Activation timing of postural muscles during bilateral arm flexion in self-timing, oddball and simple-reaction tasks.

Background: We investigated the effect of time constraint on activation timing of postural muscles during bilateral arm flexion in self-timing, oddball and simple-reaction tasks.

Methods: Thirteen healthy adults flexed their arms from a suspended position with maximum speed and stopped at the shoulder level. For erector spinae-longissimus (ES), biceps femoris (BF), and gastrocnemius (GcM), onset timing of burst activation with respect to the anterior deltoid (AD), and the displacement of the center of pressure in the anteroposterior direction (CoPy) were analyzed.

Results: AD reaction time was significantly shorter in the simple-reaction task than oddball task, suggesting that time constraint would be lower in the task order noted above and affected the state of postural preparation. The following properties were found in the onset timing of postural muscle: (1) the onset timing of BF and GcM were earlier in the task order noted above, and (2) the earliest activated distal muscle in the self-timing task was activated later in the simple-reaction task. CoPy displacement was smaller in the same task order as the onset timing.

Conclusions: It appears that with sufficient postural preparation, the focus of postural control is on the reduction of postural disturbance and earlier lower leg muscle activation.