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Effects of postural bias during support surface displacements and rapid arm movements.

The purpose of this study was to evaluate body-sway characteristics during anticipatory and reactive postural compensation. Mechanisms of postural control were studied during self-initiated rapid arm movements (RAMs) and unexpected support surface displacements (SSDs). Ten healthy subjects, 5 men and 5 women with a mean age of 28 years, were blindfolded and stood symmetrically on a moveable force platform or with weight shifted right (R) or left (L). Averaged electromyographs were obtained from L anterior deltoid, L ipsilateral biceps femoris (BFi), R contralateral biceps femoris (BFc), R quadriceps (Q), R medial gastrocnemius (G), and R tibialis anterior (TA) muscles. During SSD, the relative timing of distal-proximal synergists was altered during all biased postures. In L biased standing, functional pairs, TA-Q and G-BF, discharged at their shortest latencies. Anterior deltoid onset times during RAM were most preparatory during R weight shift. Sway latencies were shorter also in biased standing. Absolute sway latency, peak sway, and time-to-peak sway were significantly longer in SSD conditions in comparison with RAM. When disturbances in balance cannot be predicted, lower extremity preloading facilitates contralateral muscle-discharge patterns at shorter latencies. In some neurological conditions that result in incorrect body alignment, the mechanical effects of postural bias rather than the abnormal control mechanisms may at least contribute to alterations in the postural response. These results provide a basis for evaluating balance in patients with neurological deficit.