Objects of different sizes were lowered from a certain height by subjects manually. Then the same objects were lowered from the same height by the same subjects with a power assist system. We then analyzed subjects' heaviness perception, load forces and objects' motion features for the manually lowered objects and compared them to that for the power-assist-lowered objects. The distinctions in perceived heaviness, load forces and motion features were proposed to use to determine an appropriate human interactive control method for power assist systems for lowering heavy objects in various industries such as manufacturing and assembly, transport and logistics etc. that may enhance human operator's interactions with the power assist systems in terms of maneuverability, safety, stability, naturalness, situational awareness, motions, haptic and tactile feelings etc.