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This paper presents an investigation focused on cooperation of a small industrial robot and a human operator. During their cooperation the collision is expected only between the robot end-effector and the lower arm of the human worker. To ensure safe and comfortable human-robot interaction the impact must not cause any injury or considerable pain to the human operator. An investigation with human volunteers was performed, to study the impact between the robot and the man. To determine a safe range of the impact intensity with human subjects, a preliminary investigation was performed on one of the authors of this paper. During the experiment, the volunteers determined the pain they felt after each impact using a graphical user interface. In the experiments, the robot end-effector collided with the volunteer's lower arm perpendicularly at a constant deceleration. The point of impact was positioned midway between the wrist and the elbow on the dorsal aspect of the lower arm. The robot end-effector was displaced from the initial point toward the point of impact along a straight line. Several tests were carried out at maximal velocity, different robot decelerations, various end- effector shapes, and varying depth of stop point with regard to the arm surface. The impacts between robot end-effector and human arm were evaluated using a measuring system comprised of a force sensor and an optical measuring system. The goal of the investigation was to determine the correlation between the pain intensity felt during the impact and the impact energy density.