Field-Based Biomechanical Analysis of Preparation Timing and Ball–Racquet Coordination in Tennis Forehand Groundstrokes Across Incoming-Ball Speeds and Skill Levels

This field-based study examined whether bounce-referenced preparation timing during tennis forehand groundstrokes was associated with incoming-ball speed and skill level, and whether this timing descriptor related to local ball–racquet dominance around bounce. Thirty male university players (15 high-skill, 15 low-skill) performed continuous baseline forehand topspin rallies. Analyses included trials with valid incoming-ball speed within a common-support range of 60–110 km/h and ball landings within a predefined central target zone. Preparation timing was quantified using an Index of Preparation (IdP), defined as the signed offset between racquet set completion and ball bounce within normalized stroke progress. Trial-level IdP and bounce-window ball-dominant occupancy (BDO), derived from coupling angle mapping, were analyzed using participant-clustered models; Gaussian mixture summaries were retained as secondary descriptive analyses. In low-skill players, higher incoming-ball speed was associated with lower IdP, whereas this decrease was attenuated in high-skill players. Lower IdP was also associated with higher BDO, suggesting that delayed preparation was accompanied by greater local ball-dominant organization around bounce. These findings support IdP as a field-based performance analysis descriptor, while BDO-derived measures should be interpreted as exploratory descriptors requiring further validation against simpler coaching indicators and direct performance outcomes.