2014 Volume 59 Issue 2 Pages 755-770
This study aimed to clarify the importance of strength and jump ability of both the kicking and supporting legs for increasing the speed of a soccer ball during instep kicks at various approach speeds. Twelve male university soccer players performed instep kicks using different approach lengths (1 m, 3 m, 7 m, and free length). Maximal isokinetic and concentric muscular strength was measured in terms of knee extension/flexion, hip extension/flexion, and hip abduction/adduction using an isokinetic dynamometer. Jump ability was measured using countermovement jump, double-leg rebound jump, and single-leg rebound jump with the kicking leg and supporting leg. For the instep kick, kinematic and kinetic data were recorded using the Vicon T20 system (250 Hz) and force platforms (1000 Hz). The results of the analyses were as follows:
1. The approach speed increased as the approach length increased. Moreover, the time between the moment when the foot touched the ground and the moment of ball impact became shortened, and the ground reaction force at all axes increased as the approach speed increased.
2. Foot speed under all the approach conditions was correlated with hip extension and the abduction strength of the supporting leg.
3. Foot speed for the 3 m, 7 m, and free-length conditions was correlated with the rebound jump ability of the supporting leg. Moreover, foot speeds for the 3 m, 7 m, and free-length conditions was correlated with hip adduction strength of the supporting leg.
4. Similar results were obtained for relative foot speed (calculated by dividing foot speed by the speed of the body's center of gravity at the moment of ball impact).
5. The rate of change in the relative foot velocity (for an approach of 1 m to 7 m) was correlated with the rebound jump ability of the supporting leg.
These results suggest that it is important to improve hip extension and the abduction strength of the supporting leg in order to increase the ball speed, regardless of the approach speed. In addition, it is important to improve the hip adduction strength, especially the rebound jump ability of the supporting leg in order to increase the ball speed when a high approach speed is employed.
