Effects of vibratory stimulation on muscular pain threshold and blink response in human subjects.

The effects of vibratory stimulation on muscular pain threshold were investigated in 28 healthy subjects. Pain sensation was evaluated by the subjects' verbal reports in response to electrical stimulation of the vastus medialis muscle. Concomitant variations of blink response evoked as a component of the startle reaction were also studied. In all the subjects tested, high frequency vibration (110 Hz) induced a marked and long lasting elevation of the muscular pain threshold but only when vibration was applied to the skin overlying the ipsilateral quadriceps tendon or neighbouring areas and not when applied to remote ipsi- or contralateral regions. This effect was prevented either when tonic vibration reflex (TVR) of the quadriceps muscle was elicited or the skin underlying the vibrator was anaesthetized. Vibratory stimulation at low frequency (30 Hz) failed to produce any consistent effect on muscular pain threshold. Variations in threshold for blink response, as a rule, closely followed those of muscular pain threshold. However, a facilitation of the blink response, not accompanied by changes in pain sensation, was observed during the first period of both high and low frequency vibratory stimulation. The effectiveness of high frequency vibration in raising the muscular pain threshold is coherent with previous results showing that vibration is able to affect pain sensation. Present results suggest a role for rapidly adapting receptors (RA) and/or pacinian corpuscles (PC) in this effect and support the hypothesis of an inhibition of nociceptive messages, possibly at spinal segmental levels, by volleys in large myelinated afferent fibres.