A METHOD TO DETERMINE BALL IMPACT LOCATION AND ITS MOVEMENT ACROSS THE STRINGS OF A TENNIS RACKET

Abstract

INTRODUCTION: The impact location of a ball on the string plane area of a given tennis racket and its movement across the strings influences ball speed, direction of flight, and the resulting spin of the ball. The accuracy and resolution of cinematographic recordings are limited because the racket head area is small in comparison to the necessary object space for filming. Because the interaction of the tennis racket with the ball is of fundamental importance for the game, an instrumented tennis racket was developed to electronically determine the point of ball impact on the strings. METHODS: A “Kuebler Inertial Light" tennis racket was used for this study. 32 very thin steel wires were woven around 14 longitudinal and 18 transverse string sections of the racket head. This resulted in a 14 by 18 wire matrix, covering a large area of the racket head. Each steel wire was electrically insulated and connected to a charge amplifier by a thin, shielded cable. The thin cable bundle from the 32 sensors ran along the racket handle to a small electronic unit, which was attached to a belt and carried by the subjects. Total racket weight was increased slightly by the wires, but was still well within the range of weights found in commercially available tennis rackets. Through its friction with the ground and during its flight through the air, the tennis ball was electrically charged before it made contact with the racket head. At ball contact, an electrostatic charge was detected by the steel wires and electronically processed by their charge amplifiers. Using a data acquisition system with a high sampling frequency, each wire was sampled with 5 kHz. In a pre-trigger mode, data were collected for a total of 12 ms, beginning 4 ms before initial charge detection by the sensors. Following data collection, further mathematical processing was performed by multiplying the charge values of each of the longitudinal sensors with all transverse sensors, resulting in a 14 by 18 matrix of numbers. Geometric averaging of all matrix values determined the point of contact on the string area. RESULTS AND CONCLUSIONS: According to the judgment of experienced tennis players, the handling of the instrumented racket was very similar to a regular racket for forehand and backhand strokes, as well as for the serve. Only for topspin and slice strokes was a slightly increased spin production on the ball recognized. Subjects felt comfortable with the racket, some of them achieving ball speeds over 190 km/h for their serves. The matrix sensor arrangement and the time resolution of 0.2 ms guaranteed an accurate determination of the contact location of the ball and its movement across the string area. From these data, ball movements on the racket head can be shown for typical examples of forehand, backhand, topspin and slice strokes, as well as for serves.