This paper presents the design of a 1-DOF (horizontal forward-backward translational motion) power assist system (PAS) for manipulating objects in horizontal direction based on human operator's perception of object weight. We adopt a hypothesis that pertains to human's weight perception. The hypothesis means that the human must consider the mass parameter for the inertial force different from the mass parameter for the gravitational force when programming (feedforward) the load force (tangential to grip surfaces) for manipulating an object with a PAS because the perception and the reality regarding the object weight are different in this case. We simulated the system using Matlab/Simulink. Five subjects manipulated objects of three different sizes with the PAS during the simulation. Subjects subjectively determined the optimum values for the mass parameters of the inertial and the gravitational force components. Optimum mass parameters resulted in optimum maneuverability. Finally, we proposed using the findings to develop humanfriendly PASs for manipulating heavy objects in industries such as manufacturing and assembly, mining, logistics and transport, construction, disaster management, military operations etc.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.