**2. Factors influencing human perceptual system of heaviness**

Figure 1 shows a schematic that provides an overview of three categories of factors influencing human perceived heaviness: (1) factors related to the sensorimotor system responsible for lifting and holding an object (Object Lifting Phase), (2) factors or physical properties relating to an object itself (Object), and (3) factors relating to perceiving or judging heaviness (Perceiving Heaviness Phase). These factors act not only individually but also interactively. It should be emphasized here that the human psychological unit of heaviness differs from the physical unit of measuring weight.

Fig. 1. Factors influencing human perceived heaviness. Question marks indicate whether or how humans use perceived heaviness for subsequent object lifting. This is controversial and requires further investigation (See Sec. 5.1).
