Effect of Muscle Pain on Motor Control: A Human Experimental Approach

Abstract

Deep pain has the potential to affect daily movements. The effect of muscle pain per se on co-ordination during movement is not fully understood. Experimental muscle pain induced in healthy subjects might help to elucidate the basic interaction between muscle pain, muscle activity, and muscle co-ordination. Mechanical, chemical, thermal, or electrical stimuli can experimentally activate muscle receptors that signal noxious input. In this paper, endogenous and exogenous methods to induce human muscle pain are discussed. The experimental model mainly used is intramuscular (i.m.) injection of hypertonic saline, as it is safe and it induces local and referred muscle pain that can be quantified. This experimental model of muscle pain has been used extensively to assess the interaction between muscle pain and motor control in order to obtain a better understanding of the basic pain mechanisms. The interaction between muscle pain and motor control depends on the motor task (rest, static, and dynamic contractions). There is a previous hypothesis concerning the accompaniment of muscle pain by muscular hyperactivity, but this so-called ?vicious cycle theory? has not gained much support in the scientific literature. In addition, saline-induced muscle pain causes no increase in EMG activity at rest. Muscle pain reduces maximal voluntary contraction and endurance time during submaximal contractions. Moreover, muscle pain causes changes in co-ordination during dynamic tasks with increased antagonistic muscle activity and decreased agonistic muscle activity. This pattern is in accordance with the so-called ?pain-adaptation model?, predicting a decrease in movement amplitude and velocity by muscle pain. The neural mechanisms underlying pain-evoked changes in dynamic motor function can best be related to alternating inhibition and excitation of motoneurones, but the exact pathways responsible for the modulation are not known. The new knowledge from human experimental muscle pain studies provides a better understanding of the basic effects of muscle pain and may allow the development of better and more evidence-based strategies to manage or reduce the risk of work-related musculoskeletal pains.