Models of EC‐induced necrotizing or non‐necrotizing muscle injury

The models of EC‐induced necrotizing and non‐necrotizing injuries were based on muscle injuries at the age of 10 and 14 weeks, respectively. The rats were anesthetized with isoflurane and firmly fixed on a custom‐made isokinetic dynamometer platform in the prone position. The right gastrocnemius muscle was electrically stimulated with electrodes attached to the muscle belly and the Achilles's tendon percutaneously. Stimulation voltage was set to achieve maximal twitch torque, and tetanic stimulation was given for 0.3 and 2.0 sec with a train of 4 msec rectangular pulses at 10 msec intervals in the models of necrotizing and non‐necrotizing injuries, respectively. At the onset of the electric stimulation, the ankle joint was isokinetically dorsiflexed to cause an EC. The speed and range of the forced lengthening were 180 and 30° per second and from 60 to 125° and 60 to 120° of the ankle joint angle in the models of necrotizing and non‐necrotizing injuries, respectively. ECs in the necrotizing injury model consisted of 60 tetanic contractions at 2 sec intervals. On the other hand, ECs that cause non‐necrotizing injury consisted of five sets of 10 tetanic contractions at 8 sec intervals with a 3 min interval between the sets. Edema and limping were visible only immediately after the ECs. Thus, the rats were not given postprocedure pain relief medication because interventions that may affect regeneration after injury should be excluded.