Quantification of local and referred pain in humans induced by intramuscular electrical stimulation

The basic knowledge related to referred muscle pain is limited. To study referred pain, an experimental model using intramuscular electrical stimulation has been developed. Four experiments were performed: (1) the thresholds for eliciting local (LPT) and referred pain (RPT) were determined; (2) stimulus-response functions relating stimulus intensity, pain intensity ratings and size of pain areas were determined; (3) inter- and intrasession variabilities were assessed; and (4) prolonged stimulations were given with a duration of 10 min to evaluate temporal aspects of the referred muscle pain. Intramuscular electrical stimulation of the tibialis anterior muscle elicited pain at the stimulation site in 94% of the subjects, and referred pain in 78% of the subjects. Referred pain was located in the anterior part of the ankle. The mean RPT was 72% higher than the mean LPT (p<0.01). Correlation was found between stimulus intensity, sensory/pain rating scores and size of pain areas (0.74< or =r< or =0.98,p<0.04). Size of pain areas and sensation/pain rating scores were correlated (0.86< or =r< or =0.97, p<0.01). Intersession variability showed that the LPTs were not significantly different (p>0.16), but the RPTs were disparate (p<0.02). Intrasession values revealed a significant difference between the five LPTs, RPTs, local and referred pain rating scores. The size of the local and referred pain areas remained constant. Prolonged stimulation at 150% of RPT showed that the onset (the first occurrence of pain) of referred pain occurred significantly later (43 s +/- 80 s) than at the local pain site (p<0.03). This study showed that local and referred muscle pain can be elicited by intramuscular electrical stimulation, and indicated that temporal and spatial summation may be involved in the elicitation of referred muscle pain.