Achilles tendon vibration shifts the center of pressure backward in standing and forward in sitting in young subjects.

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±1.9 years), posturographic data to AT vibrations (85 Hz) were recorded over 30 s standing and 40 s 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±11.3 mm (with respect to baseline P<0.001) 2.8±0.2 s after vibration onset. In sitting: surprising systematic forward shift of the CoP throughout the duration of AT vibration with a maximum of 27.9±18.9 mm (P<0.05 with respect to the baseline) 19.6±0.3 s 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.