Comparison of human pain sensation and flexion withdrawal evoked by noxious radiant heat.

The purpose of this study was to determine the reliability of flexion withdrawal magnitude as an indicator of pain sensation. In 10 healthy human volunteers, we compared the magnitude and latency of integrated biceps EMG with the subjects' rating of pain, using a visual analog scale, elicited by noxious radiant heat stimuli applied to the dorsal forearm. The magnitude and inverse latency of withdrawal, although variable, increased exponentially as a functions of stimulus temperature. The stimulus response functions for mean withdrawal magnitude and mean pain intensity were similar for lower stimulus temperatures, but at higher temperatures the withdrawal continued to increase exponentially whereas pain intensity reached a plateau. The pain intensity and withdrawal magnitude for each stimulus were poorly correlated. Under the conditions of this experiment, mean pain intensity and mean withdrawal magnitude were both well correlated with stimulus temperature, but the magnitude of withdrawal did not reliably reflect the intensity of pain sensation.