In order to better understand the role of proprioception in postural adjustments on unstable surfaces, we analyzed the effect of vibration on the pattern of muscle activity and joint displacements (ankle, knee and hip) of eight intellectually normal participants (control group-CG) and eight individuals with Down syndrome (DS) while balancing on seesaws of different heights. The individuals with DS adopted a pattern of co-contraction and were not able to modulate the magnitude of postural response with the seesaw's height. The vibration affected neither the ability of individuals with DS to maintain balance nor the pattern of muscle contraction. On the other hand the control group changed the strategy with vibration, adopting one similar to that used by individuals with DS. Collectively, our findings suggest that proprioceptive information is essential for the motor control system to select the appropriate motor strategy of reciprocal activation among the agonist and antagonist to efficiently balance. Also, these findings suggest that a proprioceptive deficit could explain the atypical motor strategy observed in individuals with DS during balance on seesaw. (Contains 3 figures and 2 tables.)