The Role of Toss Zenith and Impact Height in the Relationship Between Shoulder Rotation Strength and Serve Speed in Junior Tennis Players

<b>Background:</b> Serve speed in tennis can be influenced by physical strength and toss kinematics, yet their interaction remains unclear in junior athletes. This study investigated the relationships between toss-related variables (zenith height, impact height, and zenith-impact differential), shoulder rotation strength, and serve speed in junior tennis players. <b>Methods:</b> Thirteen nationally ranked junior male players (mean age: 15.8 ± 0.9 years; body mass: 65.9 ± 5 kg; height: 181 ± 7 cm) participated. Isokinetic shoulder internal and external rotation strength (concentric/eccentric at 210°/s and 300°/s) were measured. High-speed 3D motion capture (Qualisys) recorded toss zenith, impact height and zenith-impact differential during flat serves directed into a designated 1 × 2 m target zone. Serve speed was measured by a radar gun. Correlation, mediation, and moderation analyses were conducted (<i>n</i> = 13; sensitivity ρ ≥ 0.69 for 80% power). <b>Results:</b> Impact height showed a significant positive correlation with successful (ρ = 0.746, <i>p</i> = 0.003) and unsuccessful serve speed (ρ = 0.776, <i>p</i> = 0.002). Moderation analyses showed that lower variability in impact height (<i>p</i> = 0.043) and in zenith-impact differential (<i>p</i> = 0.048) significantly strengthened the association between shoulder-rotator strength and serve speed. <b>Conclusions:</b> Moderation analyses revealed that lower variability in impact height and in the zenith-impact differential strengthened the relationship between shoulder strength and serve speed. Preliminary evidence suggests that consistent toss execution could enhance the translation of shoulder-rotation strength into serve speed in junior players.