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This paper deals with the design and control of a power assist system for lifting objects. It was hypothesized that an operator's perception of weight due to inertial force might be different from the perceived weight due to gravitational force. The system was simulated for different sizes of objects. Results show that an increase in mechanical time constant reduces perceived heaviness and load forces. The results also show the technique for simultaneous optimization of motion and perceived heaviness. Psychophysical studies show that the power assist system reduces the weights of the lifted objects to 40%, 30%, and 25% of the actual weights for unimanual, bimanual, and cooperative lifting tasks, respectively. The studies also show that operators apply 6.30, 6.22, and 6.14 times larger than the actually required load forces for the unimanual, bimanual, and cooperative lifting tasks, respectively. Therefore, a novel control strategy was proposed that reduced the excessive load forces and thus enhanced maneuverability, safety, naturalness, etc., of the system. The feasibility of zero gravity and zero inertia was studied. The operator's load force characteristics in some worst cases were studied. The effects of friction between an object and the operator's hand on weight perception and load forces were also studied. Power-assisted manipulation of objects in harmonic motion was also considered. Finally, it was proposed to use the findings to design power assist systems for lifting heavy objects in industries such as manufacturing, logistics, transport, construction, military and rescue operations, mining, etc., that would enhance interactions with human users.