Does symmetrical upper limb task involve symmetrical postural adjustments?

This study tested the hypothesis that, in young healthy subjects, a symmetrical upper limb task involves asymmetrical postural adjustments with respect to lower limb dominance. Subjects (N=10) purposely performed a series of bilateral forward-reach tasks (BFR) while standing unilaterally on the dominant leg, on the non-dominant leg (unilateral conditions) or on both legs (bipedal condition). For each leg, the integrated electromyographical (iEMG) activity per 20-ms periods ranging from 300 ms before BFR onset (t0) to 1000 ms after t0 was compared between the unipedal and bipedal stance conditions. This time-window included "anticipatory", "on-line" and "corrective" postural adjustments, i.e. those postural adjustments occurring before (anticipatory postural adjustments, APAs), during (on-line postural adjustments, OPAs) and after (corrective postural adjustments, CPAs) the BFR. During the APAs, results showed that, for each leg, changing the stance condition from bipedal to unipedal did not elicit any iEMG changes in any of the postural muscles investigated. In contrast, during the OPAs, an early increase in the excitation level of the semitendinosus for the dominant leg and a late increase in the excitation level of the soleus for the non-dominant leg were detected. During the CPAs, an increase in the excitation level of the soleus, tibialis anterior and semitendinosus was detected in the non-dominant leg whereas, in the dominant leg, no change in the excitation level for any postural muscle recorded was observed. These results support the concept of side dominance in the postural component of a symmetrical upper limb task.