Geometrical conditions for ballistic and visually controlled movements.

The ballistic movement is defined as a rapid voluntary movement which is motor programmed and in which visual feedback for path correction is not possible. A discrete ballistic movement is of short movement duration (less than about 200 msec) with a triphasic pattern in the electromyograph (EMG) for muscle activity and has an acceleration pattern which approximates a sinusoidal waveform. When the Index of Difficulty for such a movement is less than 3·0, the movement time is found to be related to the square root of the movement amplitude. The occurrence of ballistic movement is determined by the combined factors of Fitts' Index of Difficulty and movement amplitude.