A recruitment theory of force-time relations in the production of brief force pulses: the parallel force unit model.

A theory, the parallel force unit model, is advanced in which the buildup and decline of force in rapid responses of short duration are assumed to reflect variability in timing of several parallel force units. Response force is conceived of as being a summation of a large number of force units, each acting independently of one another. Force is controlled by either the number of recruited force units or the duration each unit contributes its force. Several predictions are derived on the basis of this theory and are shown to be in qualitative agreement with empirical findings about both the mean and variability of brief force impulses. The model also has consequences for the temporal properties of a response. For example, under certain circumstances, it predicts a reciprocal relation between reaction time and response force. Although the theory is proposed as a psychological account, relations between the assumptions and basic principles in neurophysiology are considered. Possible future applications and generalizations of the theory are discussed.