The predictions of a movement control model which states that the degree of motor program control decreases as movement duration increases, were tested in three experiments. The findings indicated that the standard deviations of movement times increases as movement duration increased. However, at longer movement durations, the presence of a visual feedback display decreased the standard deviations of movement times. In Experiment III, the rate of movement was manipulated by varying movement distance while holding movement duration constant. Movement distance did not significantly affect the standard deviations of movement times, which suggests that the degree of motor program control is a function of the duration and not the rate of movement.
The effects of movement duration, distance, and the presence of a visual feedback display on the means and standard deviations of start times - the interval from a signal to initiate movement until movement began - were also assessed. Distance did not significantly affect either the means or the standard deviations of start times.
However, the shortest movement duration always produced the shortest and least variable start times. Continuous visual feedback increased the standard deviations of start times as compared to a terminal feedback or knowledge of results condition. It was suggested that the presence of continuous visual feedback "interfered" with the initiation of movements.
The effects of the independent variables on the ratio of the standard deviations of the two temporal components of the response - SD (start time)/ SD (movement time) - were reported.
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