Postural fluctuations during pointing from a unilateral or bilateral stance.

An experiment was conducted to compare the effects of bilateral and unilateral stance on postural fluctuations and intralimb coordination during active balance control. Fifteen participants stood bilaterally and unilaterally while conducting a pointing task with an outstretched arm. Excursion of center of foot pressure (CoP) and limb movements were recorded with a force plate and eight dual-axis accelerometers, respectively. Compared to bilateral stance, unilateral stance resulted in wider CoP trajectories and greater postural fluctuations, especially in the lower limbs. The limb-dependent postural fluctuations during unilateral stance were associated with an increased coupling between the upper limb segments and a decreased coupling between the segments of the stance leg. Unilateral stance further resulted in greater regularity and spectral changes in postural fluctuations of the trunk and lower limb due to increased central oscillations (8-15 Hz). The observed structural differences in postural fluctuations between unilateral and bilateral stance strongly suggested that the postural control system modulates joint stiffness in a stance-dependent manner. Probably, in unilateral stance, attentive control was shifted to the stance leg at the expense of increasing arm stiffness to reduce movement redundancy.