Isometric force matching asymmetries depend on the position of the left hand regardless of handedness

Several studies highlighted differences in behavioral performance between the two hands, either due to hand dominance or to specialization of the brain hemisphere. In a previous study, right-handed individuals performed a bimanual isometric force-matching task with the arms in different configurations. There we found that the accuracy of the performance depended on the position of the left hand. Matching performance was worse when the left hand was in the lower position, regardless the symmetry of the arm configurations. In the present study, we tested the hypothesis that this effect is related to handedness, i.e., that in both right- and left-handed individuals the performance depends on the position of the non-dominant hand. Left-handed and age-matched right-handed participants were required to apply simultaneously the same amount of force in the upward direction, with the arms in symmetric or asymmetric configurations. No visual feedback of limb positions was provided. We found that for both groups the absolute and the signed (bias) difference of force between the sides depended on the position of the left hand. Thus, this role of the left arm was not determined by handedness, but likely by the specialization of the brain hemisphere. However, handedness influenced the performance: left-handers had a higher absolute error than right-handers in almost all conditions. No main effect of the left hand position was found for the variable error, but left-handers in most configurations had higher variable error when the left hand was in the lower position.