INTRODUCTION: Beachvolleyball is one of the most successful competitive sports in germany besides indoor. Even though kinematic and kinetic differences between indoor and sand condition for different jump types could be proven (1,2), the influence of sand type on those jumps and on injury potential is unknown (3,4). Hence, the aim of this study is to investigate biomechanical variables for the hard floor condition and two sand types differing in degree of hardness. METHODS: 18 young elite volleyball players (1m/17w) were analyzed. Six different jump conditions including volleyball specific jumps (block, 3 spike forms) as well as standard jumps (CMJ with/without arms) were analyzed on the three grounds. Kinetics and kinematics were acquired synchronously using insole plantar pressure measurements and IMU-based motion capturing. Kinetic variables include stance time, maximal force and rate of force development (RFD) at takeoff for each limb. Jump height and maximal force for the landing phase were calculated. Kinematic variables include maximal knee flexion angle at takeoff and landing as well as knee flexion at IC. RESULTS: Kinematic and kinetic differences on the three grounds were not consistent between general and volleyball specific jump types. There was no difference of contact times in the general jumps between grounds, but the closing time in all three spike jump conditions was reduced by 10% in both sand conditions. Contact time of the leading leg didnt differ. The longer contact time of the providing leg goes along with a deeper knee flexion angle of about 15% which wasn`t shown in the general jumps. Secondly RFD for both limbs was highest on the hard sand in the general jumps, but highest in hall for the spike jump conditions. Thirdly jump height reduced on the sand conditions in the general jumps by about 3 cm and in all spike jump conditions by about 10 cm. Fourthly players tended to land equally on both legs in the general jumps, but landed primarily on the providing leg after the spike jump on all the grounds. In all jumps maximal forces in the landing phase was highest on hard floor. In contrast in all jump conditions maximal force of the leading leg in the take-off phase was highest in the hard sand condition and lowest in the soft sand condition. Consequently, differences between sand type for take-off was highest. CONCLUSION: Our results show that general jumps are limited to indicate kinetic and kinematic differences in spike jumps between the three grounds. Furthermore, it has been shown that sand type may have a great influence on maximal force and RFD, but not on knee flexion angle and flight time. Since high maximal forces and high RFDs in the take-off phase might be a risk factor for chronic knee pain in volleyball players (5), further studies should examine the influence of sand type on movement mechanics.
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