An inverted-–U relation between spatial error and force requirements in rapid limb movements: Further evidence for the impulse-variability model.

Earlier we showed that there is an inverted-U relation between the force produced and the within-subject variability in that force. This finding is used together with a simple model to predict that the spatial inconsistency (in the vertical plane) of a rapid horizontal arm-swing movement should show an inverted-U relationship with the force requirements employed. Three experiments examined this prediction. The first two involved changes in the mass to be moved at a fixed movement time; the third involved the variation in movement time with constant mass. In all three studies as the force requirements of the response were increased, increased inconsistency occurred in the spatial dimension, but only to about 60%70% of the subject's maximum. Further increases in force re-requirements (from increased mass or decreased movement time) resulted in decreased spatial variability. These findings are not consistent with the general ideal of speed-accuracy trade-offs, but they are predictable from a modified impulse-variability model for rapid programmed actions.