The purpose of this study is to investigate the relation between the isokinetic strength and the ball speed, which has not yet been studied. In addition the relation between the ball speed and the muscle strength was analyzed when the kick was done explosively with a small swing range while changing the approach distance of the supporting leg.
　The subjects are 14 males having experience in soccer, including 3 players of the National team level. The muscle strength was measured with Cybex II, the leg being extended while sitting on a chair. The measurements were done for the isometric strength and the isokinetic strength with measuring speeds of 300, 240, 180, 120 and 60 degrees per second. The maximal speed of the ball kicked with the instep was measured by means of photocells. The approach distance was varied in 7 levels: no step. 30, 60, 90, 120, 150cm and no restriction (the maximal distance).
　The results are as follows;
　1) A high correlation (r=0.935) was seen between the isometric strength and the ball speed. It indicates that the leg extension power is an important factor of the instep kick.
　2) Significant correlations were seen between the isokinetic strength and the ball speed at the respective measuring speeds. The correlation coefficient was relatively high (r=0.886) at the low speed of 60 degrees per second, and low (r=0.682) at the high speed of 300 degrees per second. It shows that the ball speed is greatly influenced by the low-gear power.
　3) The dynamic strength was found to be necessary for an explosive kick at the small approach distances (no step and 30cm).
　4) The players of the National team level were superior in the isometric strength, the isokinetic strength and the ball speed. They tended to have a higher ball speed even if their muscle strength was equal to that of other subjects.

DESCENTE SPORTS SCIENCE Vol.29/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE