Training junior tennis players to increase knee flexion improves their service performance

This study aimed to assess whether training to increase knee flexion during the tennis serve improves performance and to explore the associated biomechanical changes across the body. Twenty junior tennis players were randomly allocated into control (standard in-season training) and training groups (received training to increase knee flexion during serve). Inertial sensors tracked full body and racket kinematics during five serves performed in pre- and post-training assessments. Racket velocity, impact height, and lower- and upper-body kinematics were compared. Training increased serve knee flexion by 31° (p < 0.001), leading to a 1.38 km/h increase racket velocity (p = 0.036) without affecting impact height (p = 0.331). Additionally, training increased: range of front leg knee extension (MD = 23.46°, p < 0.001) and extension velocity (MD = 54.28°/s, p = 0.008), hip range of motion (front: MD = 53.60°/s, p = 0.003; back: MD = 57.28°/s, p = 0.015), pelvis upward velocity (MD = 0.27 m/s, p < 0.001), and trunk contralateral flexion velocity (MD = 23.18°/s, p = 0.025). No main effects were found for shoulder internal rotation (p = 0.304) and elbow extension (p = 0.214) velocities. No changes were observed in the control group other than a decreased trunk contralateral flexion velocity (MD = -28.98°/s, p = 0.007). Specific training can, therefore, increase serve knee flexion. This study highlights that specific training to increase knee flexion can enhance serve performance by increasing racket velocity, without increasing upper limb joint contribution.