We studied the changes in the anticipatory postural adjustments (APAs), associated with dropping a load from extended arms and during fast bilateral shoulder flexion movements, when movements were performed in a self-paced manner and under a simple reaction-time instruction. The latter instruction applied time pressure and did not allow the regular pattern of APAs to be used. In particular, the following questions were asked: (1) are there changes in the relative timing of APAs under the reaction time condition; (2) are changes in the relative timing of APAs associated with changes in APAs themselves; (3) can different postural strategies be used to maintain stability under self-paced and reaction time conditions; and (4) are changes in APAs related to actual reaction time or to a change in the instruction? In particular, under reaction-time conditions, APAs occurred later in time, typically simultaneously with the initiation of the focal movement. Additional changes in electromyographic (EMG) patterns in postural muscles included an increase in the amplitude of EMG bursts and 'speeding-up' some of the tri-phasic patterns in postural dorsal-ventral muscle pairs. This was accompanied by a smaller early shift of the center of pressure followed by its more rapid delayed displacement. There was considerable variability in the changes of EMG and dynamic characteristics across subjects. Some of the changes in the EMG patterns in postural muscles depended on actual reaction time, while others were related to a change in the instruction and occurred even if actual reaction times were long enough to allow for the typical self-paced APA patterns to occur. These findings can be interpreted as supporting the parallel control hypothesis for the focal movement and postural adjustments. Alternatively, they can be interpreted within a framework that implies the generation of a single control function, which is transformed into two components, one directed at the focal muscles/joints and the other directed at postural muscles/joints.
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