Relevance of hand dominance to the bilateral deficit phenomenon

During maximal voluntary contractions, the sum of forces exerted by homonymous muscles when activated unilaterally (UL) is, typically, larger than the sum of forces when activated bilaterally (BL). This phenomenon is known as the bilateral deficit (BLD). Our purpose was to determine if the dominant limb would be inhibited to a greater degree in the BL condition, thereby reducing any disparity in force output between the limbs. Maximum voluntary handgrip strength was measured in 40 left-handed and 40 right-handed individuals under both BL and UL conditions. The right-handers displayed 10.4 % greater right hand strength in both conditions; the left-handers exhibited 5.5 % greater left hand strength in the UL and 4.3 % in the BL condition. A BLD (-1.30 %) was present in the left-handed group only but a reduction in the force disparity between the hands was not evident. It was observed, however, that seven individuals from each group exhibited greater UL force with their non-dominant hand. Accordingly, we re-analyzed the data after rearranging the groups based on unilateral hand grip strength dominance. A significant reduction in force disparity between the hands occurred for the left-handed group only, the result of a significant inhibition of the stronger left hand. A trend towards a similar reduction occurred for the right-handers because of a significant force reduction of the stronger right hand. Consequently, it appears that for maximum handgrip contractions, the BLD may be related to preferential inhibition of the stronger hand, especially for individuals who are left-hand-strength-dominant in terms of unilateral force output.