Abstract
This study examined the effects of different number of eccentric contractions (EC) on histological characteristics, surface electromyogram (EMG) parameters (integral EMG, iEMG; muscle fiber conduction velocity, MFCV; and action potential waveform) and isometric peak torque using the rat EC model. Male Wistar rats (n = 40) were anesthetized, and EC were initiated in the tibialis anterior muscle via electrical stimulation while the muscle was being stretched by electromotor. The rats were grouped according to the number of EC (EC1, EC5, EC10, EC20, EC30, EC40 and EC100). Three days after EC, surface EMG signals and isometric peak torque were measured during evoked twitch contractions via electrical stimulation of the peroneal nerve. The muscle damage was evaluated from hematoxylin-eosin (HE) stained cross-sections as relative number of damaged fibers to intact fibers. Intense histological muscle damage (approximately 50 to 70% of the fiber), loss of isometric peak torque, disturbance of action potential waveform and depression of iEMG (approximately −60 to −70%) were observed at EC20, EC30, EC40 and EC100. On the other hand, the MFCV did not change in any EC groups. Though muscle damage and pathologic surface EMG signals were not found at EC10, isometric peak torque was reduced significantly. In conclusion, the extent of histological muscle damage is not proportionally related to the number of EC. Muscle damage was reflected by iEMG and action potential waveforms, but not by MFCV which remained unaffected despite approximately 50 to 70% of the fiber demonstrating injury.
