Quantitative Evaluation of an Isometric and a Position Joystick for Children with Cerebral Palsy

Study of the interaction between individuals with disability and the computer has implications in the design of input devices, such as the mouse and keyboard, and leads to greater knowledge about movement control. The present work evaluated the performance of non-disabled individuals and those with cerebral palsy in the use of a spring-centered position joystick versus a position-control isometric joystick. The subjects acquired differently sized targets at different distances by positioning a cursor on the target. Performance with the position joystick was superior when compared with the performance with the isometric joystick. It was found that due to the high stiffness of the isometric joystick, physiological noise was recorded. This led to uncontrolled movement of the cursor on the screen, which added to the distance traversed by the cursor and increased the time needed to reach the target. With the position joystick, the inertial load due to the displacement of the stick and the elastic action of the spring provides damping of the physiological noise. Movement time was directly proportional to the cursor-target distance, and inversely proportional to the target size. The phase-plane plot and trajectory of the cursor movement revealed the presence of one primary movement that accounts for the peak velocity, and several submovements. Fitts' paradigm was a good predictor of cursor positioning task for the non-disabled individuals. Subjects chose to move more slowly towards the smaller and closer targets, and they increased speed for larger and more distant targets.