Subjective Pain Sensation is Linearly Correlated with the Flexion Reflex in Man

In an attempt to define the relationship between physiological and psychological correlates of pain during low-level voluntary contraction, electrical stimuli between threshold and maximally tolerable intensities were delivered to the sole of the foot in 7 normal subjects. The two measures used to assess the response magnitude are: (1) the flexion reflex (FR) in the ipsilateral biceps femoris (BF), and (2) the estimate of perceived intensity reported on a visual analog scale (VAS). Our results showed that both BF FR area and VAS ratings bore a direct linear relationship with stimulus intensity and with each other, suggesting that at least under our paradigm, the sensory component of a nociceptive stimulus may already be largely set at the spinal interneuronal level.