An active servo-system was used to change the stiffness of a manipulandum used in a position-control pursuit-tracking task. The elastic stiffness of the manipulandum connected to the forearm was set by a computer at one of five levels ranging from 0 N/m to 2000 N/m. Subjects were required to track, either by moving their forearm or by generating a force isometrically, a visually presented target whose position changed randomly every second for 100 s. Nonparametric and parametric impulse response functions were calculated between the input (target) and output (force or position) in each tracking condition, and revealed that for all subjects force control was faster than position control when the stiffness of the manipulandum was set at 0 N/m. Subjects were also consistently faster in reaching the target when the stiffness was greater than zero, and were more accurate (steady-state response) when the stiffness of the manipulandum was set at lower rather than higher amplitudes. The parametric impulse response functions revealed that the human operator system was underdamped (0.7) with a natural frequency of approximately 8 rad/s. These findings were interpreted in terms of the responses of the various subsystems (visual, cognitive, contractile, limb mechanics) that comprise the human operator's response.