A feedback control based on the visual perception self-motion contributes to postural stability, however little is known about the visual modulation of postural muscles. Aim of this study was to investigate the effect of optic flow direction on the muscle activation and to verify the correlation between visual stimuli, muscle activity and body sway. We used surface electromyography (EMG) and stabilometry on 24 right-handed young adults. Center of pressure (COP) and EMG data were acquired at once. We analyzed the bilateral activation of tibialis anterior and gastrocnemius medialis during full field expansion or contraction optic flow stimulation. A stimulus made of dots moving in random directions was used as control. The activity of the tibialis anterior and gastrocnemius medialis resulted well correlated to the antero-posterior COP displacement. We analyzed the highest EMG amplitude values to evaluate if optic flow differently modulated the muscle responses. The stimulus effect was significant in 23 subjects (96%). In the tibialis anterior we found a laterality effect in males (p<0.001) and a gender difference (p<0.05). Random control stimulus activated the muscles as well as the optic flow, however the COP displacement during random was much larger and less organized with respect to radial optic flow. This may suggest that a visual stimulus always evokes an increased excitatory input on postural muscles, but the stimulus structure produces different postural effects.