The purpose of this study was to determine the factors that influence the co-modulation of motor unit discharge rate in soleus muscles of both legs during upright standing. Single motor units were recorded from the left and right soleus muscles under three experimental conditions: 1) standing quietly with the eyes open and closed, 2) standing with the eyes closed while vibration was applied to one Achilles tendon, 3) swaying voluntarily or producing variable low-force isometric contractions at a frequency of 0.05 Hz. Correlations in motor unit discharge rate between left and right soleus motor units were assessed using common drive analysis. The results showed that common drive to motoneurones of the two muscles did not differ between standing with the eyes open or closed, but there was an order effect, with the second task having significantly lower common drive than the first. Common drive was also significantly lower when vibration was applied to one leg compared to when no vibration was applied. Common drive was higher as subjects swayed anteriorly as compared to when they swayed posteriorly. There were no significant differences in common drive across phases of the variable isometric force contraction. Common drive was higher during voluntary sway than during variable force production; both of these values were significantly lower than those derived from the quiet standing task. These results suggest that proprioceptive and sub-cortical inputs contribute to the co-modulation of the firing rate of soleus motor unit pairs of the left and right leg during standing posture.