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

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Abstract
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≤r≤0.98, pr≤0.97, p0.16), but the RPTs were disparate (pp<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.