Pain sensitivity, exercise and stoicism.

Abstract

Habitual recreational runners engage in strenuous physical activity undeterred by fatigue, pain, or adverse environmental conditions. Some continue to run despite stress fractures1 or the onset of myocardial infarction2. Even without injuries, however, regular training produces regular discomfort. These observations raise two questions: are athletes generally less sensitive to pain than normally active persons, and does exercise reduce pain sensitivity? As the presence or magnitude of allegedly noxious natural stimuli cannot be controlled, laboratory pain tests are employed. By presenting uniform, calibrated noxious stimuli to athletes and controls, or to athletes under different experimental conditions, one can reasonably study differences in pain responsivity. This paper reviews evidence of differences in pain responsivity between habitual exercisers and normally active people and of changes in pain responsivity during exercise. It further discussses the notion of 'stoicism' with regard to these findings. Various laboratory pain tests have been utilized to document the effects of exercise on pain behaviour. To facilitate later discussion, these stimuli will be described briefly. The tourniquet ischaemia pain test3 usually involves inflating a pressure cuff above systolic pressure on the upper arm, after which the subject exercises the hand for several minutes. Sensation and pain reports are elicited at regular intervals, and the subject is encouraged to tolerate the stimulus for as long as 10 or 15 min. Cold pain is produced by the cold pressor test, in which the subject immerses his hand in an icewater slurry for as long as 3 to 5 min. During this immersion, sensation and pain reports are elicited at regular intervals, and the time to withdrawal is noted. Heat pain is often delivered by a contact thermode4, but also with focused light5 and lasers6, either in a continuous ramp or as temporally discrete stimuli. The subject either rates each discrete stimulus on a rating scale, or indicates when particular levels of sensation or pain have been achieved along the ramp. Electrical stimuli are delivered to the skin, usually of the fingers or forearm7, but also to the teethV10. The stimulation and response protocols are analogous to those described for thermal stimulation. Pressure pain is usually produced by placing the subject's finger under the dulled edge of a lucite block; different forces press the edge against the dorsal surface of the finger, and the subject reports when particular levels of sensation or pain have been achievedl 1. Responses are then summarized by various psychophysical measures. Most commonly, the pain threshold is taken as the mean stimulus intensity evoking the minimal report of pain. Tolerance is taken as the mean maximal intensity endured on the trials. Another model, sensory decision theory (SDT) 2, provides two indices of perceptual performance. Discriminability indexes the accuracy in separating higher from lower intensity stimuli, and response bias indexes the most probable report category. Since most of these indices can vary independently of one another, the use of SDT separates measures of average report level from sensitivity to differences in level. Analgesics such as local nerve blocks or IV morphine5 reduce both the number of pain reports and discriminability, while placebos only reduce the number of pain reports13.