Terminal accuracy of unexpectedly loaded rapid movements: evidence for a mass-spring mechanism in programming.

Experiments using rapid-positioning movements in humans, where the subject is suddenly and unexpectedly provided with a change in the load characteristics of the limb, are described. Taken together, the pattern of results supports a mass-spring model of unidirectional limb action, where the limb moves to a position defined by the relative tensions in the agonist and antagonist. As well, various results provide evidence contrary to predictions from an impulse-timing viewpoint, where the motor program times the onset of impulses to the musculature, and against a feedback-processing viewpoint, where limb position is defined by minimizing positioning error indicated by feedback. The evidence suggests that the role of phasing in motor programs may be different for unidirectional actions on the one band and multi-directional and/or multi-component actions on the other.