Kinematics differences between the flat, kick, and slice serves measured using a markerless motion capture method

Tennis injuries have been associated with serving mechanics, but quantitative kinematic measurements in realistic environments are limited by current motion capture technologies. This study tested for kinematic differences at the lower back, shoulder, elbow, wrist, and racquet between the flat, kick, and slice serves using a markerless motion capture (MMC) system. Seven male NCAA Division 1 players were tested on an outdoor court in daylight conditions. Peak racquet and joint center speeds occurred sequentially and increased from proximal (back) to distal (racquet). Racquet speeds at ball impact were not significantly different between serve types. However, there were significant differences in the direction of the racquet velocity vector between serves: the kick serve had the largest lateral and smallest forward racquet velocity components, while the flat serve had the smallest vertical component (p < 0.01). The slice serve had lateral velocity, like the kick, and large forward velocity, like the flat. Additionally, the racquet in the kick serve was positioned 8.7 cm more posterior and 21.1 cm more medial than the shoulder compared with the flat, which could suggest an increased risk of shoulder and back injury associated with the kick serve. This study demonstrated the potential for MMC for testing sports performance under natural conditions.