Aim
  The study aimed to develop an appropriate in vivo rat model of muscle strain to investigate whether electroacupuncture is useful for improving muscle strain.
Methods
  An in vivo muscle strain model was developed with Wistar rats by the eccentric contraction (EC) evoked by pulling their hind limb with a stainless-steel wire that was connected to a weight; the EC was given under anesthesia during tetanic contraction in their gastrocnemius evoking by electrical stimulation of the sciatic nerve. Development of muscle strain was evaluated by comparing the muscle tension induced by the twitch of the gastrocnemius, pain thresholds measured using the Randall-Selitto test, and the serum creatine kinase (CK) activity between the following three experimental groups : EC×1 group, EC was temporarily evoked to their hind limb only once (n=10); EC×5 group, EC was temporarily evoked to their hind limb five times consecutively (n=6); and control group, EC was not evoked to their hind limb (n=6). After confirming the absence of a significant difference in the parameters between the two EC conditions, the effect of electroacupuncture (EA) was evaluated using the model of one-time EC (EA group n=10). At 12 hours after EC, EA was carried out at a frequency of 50 Hz and an intensity of 0.5 mA for 15 min; this stimulation was given for six consecutive days.
Results
  Muscle tension, pain thresholds, and the CK activity showed no remarkable changes in the control group during the experimental period. A significant decrease in the muscle tension was observed after EC in the EC×1 and EC×5 groups, and the decrease sustained until 48 and 24 hours after EC, respectively. Pain thresholds in the EC×1 and EC×5 groups were significantly lower than those in the control group at each of the time periods studied after EC, and were sustained at least until 120 hours after EC. The CK activity increased in the EC×1 and the EC×5 groups at 30 minutes after EC. Recovery in muscle tension after EC was faster in the EA group than in the EC×1 group. Pain thresholds also showed faster recovery in the EA group than in the EC×1 group, indicating statistically significant differences at 72, 96, and 120 hours after EC.
Discussion
  The in vivo rat model, which was developed in the present study, was considered to be an appropriate model of muscle strain because it clearly showed a decrease in the muscle tension and pain thresholds, and an elevation in the CK activity. EA was considered to be capable of accelerating the recovery of muscle strain as it was found to improve muscle tension and pain thresholds in the model.