User-specific Tennis Racket Selection Method by Use of Motion Capture and Dynamic Analysis

Abstract

Among many properties of tennis racket, moment of inertia (MOI) is a crucial property to consider when choosing a racket, as it may not only affect the power of a stroke, but also potential for injury. A careful choice of the property can be made by feeling the racket weight while swinging it, but this process is too subjective and is not accurate for nonprofessional tennis players. Therefore, more objective and generalized method is needed for tennis players of various playing levels. The purpose of this study is to propose a new method of selecting the optimal MOI of a racket that maximizes the post-impact ball speed while preventing upper limb injuries, through kinematic analysis and dynamic analysis of the forehand swing motions with different racket MOI. Motion capture was performed on forehand motion of 3 skilled tennis players for 5 different racket MOI, and then kinematic analysis and dynamic analysis were done to determine the racket swing speed at the time of impact and the peak torque applied on shoulder, elbow, and wrist joints. A simple racket-ball collision model which follows conservation of momentum law was used with the racket swing speed to predict post-impact ball speed. Isokinetic measurement was done on the shoulder, elbow, and wrist joint motions to normalize the peak torque values determined from the dynamic analysis. This study discusses how these post-impact ball speed and the normalized peak torque values can be used to evaluate racket MOI on each individual player. This study proposes an idea of normalized joint torque limit, which is the percentage of the joint strength allowable in repetitive sports such as tennis, and this idea was implemented to set the joint torque limit for each player. By doing so, the racket MOI with maximum post-impact ball speed while below normalized joint torque limit could be found for each subjects. The value of the normalized joint torque limit needs to be corrected based on experimental data in the future study, and this would improve the method for more accurate racket selection.