It was reported that when the upper or lower limbs were used in a simultaneously bilateral manner, bilateral deficit was observed compared to that in a unilateral manner (Vandervoort et al., 1984;Koh et al., 1993). This study were to determine the effect of resistance training or practice for shortening the response speed on the bilateral deficit and to investigate the mechanism mediating it. Findings from the study were as follows. Force exerted bilaterally was increased more by bilateral training and force exerted unilaterally was increased more by unilateral training. Therefore, lateral specificity exists in the effect of resistance training. Bilateral indices (BI) (Howard & Enoka, 1991) for the untrained upper/lower limb of the unilateral training group were shifted in a negative direction as for trained lower/upper limb. However, BIs for the untrained upper/lower limb in the bilateral training group were not shifted in a positive direction as these indices are for trained lower/upper limb. Practice in shortening the response speed improved the reaction time. However, BI was not changed because the bilateral reaction time shortened accompanying with unilateral left reaction time. It was suggested that a program for unifying responses of the left and right hand exists in the right hemisphere and that bilateral reaction time was elongated by this added process. Though bilateral deficit in strength and reaction time seemed to be the same phenomenon, mechanisms mediating both phenomena were not suspected to be completely same.