Kinematic comparison among players of different level in tennis serve

The tennis serve was studied by many authors (Elliott 1995; Reid 2008). Although, in this previous works, kinetics and kinematics aspects were studied, but they did not analysed the differences of the body displacement as regards the racket and the body angles at the instant of impact, among players of different level. The aim of this study is to compare the ball, racket and body kinematics, in order to find the technical differences among Experts, Intermediates and Beginners players. Methods 18 right-handed subjects performed 16 serves each one, during a training session. The players were divided in 3 groups (based on the national ranking), 8 Experts 5 Intermediates, 5 Beginners. Three 50Hz cameras were located outside the court, with 120° between the optical axis angles. The serve area was calibrated with a 3D object with known dimensions (1,10 x 1,10 x 2 m.), using 16 points. Simi Motion Analysis System was used for a manual digitalization to obtain 3D coordinate data. 25 points were digitized from the same operator (19 for the body, 5 for the racket and 1 for the ball). Excel, Matlab and Spss softwares were used to analysed the data. The determination of the instant of impact was made by Camorani et al. (2007,P. 173). The variables were compared among the 3 groups of players using the Kruskal Wallis Test (p<0,05). Results Significant differences were found in ball velocities after impact, in all its directions among all groups, except for the lateral velocity between Intermediates and Beginners players. Regarding the racket differences in linear velocity were found only in the net direction among all groups. The racket angular velocity at impact were significant differences between Experts and Beginners and between Intermediates and Beginners players. The height of the ball impact was statistically different among all groups. The ball throw test (200g), used as explosive strength index, gave the same results of the racket angular velocity. Only 4 variables on 16 analyzed showed statistically significant differences among the groups. In particular, the wrist position (dominant arm) respect the racket centre on the sagittal plane at impact, resulted significant different between Experts and Beginners and between Intermediates and Beginners players. The lateral trunk flexion at impact did not show significant differences only between the Experts and Beginners groups. The left knee flexion (anterior knee) resulted significant only when the racket arrive at its lowest point during the cocking phase (RLP), and this significant difference was between Experts and Beginners. The racket longitudinal angle resulted significant only between Experts and Beginners. The sequent variables were not statistically different: the position of the shoulders respect the racket on the frontal and saggital plane, the position of the wrist respect the racket on the frontal plane, the shoulders angles between the frontal and saggital plane, the elbow angle, the wrist angle, the trunk angle between the frontal plane, the trunk torsion angle, the maximum knee flexion angle, the knee angle at impact, the racket trasverse angle. Discussion As expected the performance indexes are significant different among the groups. The anthropometric data and the height of impact of the ball, the muscular strength (ball throw test) can give an important explanation for significant differences showed by the ball and racket velocities. As appears from this study, the 3 groups use different techniques strategies during the cocking phase and at the instant of impact. During the cocking phase the maximum knee flexion is similar among the groups, but at RLP the knee angle shows a significant difference between Experts and Beginners. The up word force is more utilized from the Experts (confirmed by knee angle at impact). The Beginners cannot use the leg cocking, for the up force, in the right time section. Comparing the lateral trunk flexion, th...