AMPLITUDE AND DURATION SCALING OF BRIEF ISOMETRIC FORCE PULSES

The predictions of 2 rival models for the production of brief isometri c force pulses were tested in 3 experiments. The prototype-function mo del assumes that an observable force pulse emerges from an underlying prototypical function by scaling its duration and amplitude. The paral lel-force unit model (PFUM) assumes that force pulses emerge from the concerted activity of a large number of force units with invariant for m. Experiment 1 showed that the shape of single-trial force-time funct ions depends mainly on their rise times but not on their amplitudes. E xperiment 2 tested the same predictions for mean force-time functions at 3 levels of rise time and 3 levels of peak force. Experiment 3 exam ined shape constancy for less forceful pulses. In sum, the results ind icate that the shape of force pulses is approximately preserved if the y vary in amplitude but not in duration. They become more symmetrical as rise time increases. This finding does not support a prototype-func tion model and supports the predictions of PFUM. Some additional findi ngs on force variability are discussed and compared with the predictio ns of PFUM. The data suggest some modifications of PFUM.