Postural readjustment to body sway induced by vibration in man

Summary

Postural readjustment to body sway, induced by vibration applied to leg muscles for 30 or 40 s, was investigated in healthy subjects standing erect on a gravicorder. The vibratory stimulus induced involuntary body sway in the same direction as the vibrated side, and after cessation of vibration, the body swayed in the opposite direction beyond the initial standing position. Body sway consisted of a sustained displacement with gradual onset, upon which were superimposed some oscillatory movements.

An increase in discharge of the soleus muscles was not obvious when vibration was applied to the Achilles tendon of a standing subject. However, vibration could provoke a tonic activity in the soleus muscles under the following conditions: (1) when the muscle was maintained in an isometric contraction, or (2) when the muscle was stretched voluntarily by leaning forward. It was found that EMG activity in the soleus muscles correlated well with the position of the center of gravity of the body. During vibration, an enhancement of the EMG activity was roughly proportional to the magnitude of the body inclination. These enhancements can be explained as the tonic vibration reflex (TVR), and body inclination induced by vibration may be partially explained as the TVR phenomenon.

An ischemic nerve block of the legs reduced the vibration-induced body sway and the Achilles tendon reflex (ATR). The time courses of suppression and recovery after the release of the nerve block were compared: (a) the backward body inclination induced by the Achilles tendon vibration became obscure after 9–12 min of ischemia, and it took 6–12 min for recovery, and (b) the amplitude of the ATR decreased more gradually than that of body inclination and diminished after 20 min of ischemia, but it recovered very rapidly (within approximately 1 min). The discrepancy between these time courses suggested that afferent fibers which take part in body sway are not only group Ia fibers, but also smaller ones such as group II and cutaneous afferents.

Frequency analysis of body sway during postural readjustments to the vibratory stimulus showed: (1) a marked increase in amplitude for the low frequencies (less than 0.2 Hz) that reflected body inclination; and (2) sharp spectral peaks between 0.2 Hz and 0.8 Hz that reflected oscillatory movements.