In designing a human-computer interface, it is vital to understand the principle behind motion generation in humans. Previous research has mostly focused on kinematics and not kinetics. In this paper, we present the characteristics of human behavior while performing force tracking tasks. Here we present a preliminary study in which three type of measurements were conducted with three healthy young adult male subjects to characterize force modulation (discrete, rhythmic, and target transition). Subjects were instructed to control the force exerted to the load cell during a force pursuit tracking task. The visually-guided discrete attempt investigated human behavior when the target changed discretely (every 5 seconds). The rhythmic attempt investigated human behavior when the target changed continuously and periodically (every 1 second). The target transition required subjects to alter the behavior of the input force. The results suggest the following: the differential value of the force generated by a human is sufficiently large, while the response speed is based on visual information; humans can predict future reference to a small degree and attempt to improve or correct their force; and the rule of superimposition comes into effect with regard to the differential value of force.