This study was designed to examine how baseball batters adjust their batting actions to slow-speed and fast-speed pitches by measuring the ground reaction forces. The followings were revealed during different phases of the batting action. 1) At the take-back motion phase, the weight was transferred onto the catcher-side foot. The ground reaction force of the left foot, i.e., the pitcher-side foot, was increased immediately to fix the left foot firmly on the ground at the bat-swing phase. 2) In batting the slow-speed pitches, fixing the stepped left foot on the ground was remained momentarily before the bat-swing action. 3) In batting the slow-speed and fast-speed pitches, the ground reaction force curves of the left foot at the bat-swing phase were almost the saae. 4) The bat swing was started after fixing the left foot on the ground. 5) In each batting for the slow-speed and the fast-speed pitches which were mixed at random, the beginnig-phases of the batting motions were started at the almost same time. These analyses suggest the followings: 1)Ddecision on hitting the pitch is made before fixing the stepped foot firmly on the ground. 2)By momentary fixation of the stepped foot on the ground, the bat-swing action is synchronized to the pitch for correctly timed hitting. 4)When it is difficult for batters to predict whether the next pitch will be fast-speed or slow-speed, batters' actions may be prepared to hit the fast-speed pitches. And when the pitched ball is slow-speed, batters may adjust the start of the bat-swing action by momentary fixation of the stepped foot on the ground.
