In the computer analysis of mechanisms it is usual to choose the actuators that drive them in terms of the ease with which they can be introduced into the definition of the system rather than in terms of describing the reality. Once the force of the replacement actuator used has been obtained, that of the real actuator must be determined and some of the calculated constraint forces differ from the real ones. This work studies the situation described in the inverse dynamic analysis of mechanisms of 1-DOF. It will show how to obtain the force of the real actuator from the force of the actuator used in the simulation process, and proposes the use of auxiliary mechanisms which aid in the description of the real actuators. It also provides criteria for determining which constraint forces obtained in the simulation process with the replacement actuator are different to the real ones.