Histological Skeletal Muscle Damage and Surface EMG Relationships Following Eccentric Contractions

Abstract

This study examined the effects of a different number of eccentric contractions (ECs) 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 ECs 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 ECs (EC1, EC5, EC10, EC20, EC30, EC40, and EC100). Three days after the ECs, 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 a 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 group. Although muscle damage and pathological 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 ECs. Muscle damage was reflected by iEMG and action potential waveforms, but not by MFCV, which remained unaffected even though approximately 50% to 70% of the fiber demonstrated injury.

Eccentric muscle contraction and stretching evoke mechanical hyperalgesia and modulate CGRP and P2X3 expression in a functionally relevant manner

“…This reduction is attributable to both fatigue and muscle injury, although injury produced by EC is likely independent of fatigue [64]. While a substantial number of muscle contractions were used in this study, other masseter studies have employed more ECs [32] or contraction for 30min per day for 14d [89] and hindlimb studies have used 100–500 ECs [35,46,47,95,96]. …”

Impaired action potential conduction at high force levels after eccentric exercise

“…Although Kano et al (2008) did show a clear disturbance of action potential waveforms in the presence of extensive muscle damage, CV remained intact. As CV was only measured three days post-exercise, any earlier reductions in CV would not have been detected.…”

“…Strenuous eccentric animal models using tetanic electrical stimulation have not been able to show a major dysfunction in the gross sarcolemmal function after eccentric contractions based on power spectral parameters (Warren et al, 1999) or CV (Kano et al, 2008). Although Kano et al (2008) did show a clear disturbance of action potential waveforms in the presence of extensive muscle damage, CV remained intact.…”

Past injurious exercise attenuates activation of primary calcium-dependent injury pathways in skeletal muscle during subsequent exercise

“…Excessive exercise activates calcium‐dependent injury pathways, results in a prolonged muscle strength deficit, and may induce necrosis of muscle fibers in severe cases (Kano et al. ). As for the injury at the age of 10 weeks in the RI group, we have previously shown that about 10% of muscle fibers undergo necrosis (Takagi et al.…”

“…The injury induces a prolonged muscle strength deficit; furthermore, muscle fibers can undergo necrosis in case of severe injury (Kano et al. ). As a skeletal muscle adaptation to the injury, injured skeletal muscle reduces the severity of the symptoms of subsequent muscle injury, a phenomenon called the “repeated bout effect” (RBE) (Nosaka et al.…”