PHYSIOLOGICAL RESEARCH ISSN 0862-8408  2007 Institute of Physiology, v. v. i., Academy of Sciences of the Czech Republic, Prague, Czech Republic Fax +420 241 062 164 E-mail: physres@biomed.cas.cz http://www.biomed.cas.cz/physiolres Physiol. Res. 56: 829-832, 2007 RAPID COMMUNICATION Velocity of Body Lean Evoked by Leg Muscle Vibration Potentiate the Effects of Vestibular Stimulation on Posture O. DZURKOVÁ, F. HLAVAČKA Institute of Normal and Pathological Physiology, Slovak Academy of Sciences, Bratislava, Slovak Republic Received September 17, 2007 Accepted October 1, 2007 Summary To investigate the vestibular and somatosensory interaction in human postural control, a galvanic vestibular stimulation of cosine bell shape resulting in a small forward or backward body lean was paired with three vibrations of both soleus muscles. The induced body lean was registered by the position of the center of foot pressure (CoP). During a quiet stance with eyes closed the vibration of both soleus muscles with frequency (of) 40 Hz, 60 Hz and 80 Hz resulted in the body lean backward with velocities related to the vibration frequencies. The vestibular galvanic stimulation with the head turned to the right caused forward or backward modification of CoP backward response to the soleus muscles vibration and peaked at 1.5-2 s following the onset of the vibration. The effect of the paired stimulation was larger than the summation of the vestibular stimulation during the quiet stance and a leg muscle vibration alone. The enhancement of the galvanic stimulation was related to the velocity of body lean induced by the leg muscle vibration. The galvanic vestibular stimulation during a faster body movement had larger effects than during a slow body lean or the quiet stance. The results suggest that velocity of a body postural movement or incoming proprioceptive signal from postural muscles potentiate the effects of simultaneous vestibular stimulations on posture. Key words Vestibular system • Postural control • Muscle vibration • Sensory interaction The role of the human stance control is to maintain the vertical body’s centre of gravity within the area of the feet support. Multisensory information, mainly from vestibular, somatosensory and visual afferents are integrated in a continual process of adjusting the body‘s centre of gravity in relation to an internal space reference frame (Maurer et al. 2006, Horak and MacPherson 1996). Galvanic vestibular stimulation (GVS) can induce postural reactions in standing subjects that are useful in determining the influence of vestibular function on balance (Fitzpatrick and Day 2004). Application of electric current between the human mastoid processes during the stance with head turned to the right or left evokes the body lean mainly forward or backward towards the side of the anode (Lund and Broberg 1983). Proprioceptive input from postural muscles, particularly from leg muscles, is also important for balance control. In standing subjects, vibration of leg muscles results in a postural response known as the vibratory induced falling. These postural reactions can be characterized by body