The subject of this paper is about the relationship between the stiffness and the transparency of Cable-Driven Parallel Robots (CDPRs) used as human-machine interfaces in object comanipulation tasks. An index quantifying the transparency of a CDPR is first introduced. The stiffness of the robot is determined in simulation which parameters have been experimentally identified. Particular attention is paid to the effect of the Moving-Platform pose and cable tension management on CDPR stiffness. Then, the relationship between the stiffness and the transparency is analysed. Finally, the transparency index is traced throughout the constant-orientation static workspace and throughout the cable tension feasibility polygon for a given MP pose.