We examined mechanisms of coordination that enable skilled recreational baseball players to make fast overarm throws with their skilled arm and which are absent or rudimentary in their unskilled arm. Arm segment angular kinematics in three dimensions at 1000 Hz were recorded with the search-coil technique from the arms of eight individuals who on one occasion threw with their skilled right arm and on another with their unskilled left arm. Compared with their unskilled arm, the skilled arm had: a larger angular deceleration of the upper arm in space in the forward horizontal direction; a larger shoulder internal rotation velocity at ball release (unskilled arms had a negative velocity); a period of elbow extension deceleration before ball release; and an increase in wrist velocity with an increase in ball speed. It is suggested that some of these differences in arm kinematics occur because of differences between the skilled and unskilled arms in their ability to control interaction torques (the passive torque at one joint due to motion at adjacent joints). It is proposed that one reason unskilled individuals cannot throw fast is that, unlike their skilled counterparts, they have not developed the coordination mechanisms to effectively exploit interaction torques.