Are amplitude and duration of anticipatory postural adjustments identically scaled to focal movement parameters in humans?

Anticipatory postural adjustments (APA), are known to precede the onset of voluntary movement. The aim of this study was to determine whether APA amplitude and duration are programmed identically according to the voluntary movement parameters. Subjects performed shoulder flexions at maximal and sub-maximal velocities, with and without an additional inertial load. APA amplitude and duration were plotted against kinetic energy and work of the forthcoming voluntary limb movement and showed a significant linear relation. When APA duration was plotted against mechanical work, two distinct APA duration values corresponded to a given value of work depending on the weight of the load, but this was not the case for APA amplitude. It is suggested that the APA amplitude and duration are scaled according to the same movement parameters, but not in the same way, APA duration being additionally sensitive to the postural effect which results from an additional weight.