Approach to the peripheral mechanism of muscular pain using a model of delayed onset muscle soreness

Muscle pain is quite common, but its mechanism is not well understood. For analyzing peripheral mechanism of muscle pain, importance of non-invasive method evaluating pressure pain threshold (mechanical withdrawal threshold in animals) was discussed. For experimental model analyzing muscle pain mechanism, delayed onset muscle soreness model was developed by applying lengthening contraction (LC) to rats, and results obtained in this model was introduced. Existence of mechanical hyperalgesia in this model was demonstrated by measuring mechanical withdrawal threshold by Randall-Selitto apparatus equipped with a large probe with tip diameter of 2.6 mm and by examining c-Fos expression in the superficial dorsal horn. Increase in mechanical sensitivity (decrease in threshold and increase in response magnitude) of muscle C-fiber nociceptors was demonstrated by single nerve recording in vitro in the muscle which were hyperalgesic 2 days after LC, and this change is considered to be the peripheral mechanism for mechanical hyperalgesia after exercise. Absence of muscle damage and inflammation, existence of taut band and referred pain by compressing the taut band in this model suggest that this model will be suitable and useful for the study of neural mechanism of clinically important muscle pain conditions.