Poor Joint Work in the Lower Limbs during a Tennis Forehand Groundstroke after a Cross-Over Step Inhibits an Increase in the Racket Speed.

A forward run-up and stepping are used to accelerate hitting tools or throwing objects in sports. This study aimed to investigate the effect of a forward cross-over step on the speed of a hitting tool by analyzing the joint work and mechanical energy of the whole body and the hitting tool using inverse dynamics. Thirteen advanced tennis players performed forehand groundstrokes at maximum effort with and without a forward cross-over step. From the whole body plus racket perspective, the body-weight-normalized mechanical energy at the start of the hitting motion increased by 1.74 ± 0.42 J·kg^-1 due to the cross-over step. However, the increase in the magnitude of total negative joint work, primarily attributed to the lower limbs, was 1.38 ± 0.31 J·kg^-1 due to the cross-over step, conventionally regarded as energy absorption. Consequently, the mechanical energy of the whole body plus the racket at ball impact was comparable between the conditions. Nevertheless, from the segmental perspective, the mechanical work performed by the net shoulder joint force of the playing upper limb with the cross-over step during the hitting motion was greater than that without the cross-over step. Subsequently, the slight increase in the mechanical energy of the playing upper limb plus racket (0.25 ± 0.21 J·kg^-1) resulted in increased racket speed (4.3%). Considering the comparable total mechanical energy and a resultant increase in racket speed, players and coaches should not overestimate the effect of the forward step on racket speed.