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Abstract
A model is proposed in which movement accuracy is regulated by means of corrective
actions taken at discrete intervals throughout the course of a movement. A movement,
as represented by its tangential velocity profile, cna be decomposed into a series
of one or more submovements. Each submovement consists of a prototype velocity profile
which can be scaled in magnitude and duration. For planar two-joint movements, we
demonstrate that these submovements can be mathematically represented either in terms
of velocity profiles or in terms of the underlying joint torque profiles. In either
case, the submovements superimpose linearly to produce the composite movement. The
model provides a very good fit to tangential velocity profiles recorded from human
subjects during three-dimensional arm movements with constraints on accuracy and speed.
The model assumes that when a submovement is present, its onset is associated with
a change in the direction of the hand path and/or a zero crossing or inflection in
at least one of the components of the velocity vector. The model is consistent with
a strategy in which precision is achieved by periodic discrete actions which redirect
the moving arm in order to bring the hand closer to the target. Since submovements
were also observed in slow movements where accuracy constraints had been relaxed,
we hypothesize that the strategy of superimposing a series of submovements to make
one composite movement may be a general one. We suggest that it would be particularly
appropriate for the process of learning a new motor skill.