Transient hypoalgesia under physical exercise--relation to silent ischaemia and implications for cardiac rehabilitation.

A series of studies with humans as well as experiments carried out on animals could show that physical exercise leads to temporary hypoalgesia. Reduced sensitivity to pain is not only demonstrable after long-distance exercise (such as marathon run) but also after intensive physical exercise on a laboratory ergometer. Pain threshold elevation is most pronounced during maximal exertion, but hypoalgesia remains present also after exercise is stopped demonstrating that a systemic analgetic effect is induced by the exercise process. Pre-exercise pain threshold level is returned to approximately 60 minutes after the exercise. The cause of the exercise-induced hypoalgesia is probably an activation of central pain inhibitory mechanisms by the "stimulus" of physical exercise (stimulation- or stress-induced analgesia). Central pain inhibitory systems are thereby triggered by the stimulation of afferent nerve endings (group III and IV) in the skeletal muscle. The same trigger mechanism also plays a role as a release stimulus for hormones such for beta-endorphin which is increased under physical exercise. Plasma-beta-endorphin is probably not directly involved in the exercise-induced hypoalgesia but is rather a "marker" for the activating of central analgesia mechanisms. Stress-induced hypoalgesia plays also a role in the coronary heart disease. The activation of endogenous analgetic mechanisms leads to a part of the myocardial ischaemia provoked by exercise being silent under exercise. Completely asymptomatic myocardial ischaemia patients display a generalized hypoalgesia which is demonstrable independent of an exertion stimulus and which indicates a central set-point change in the antinociceptive system.