When a robot system collides with environment not only the contact point but also the joints of the robot system experience impulsive forces or moments. However analysis on the so-called internal impulses acting on the joints has not attracted much attention although excessive impact may cause damage to the joint. In this work, we propose a closed-form internal impact model applicable to general classes of robotic mechanisms. The characteristics of the internal impulse are analyzed through simulation for serial-chain, closed-chain, and hybrid-chain systems. It is shown that the external impulse is distributed to the internal impulses of the various structures and that configurations best for external impulses might not be best for internal impulses.