Identifying the afferents involved in movement-induced pain alleviation in man

It has been clearly established that the perception of nociceptive stimulus decreases in intensity when movement is initiated in the part of the body to which the stimulus is applied. The pain alleviation is probably at least partly due to the activation of afferents which occurs during the movement. The present experiments were carried out with a view to investigating which groups of afferent fibres is mainly responsible for the gating of the nociceptive messages which occurs during movements. In eight volunteers we investigated the changes in the amplitude of the nociceptive leg flexion reflex (RIII) when the subjects were at rest, when they were performing active or passive ankle movements and when the spindle proprioceptive pathway was mobilized by applying vibratory stimulation to the Achilles tendon. Similar experiments were also carried out with eight other volunteers after anaesthetizing the ankle skin mechanoreceptors. This method was chosen because a clear-cut correlation is known to exist between the amplitude of the nociceptive motor reflex and the intensity of the pain perceived by the subject. The data obtained clearly show that anaesthesia of cutaneous mechanoreceptors connected to the large diameter afferent fibres prevented the decrease in the motor response which otherwise accompanied both active and passive movements. Activation of the Ia fibre group by tendon vibration resulted on the contrary in an increase in the amplitude of the motor response. Movement-induced pain alleviation therefore does not mainly involve the activation of the Ia group of fibres.