# Relationship Between Ball Speed and Spin in Elite Youth Table Tennis Players Using Optical Sensors
## 4 Discussion  
The primary objective of this study was to demonstrate the presence of a correlation between linear speed and ball spin. This study allowed us to confirm the presence of a strong positive correlation between ball spin and linear speed in four-stroke conditions. The first observation showed that the player's mean linear speed ranges from 7 to 17 m.s\({}^{-1}\) and 60 to 120 RPS for rotation. These results are consistent with observed values in the existing literature, such as _[PERSON], [PERSON], and [PERSON]'s_ work, who have found similar speed and spin values [4,10,22] It is noticeable that the topspin forehand on topspin balls (exercise 3) clearly shows higher spin and speed compared to the remaining exercises due to the nature of the movement. The backhand strokes (exercises 2 and 4) display a lower speed and spin range than the forehand strokes (exercises 1 and 3). This could be due to the nature of the stroke; exercises 2 and 4 need more control to strike the ball back, whereas exercises 1 and 3 need more physical engagement from the player. This difference in ball speed depending on the stroke type was already observed in the literature, where racket velocity has been observed to be faster for forehand strokes than backhand strokes [10,23,24].  
The protocol was designed to observe the behavior of ball speed/spin on the most used offensive strokes in table tennis, but not for defensive stroke observation (e.g., the block).  
However, more extreme conditions, such as a no-spin ball with high linear speed or a no-speed ball with a lot of spin, would not follow this rule and are physically possible, but probably won't hit the table in a real match condition.  
The linear relationship between ball speed and spin is probably due to the constraint of hitting the table with the ball (ball precision), known as the speed-accuracy trade-off in arm movement [25]. This relationship could also appear due to the Magnus effect, admitted to affect spinning objects in a gas (air) and change the path of this object. A higher spin induced by the player will affect this trajectory and allow the ball to drop quickly due to the lift force created by the topspin. However, it seems effective to develop a large spin, as it will decrease the flying time of the ball and reduce the reaction time of your opponent [26]. However, the results presented in our work are taken from a static measure (single measurement points); therefore, these results were not considered in the present study but should be used for match conditions studies.  
Figure 9: Ball linear speed for each exercise is displayed as a scatter plot, box plot, and density plot.  
It is important to underline that this strong linear correlation is not affected by the stroke condition (stroke realized and incoming spin), even if linear speed has shown significant differences between stroke conditions as hypothesized in the introduction.  
Such tests have already been conducted with different test populations. In the study of [PERSON] et al. [22], the test population had a mean age of 28.4 years, with different practice levels among adults. Our test population is younger (14.6 years mean age for men) and is geared towards young players in training. Working with young players, some have already finished developing their adult bodies, while others are still in growth. However, the ranking range is large enough to have advanced to expert players, in growth or not. To our knowledge, there are few studies doing comparisons between younger and older players on different aspects of performance, thus independently of their practice level.  
Such results should be interpreted with caution due to the nature of the test group. Future studies should involve a wider range of participants by including gender parity, various practice-level groups, or even age differences.