Analysis of a Movement of a Top Table Tennis Player using a Direct Linear Transformation (DLT) Method

Abstract

We have conducted a research in order to clarify a movement of a top table tennis player using DLT method. We have tried to obtain basic data such as time intervals of various playing, times of shots, playing area, distance of movement and so on to plan a training program of a movement for table tennis.
The objects of analysis were 5 games of YOO Nam Kyu, who was the SEOUL Olympic champion, in the 9th ASIAN Table Tennis Championships in Japan. The games were filmed at 60 frames/second with 6 VTR cameras which were installed at separate locations in the gymnasium. For the purpose of this analysis, the various time intervals are defined as follows: In-play Time was from the initial movement of the service to final contact with the ball during play. It is acknowledged that this definition differs slightly from the official ITTF regurations for “ball in play” time. Rally Time was from contact with the ball on service to final contact with the ball during play. Service Time was from the initial movement of the service to contact with the ball on service. Waiting Time was from one player's contact with the ball to the other player's contact.
Two-dimensional coordinates on a TV monitor were computed with a VTR motion analyzer connected to a personal computer. These coordinates were then converted to real (X-Y-Z) coordinates using DLT method. The position of the player was estimated from the coordinates of the player's head (vertex), center of shoulders (acromions), center of hips (trochanterions) and center of ankles (malleolus fibulae points). Distance of movement and velocity of movement were computed for every body point in several directions; right to left and forward to backward (in relation to the table), and up to down. Distance of movement was a line that links the position of the player every 0.1 sec during Rally-time. Velocity of movement was calculated by dividing distance of movement by Rally-time for each rally.
The results and findings are summarized as follows;
1) Mean total In-play time was 144.63±27.69 sec (±SD). This represented about 20-30% of the games of which average was 611.75±33.50 sec long, meaning that about 70-80% of the games was excess time spent doing such things as picking up the ball or wiping off perspiration.
2) Mean Service Time, Mean Rally Time, Mean In-play Time and Mean Waiting Time was 1.05±0.37 sec, 2.56±1.79 sec, 3.60±1.83 sec and 0.66±0.19 sec, respectively. This represented about 30% of In-play Time was Service Time and about 70% of it was Rally Time.
3) Mean times of shots during 1 rally was 4.61±2.12. The distribution of times of shots during 1 rally showed that in the case of 74.6% of all rally, a player won or lost a point in 5 shots.
4) The playing area for YOO was positioned at the side of his free hand in 4 games. In 1 game, it was positioned at the side of his racket hand. His main stroke of the game was different from previous 4 games.
5) In each game, the distance of movement for YOO on a horizontal plane during Rally Time was between about 100m and 200m at the center of the ankles. And mean distance of movement for YOO on a horizontal plane during 1 rally was between about 3.5m and 4.0m at the center of the ankles. In the table tennis games, the player's repeated acceleration and deceleration in various directions was instantaneous.
6) In each game, mean average velocity of movement for YOO on a horizontal plane during 1 rally was between 0.85m/sec and 1.08m/sec at the head, 0.90m/sec and 1.05m/sec at the center of the shoulders, 0.99m/sec and 1.18m/sec at the center of the hips and 1.36m/sec and 1.51m/sec at the center of the ankles.
7) It seems that these new data and findings are very important to plan an efficient training program for table tennis coaches and players.