EFFECT OF DIFFERENT ECCENTRIC CONTRACTION NUMBERS ON MORPHOLOGICAL AND FUNCTIONAL ASPECTS IN RAT SKELETAL MUSCLE

Abstract

PURPOSE The purpose of this study was to examine the effect of increasing number of eccentric contractions on the extent of muscle fiber damage and the functional parameters of muscle force (isometric torque, electromyographic signals). METHODS The right tibialis anterior (TA) muscles of anesthetized male wistar rats (n = 4–6, each group) were subjected to 1, 5, 10, 20, 30 or 40 controlled eccentric contractions induced by electrical stimulation. The contralateral TA muscle was used as a normal control. Three days after eccentric exercise, isometric peak torque and surface myoelectric signals during twitch contraction induced by electrical stimulation were measured in control and exercised TA muscles. EMG parameters, such as a muscle fiber conduction velocity (MFCV) and integral electromyogram (iEMG), were calculated using surface array electrodes. Muscle fiber damage was quantified in transverse sections stained with hematoxylin-eosin (HE). RESULTS The TA muscles subjected to 1, 5 and 10 eccentric contractions exhibited slight muscle damage (under 2.3% damaged fibers). In contrast, TA muscles subjected to 20 (43.4 ± 12.8%), 30 (48.4 ± 12.5%) and 40 (44.2 ± 15.3%) eccentric contractions exhibited intense muscle damage. There was no difference in the percentage of muscle damage among the 20, 30 and 40 contractions groups. The isometric peak torque was significantly decreased in TA muscles subjected to more than 10 eccentric contractions, compared with control TA muscles. A significant correlation was recognized between peak torque and percentage of muscle damage (r = −0.62). Also, there was a significant correlation between peak torque and iEMG (r = 0.43). MFCV did not differ between the exercised TA muscles (3.77 to 4.19 m/sec) and control TA muscles (3.52 to 4.14 m/sec). CONCLUSION The results indicated that I) the extent of muscle fiber damage does not depend on the increased number of eccentric contractions, 2) the decline of muscle force after eccentric exercise is related to the extent of muscle fiber damage and decreased iEMG signal.