Abstract: Objcetive: By testing the variation of stability, speed of jump services and the success rate of volleyball players, the mechanism on how core training improves the speed and success rate of jump services is discussed here. Method: 36 volleyball players of various proficiency level were trained 3 times a week for 16 weeks, with five tests on side bridging,medicine ball side slamming , 1-min sit-ups, speed and success rate of jump services taken respectively before core training(H₀), by the end of the fourth week(H₁), by the end of the eighth week(H₂), by the end of the twelfth week(H₃), and by the end of the sixteenth week(H₄). Result: The 36 athletes have acquired significant improvement in core stability. Changes are significant in time spent on left and right side bridging and amount of sit-ups during phase H₂-H₁, H₃-H₂ and H₄-H₃. There is improvement on the distance of medicine ball side slamming during phase H₄-H₃ but no significant difference is noticed. Moreover, during phase H₃-H₂ there appears great increase and athletes with higher proficiency level experience more improvement. Speed and success rate of their jump services increase greatly compared to what was recorded before training. There is significant difference in the increasing of speed of jump services during phase H₂-H₁、H₃-H₂. Significant difference in the increasing of success rate of their jump services has been noticed during phase H₂-H₁、H₃-H₂ and H₄-H₃, and during phase H₃-H₂ athletes with higher proficiency level experienced less significant improvement in success rate of their jump services. Conclusion: The stability of core musculature in different horizontal planes of the human body might be the key to influence the speed of jump services. The mechanism on how core training influences the speed and success rate of jump services may lie in the joint effect of core stability of different horizontal planes of the human body.

Key words: Core Strength Training, Volleyball Athletes, Core Stability, Jump Service Speed, Success Rate