OBJECTIVE: An Achilles tendon (AT) vibration is known to disrupt the postural control in standing ("vibration-induced falling", VIF) and to backward tilt the postural vertical in restrained sitting position, suggesting a link between AT vibration and internal representations involved in postural control. A recalibration of some troubles in body orientation by oriented sensory manipulations could be of great clinical interest. In order to use the VIF paradigm in a procedure suitable for a rehabilitation context, AT vibration deserves to be more investigated in sitting, for security reasons, and first in young participants. METHOD: In 12 healthy participants (6 men/6 women; 23.3±1.9years), posturographic data to AT vibrations (85Hz) were recorded over 30s standing and 40s sitting trials. RESULTS: Surprisingly, four types of differences were found between standing and sitting AT vibrations: presence/absence of VIF and kinesthetic illusion, opposite directions of the centre of foot pressure (CoP) displacements, different temporal profiles. In standing: VIF without kinesthetic illusion, rapid backward shift of CoP with a peak of -54.6±11.3mm (with respect to baseline P<0.001) 2.8±0.2s after vibration onset. In sitting: surprising systematic forward shift of the CoP throughout the duration of AT vibration with a maximum of 27.9±18.9mm (P<0.05 with respect to the baseline) 19.6±0.3s after vibration onset, associated with a kinesthetic illusion in most subjects. CONCLUSION: The present study invalidates our idea to extend in sitting the VIF paradigm but the unexpected results open a new window about the basic mechanisms underlying muscle vibration effects.