Author(s): TESHIMA, Y., KASHIDA, K., Institution: CHIBA INSTITUTE OF TECHNOLOGY, Country: JAPAN, Abstract-ID: 2760

INTRODUCTION:
Many studies have been published on the vibration and coefficient of restitution of tennis rackets. PowerAngle, Inc., developed a tennis racket with strings strung diagonally in two directions (diagonal racket) [1] based on the idea that changing the string direction could reduce the vibrations generated when hitting the ball and transmitted to the arm. In the website, Joselow reported that this diagonal racket reduced the ball's impact acceleration by approximately 40% compared to a tennis racket strung in two directions (normal racket) [2]. Kawazoe reported that when a ball impacts the racket at 20 m/s, the coefficient of restitution changes by 1.4% with changes in string tension, whereas when the ball impacts at 30 m/s, the coefficient of restitution remains nearly constant regardless of string tension [3]
METHODS:
In this study, the string parameters were set to create a total of 24 different string conditions by varying two types of tension, two types of material, two levels of string diameter, and three levels of tension. The maximum amplitude of the impact acceleration (hereafter referred to as impact value) and the coefficient of restitution that occur when a tennis racket hits a ball were measured simultaneously, and the average values of the measured values were compared using a significant difference test. Measurements were performed 10 times for each string condition, and a t-test was performed on the obtained impact acceleration and coefficient of restitution measurements to test the significance of the average values (p<0.05 was considered statistically significant).
RESULTS:
For each impact acceleration and coefficient of restitution, we compared the average values for every pair of all 24 string conditions, for a total of 276 pairs. These comparisons revealed the following trends: When comparing rackets with only the stringing method changed, the impact acceleration was higher with the normal racket than with the diagonal racket, and the coefficient of restitution was higher with the diagonal racket than with the normal racket. When comparing rackets with only the string material changed, the impact acceleration was higher with polyester than with nylon, and the coefficient of restitution was higher with nylon than with polyester. When comparing rackets with only the string tension changed, the impact acceleration was higher with higher tension than with lower tension, and the coefficient of restitution was higher with lower tension than with higher tension.
CONCLUSION:
When compared under common conditions, the greater the impact acceleration, the smaller the coefficient of restitution, and a "negative correlation" was frequently observed. When comparing the impact acceleration and coefficient of restitution between normal and diagonal rackets, the results showed that the impact acceleration was greater with the normal racket, and the coefficient of restitution was greater with the diagonal racket.
[1] PowerAngle homepage, [2] Joselow (2008), [3] Kawazoe (2003)