Tennis: the influence of grip tightness on reaction impulse and rebound velocity.

The effect of grip firmness between a tennis racket and a mobile hitting arm on post-impact ball velocity and reaction impulse have been investigated. A pneumatically driven racket-arm was triggered by a stimulus from a photoelectric cell positioned at the exit nozzle of a ball machine so that impact occurred with the racket perpendicular to the path of the ball. Tennis balls were fired to strike each horizontally positioned racket at four locations: the geometric center of the strings, 5 cm from the center towards the distal end, 5 cm closer to the shaft from the center, and 5 cm above the center. The average horizontal velocity of the ball, both before and after impact, was determined by using stroboscope photography. Reaction impulse (the integral of the force recorded following impact over a period of 50 ms), collected using Kistler force transducers, was photographed from a storage oscilloscope and measured over the 50 ms with a digitizer. A non-significant increase in rebound coefficient and reaction impulse in the direction of the hit were recorded for central impacts with an increase in grip tension. The largest rebound coefficients, and therefore the rebounds with the highest velocity, occurred at the center of the strings with a tight grip. Decreases in rebound coefficients and increases in reaction impulses were recorded for off-center impacts when compared to the central impacts. Significant increases in rebound coefficients were recorded for an increase in grip pressure for off-center impacts.