The back pain is the most common injury in human activities where heavy objects must be lifted or must be suspended for a long time. A weight lifting exoskeleton also known as force augmentation exoskeleton is designed to reduce the strain on the back and the limbs and reduce the risk to suffer injuries. On the other hand, different kinds of controllers have been implemented to achieve whit this goal, for example, a conventional PD Control, PD Control with Gravity Compensation, PD Control with Adaptive Desired Gravity Compensation and PD Control with Robust Compensator. This paper aims to evaluate and compare the performance from the previously cited controllers used to reduce the strain in the back, through the implementation of each controller in a three Degrees Of Freedom (DOF) exoskeleton powered by pneumatic muscle actuators; some numerical simulations as well as experimental trials have been conducted and three different performance indices were used in order to determine the effectiveness of each one with respect to the simple PD controller when the mass to be lifted is unknown.