Evoked pain in the motor endplate region of the brachial biceps muscle: An experimental study

Spontaneous electrical activity, interpreted as motor endplate activity, has been reported at muscle trigger points. This study examined whether the motor endplate region, identified by electromyography, is more sensitive to noxious stimuli than other muscle sites. We induced pain in the brachial biceps muscles of 21 healthy subjects by injecting capsaicin (30 microg/0.1 ml) and NaCl (5%/0.2 ml) in the motor endplate region and at electrically silent muscle sites. Needle and evoked pain were measured by a visual analogue scale (VAS) (0-10) and the short form of the McGill Pain Questionnaire. The needle pain in the motor endplate region differed both in intensity and quality from that at other sites. The maximal pain after NaCl 5% was higher, and the VAS area-under-the-curve for NaCl 5% and capsaicin were larger, in the motor endplate region than in other sites. A higher density of muscle nociceptors in the vicinity of the motor endplate region may account for the observed differences in pain. These findings may have clinical implications for the treatment of musculoskeletal pain conditions.