# Kinematic Analysis of the Racket Position during the Table Tennis Top Spin Forehand Stroke
## 4 Discussion
[PERSON] and colleagues [15] suggested to divide the top spin stroke in four different phases, which can be defined as preparatory, backswing, forward swing and follow through. Considering this classification, the minimum height (H-Min) is included in the backswing phase. The height of the racket measured at that point (CC: 0.530 \(\pm\) 0.124 m; LL: 0.533 \(\pm\) 0.131 m) was lower than the table height (0.76 m). Moreover, the height of the center of the racket with respect to the floor at the moment of the maximum distance between racket and table (H-RT) was exactly at the end of the backswing and at the beginning of the forward swing. It was higher than the H-Min but still lower than the height of the table (CC: 0.599 \(\pm\) 0.094 m; LL: 0.605 \(\pm\) 0.093 m), showing the need to raise the racket at the beginning of the forward swing phase. Subsequently, at the moment of maximum racket velocity (H-MMV), the racket was higher than the table in both CC and LL (CC: 1.081 \(\pm\) 0.053 m; LL: 1.086 \(\pm\) 0.054 m). In the present investigation, the moment of maximum racket speed was considered as the ball-racket impact. Several investigations indeed support the idea that the moment of maximum racket velocity in table tennis shots corresponds to the ball impact [5; 6; 7; 15; 16]. Therefore, the height of the racket at the moment of the ball-racket impact should also be considered greater than the height of the net with respect to the floor (0.76 + 0.1525 = 0.9125 m). A stereotypical movement, in terms of height of the racket at the key points, seems to be also confirmed by normalized data. Moreover, it suggests that the height of the racket during the execution is independent from the height of the players, and it is more influenced by the height of the table. This finding confirms the need to raise the racket at the impact, during the forward swing phase. The maximum height of the racket with respect to the floor (H-Max) corresponds exactly to the end of the follow through phase. The results showed greater values (CC: 1.628 \(\pm\) 0.075 m; LL: 1.624 \(\pm\) 0.084 m) than the height of ball-racket impact (CC: 1.081 \(\pm\) 0.053 m; LL: 1.086 \(\pm\) 0.054 m). Recently, [PERSON] et al. [5] compared different kinematic variables in CC and LL forehand top spin executions. Significant differences were observed for lower body variables (angles). In particular, the lower body was more rotated with respect to the table when the shot played was LL compared to CC. Differences in the upper body may also be due to the more rotated position of the whole body with respect to the table. Indeed, the position of the feet seems to influence the other kinematic variables ([PERSON] et al., 2018). Furthermore, the greater inclination of the racket at the moment of the impact was measured in LL and may be also related to the final direction of the top spin. The results of the present study confirm these previous observations, showing a significant difference in the rotation of the racket in the z axis at the moment of maximum velocity of the racket (Rz). The racket inclination, that can be considered a relevant kinematic variable, was more pronounced for LL (161.8 \(\pm\) 5.3\({}^{\circ}\)) with respect to CC top spin (148.6 \(\pm\) 6.6\({}^{\circ}\)). This does support our second hypothesis regarding racket inclination differences comparing the two executions, LL and CC, respectively. The racket face angle and the direction of racket path have been analyzed comparing the forehand top spin stroke against heavy and light backspin [1]. It was shown that the racket face was more open against heavy backspin than against light backspin [1]. The present findings suggest that the whole position of the body with respect to the table has a primary effect on the kinematics of CC and LL forehand top spin strokes. These results therefore confirm a standardization of the shot and a relevant control of the movements in the examined athletes.