Flexibility of anticipatory postural adjustments revealed by self-paced and reaction-time arm movements.

In standing humans, it is not certain whether anticipatory postural adjustments associated with rapid, voluntary elbow flexion movements (focal movements) originate as a selection from preset synergies or as the result of specific planning of motor commands. We studied these muscle recruitment patterns when the same focal movement was made under behavioral conditions of a self-paced task (SPT) and a reaction-time task (RTT). While standing still, eight normal subjects performed focal movements under the SPT and RTT behavioral conditions and under three different biomechanical conditions: (1) unloaded-upright, (2) loaded-upright (holding a 3800-g metal bar), and (3) unloaded-forward leaning. Anticipatory postural adjustments were quantified using the latency and duration of electromyographic (EMG) data and the center of pressure (COP) displacement data. Postural-kinematic [joint and body center of gravity (COG) motion] data were used to quantify the anticipatory postural program's effectiveness at preventing postural movement. Focal movement kinematics and associated EMG activity differed due to biomechanical but not behavioral (SPT vs. RTT) conditions. The maximum and net displacement of the body COG measurements did not differ between the behavioral conditions. The amplitude, timing, and net movements of lower extremity joints were influenced by the behavioral conditions. However, the behavioral conditions significantly affected the phasing (including order of activation) and duration of anticipatory postural EMG activity and the phasing of COP displacements under certain biomechanical conditions. These findings support the theory that anticipatory postural adjustments are planned in detail.