element of table tennis technique is very poorly described and analyzed in the scientific literature, though, in a rather general way, the technique of execution is described in table tennis textbooks [26; 27]. The \"Section 3\" of the present article provides information concerning the coordination of movements during the execution of forehand serves. First of all, it should be noted that, as this research has shown, the technique of serving in table tennis uses the entire kinematic chain. It takes into account the coordinated movements of the entire body, starting from the joints of the lower limbs, through the torso, to the upper limb with the racket. Such coordination of movements is aimed at achieving the greatest acceleration of the hand with the racket--the most distal segment [28]. It uses the principles of proximal-to-distal sequence, stretch-shortening cycles, and velocity summation, which are described in the literature [28; 29; 30]. Bearing in mind the individual differences between players, the course of coordinated movements during both types of serves in the studied joints could be defined as follows: in the backswing phase, one can observe the external rotation of the upper body (trunk); flexion in the knee joints; extension in the hip and ankle; abduction, external rotation, and flexion at the shoulder joint; extension at the elbow joint (in LoS); and extension at the wrist joint. During the hitting phase, mostly reverse directions of movements can be observed in relation to those described, but in most joints and all players, these movements were faster than during the backswing phase. The movements \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type of Service** & **Mean** & **Standard** & **Median** & **Quartile** & **Coefficient of** & \(p\)**-Value** \\  & & **Deviation** & & **Range** & **Variation** & **of U Mann–Whitney Test** \\ \hline ShS & 6.35 & 1.01 & 6.54 & 1.40 & 15.87 \\ LoS & 6.84 & 1.60 & 6.43 & 1.82 & 23.40 & 0.07 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Values of the maximal resultant velocity of the playing hand (m/s) in the hitting phase. in the hitting phase are internal rotation of the upper body, flexion and rotation in the hip joints, and adduction and internal rotation in the shoulder joint. Without a doubt, an important movement in this phase is the movement of the hand with the racket--pronation and ulnar flexion in the wrist joint. This is most likely due to the manifestation of the above-mentioned biomechanical principles of movement execution aimed at increasing the force drive; in the case of table tennis and our study, this is the force with which the player hits the ball with the racket during services [31,32]. The differences between the types of services were found in the shoulder rotation in both the backswing and hitting phase. Differences were seen in the range of motion in both phases (between BackS and BackE and BackE and Forw), as well as the angular velocity at the moment of contact (Cont). Movement at this joint, therefore, appears to be important in achieving a high hitting speed. This confirms the observations of other authors regarding the use of this movement when performing forehand serves [33]. The observed differences in the range of movement between ShSs and LoSs seem to appear in most body joint movements. A significant difference was observed in the flexion movement at the elbow joint between the two types of serves. In the case of long serves, this movement had a higher range and was faster than in short serves during the hitting phase.