Lengthening contraction (LC)-induced mechanical hyperalgesia severely restricts physical activities in our daily living. The current study aimed to clarify which mechanical factors during LC determine the degree of hyperalgesia. Under isoflurane anesthesia, ankle extensor muscles of rats were repetitively loaded with LC of different stretch range of motion (ROM) and angular velocity (VEL). The degree of mechanical hyperalgesia was quantified by measuring the mechanical withdrawal threshold of the exercised muscle before, 3 hours, 1, 2, 3 days after LC. We found that LC at the ROM of 60, 90, and 120º significantly decreased the withdrawal threshold in a ROM-dependent manner while that at 30º did not, and that LC at the VEL of 100, 200, and 400º/s significantly decreased the threshold in a VEL-dependent manner while that at 50º/s did not. Repetitive stretching of the muscle alone without muscle contraction had no effect on the nociceptive threshold. Interestingly, when the VEL was fixed, the peak integrated torque generated during LC (PTq) was significantly correlated with ROM of LC and thus the degree of hyperalgesia was correlated with PTq. On the other hand, when ROM was fixed, PTq was inversely correlated with VEL, and instead, the speed of torque increase during LC (SpTq) was significantly correlated with VEL of LC and thus the degree of hyperalgesia was correlated with SpTq. These results indicate that different mechanical factors are involved in mechanical hyperalgesia after LC.
This work was supported by the Grant-in-Aid for Young Scientists (A) (23689033), challenging Exploratory Research (24659106), and Scientific Research (B) (25282160) from the Japan Society for the Promotion of Sciences. This work was performed in a collaborative research project between the Nagoya University and the Vitacain Pharmaceutical Co. Ltd. The authors declare that there were no conflicts of interest in this study.