For immediate motion, taking a ready posture is a promising way because it can reduce enormous possible patterns of movement. For the selection of an appropriate ready posture, it is essential to know the structure of the task that includes the physical characteristics of a robot. We investigated our previous-built badminton robot arm to decide an appropriate ready posture as a case study. We first examined the kinematic characteristics to extract possible ready postures. In this analysis, we found that postures for hitting shuttles are limited to approximately one-tenth of all possible postures. Besides, these postures could be spatially divided into two clusters, one is for forehand shots, and the other is for backhand shots. Then, we calculated reaching time from various postures and confirmed that possible ready postures by kinematics analysis shortened averaged reaching time. We also found that there are more effective ready postures if considering the dynamics of the robot. These ready postures did not resemble those calculated by kinematics analysis.