This paper reinterprets data from five recently published experiments on response complexity and programming time in which number of movement parts of rapidly executed limb responses was advanced as the principal element influencing length of response programming time. Alternatively it is argued here that programming time, in these experiments, was predominantly a function of the constraints placed upon the output of the motor system by the demand for movement accuracy. The quantification of this accuracy demand is achieved by using the metric of Index of Difficulty (Fitts, 1954). In discrete and straight-line tapping responses to circular targets, response complexity may be conceptualized in terms of the angular constraint imposed on movement initiation at the start key. When responses require changes of direction between movement parts it is proposed that programming time may be a function of the cumulative movement constraints imposed by the task. The discussion focuses on the process by which an increased accuracy demand requires a more constrained motor system output which is brought about by a larger and/or more precise muscle synergy recruitment pattern, resulting in an increase in programming time. © 1988, Elsevier Science & Technology. All rights reserved.
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