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1 Department of Neural Regulation, Division of Regulation of Organ Function, Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-8601, Japan
2 Department of Care and Rehabilitation, Division of Physical Therapy, Seijoh University, Nagoya 476-8588, Japan
Delayed onset muscle soreness (DOMS) is quite common, but the mechanism for this phenomenon is still not understood; even the existence of muscle tenderness (mechanical hyperalgesia) has not been demonstrated in experimental models. We developed an animal model of DOMS by inducing eccentric contraction (lengthening contraction, ECC) to the extensor digitorum longus muscle (EDL), and investigated the existence of mechanical hyperalgesia in the EDL by means of behavioural pain tests (Randall-Selitto test and von Frey hair test, applied to/through the skin on the EDL muscle) and c-Fos expression in the spinal dorsal horn. We found that the mechanical withdrawal threshold measured with the Randall-Selitto apparatus decreased significantly between 1 and 3 days after ECC, while that measured by von Frey hairs did not. The group that underwent stretching of the muscle only (SHAM group) showed no change in mechanical pain threshold in either test. These results demonstrated that the pain threshold of deep tissues (possibly of the muscle) decreased after ECC. c-Fos immunoreactivity in the dorsal horn (examined 2 days after ECC/SHAM exercise) was not changed by either ECC or compression (1568 mN) to the EDL muscle by itself, but it was significantly increased by applying compression to the EDL muscle 2 days after ECC. This increase was observed in the superficial dorsal horn of the L4 segment of the ipsilateral side, and was clearly suppressed by morphine treatment (10 mg kg–1, I.P.). These results demonstrated the existence of mechanical hyperalgesia in the muscle subjected to ECC. This model could be used for future study of the neural mechanism of muscle soreness.
(Received 17 November 2004;
accepted after revision 25 January 2005;
first published online 27 January 2005)
Corresponding author K. Mizumura: Department of Neural Regulation, Division of Regulation of Organ Function, Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-8601, Japan. Email: mizu{at}riem.nagoya-u.ac.jp
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  T. Taguchi, J. Sato, and K. Mizumura Augmented Mechanical Response of Muscle Thin-Fiber Sensory Receptors Recorded from Rat Muscle-Nerve Preparations In Vitro After Eccentric Contraction J Neurophysiol, October 1, 2005; 94(4): 2822 - 2831. [Abstract] [Full Text] [PDF]
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 U. Proske Muscle tenderness from exercise: mechanisms? J. Physiol., April 1, 2005; 564(1): 1 - 1. [Full Text] [PDF]
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