Eccentric exercise is easy to induce the microstructure changes of the skeletal muscle, thereby resulting in skeletal muscle microinjury and delayed onset muscle soreness, which has been confirmed by a large number of researches[13-18]. Increasing evidence has been harvested from histomorphology, serum enzymes and metabolic parameters, and various hypotheses about exercise-induced skeletal muscle damage have been proposed[13, 19-30]. However, the exact mechanism of skeletal muscle microinjury is still in the exploration.
Muscle soreness and micro-structural damage caused by sports training and physical exercise are the more common phenomena, and a delayed onset of pain has important implications for sports training and recovery, which usually occurs within 24-48 hours after exercise and sustains for 5-7 days or more, and then the pain and microdamage were relieved or disappeared gradually[31-34]. This study found that after a one-time eccentric exercise, it was obvious for us to see muscle fiber degeneration under light microscopy, blurred muscle fibers, light and dark stripes, some muscle fiber necrosis, fuzzy and discontinuous myofascia, lymphocyte infiltration and fibroblast proliferation, some of muscle fiber phagocytosis; under the transmission electron microscopy, after eccentric exercise, significant changes in the ultrastructure of skeletal muscle were found as follows: individual sarcomere, myofibrils or more continuous sarcomeres, destroyed myofibrillar structure, irregular myofilament arrangement, and even myofilament dissolution and disappearance. Sometimes, there were also a large number of glycogen granules and a small amount of mitochondria gathered. Twisted myofilaments appeared, but the stripes disappeared. I band showed structural necrosis, M line disappeared, Z line was blurred, lost, and presented with corrugated changes. Satellite cells were visible between the basement membrane and sarcolemma, which were mostly fusiform, with some of swelling. The cytoplasm was rich in ribosomes and a small amount of mitochondria, mitochondrial swelling and hyperplasia could be seen, and even the structure disappeared. Therefore, mechanical injury was dominant in the early stage of skeletal muscle injury caused by eccentric exercise, which is consistent with the literature[35-38]. Because in the muscle eccentric contraction, the muscle fiber cross-sectional area per unit to withstand a maximum tension, but this time in a lengthening contraction of muscle fibers, thick filaments and thin filaments to reduce the number of cross-bridge connection; few of motor units are recruited, muscle electrical activity reduced; and the sarcomere mechanical properties imbalances and intermediate filament protein which maintaining sarcomere structural integrity may change. This makes it prone to damage, but also the most serious. This experiment also found at different time points after eccentric exercise, skeletal muscle tissue morphology and ultrastructure have been changed in different degree, and this change has obvious phase resistance, and especially serious in 24-48 hours after exercise; it was illustrate that the skeletal muscle injury could be delayed after eccentric exercise and gradually repaired, which may be the result of gradually lifting of mechanical damage after exercise.
Bornemann et al [39], by intracardiac injection of soleus ringer's solution, found that muscle cells and childish muscle cells expressed desmin and vimentin. With the mature of muscle cells, the expression of Desmin and Vimentin were decreased, scattered and fixed in Z board level; 2-4 weeks after injury, Vimentin and Desmin in the muscle fibers still exist, indicating Vimentin is a good indicator of muscle regeneration. In order to explain the relationship between the change of intermediate filament and the structural change of skeletal muscle after eccentric exercise, Friden and co-workers[40] thought that Titin, Desmin and Vimentin played a very important role in the occurrence and development of exercise-induced muscle damage. Findings reported by Vater et al [41] showed that the relationship between intermediate filaments and muscle damage was due to the position of intermediate filaments and its role in cell decision. Sarcomere can be pulled during exercise, the change of different sarcomere length is not the same, and not exactly the same forces suffered. In order to maintain the integrity of the sarcomere structure, it is required that at the time of muscle relaxation, each muscle can regain its original position, restore the original length. This function is realized by the intermediate filaments such as Titin, Desmin and Vimentin. Once the intermediate filament structure changes, the integrity of the sarcomere structure is able to maintain and skeletal structure is bound to change.
In this study, desmin and vimentin showed a network distribution in the normal muscle tissue of rats; after exercise, desmin and vimentin were both expressed. For desmin-positive parts, myocytes exhibited clear boundary and patchy distribution; for vimentin-positive parts, myocytes had fuzzy boundaries and spotty distribution. In the quiet control group, the percentage of de-dyed cell for desmin was 0 (the highest score on immune response); after exercise, anti-desmin de-dying at varying degrees was found at different time. The percentage of de-dyed cells was increased to different extent (decreased score on immune response), which was highest at 24 hours after exercise. At 48 hours after exercise, the desmin expression began to increased but could not recover until the 72nd hour. These findings indicate that desmin expression in the skeletal muscle of rats is decreased after eccentric exercise. Therefore, the variation of intermediate filament expression may reflect that eccentric exercise can play a delayed damage to the skeletal muscle, and the damaged structure can be regenerated and repaired after exercise, which is consistent with the literature reported[42]. Lieber et al [43] reported that after eccentric exercise, the loss of desmin staining is the characteristics of muscle fiber damage caused by eccentric exercise. They also found that the loss of desmin staining in extensor digitorum longus begun to appear at 5 minutes after 30 minutes of eccentric exercise, but desmin expression in the tibialis anterior muscle had no obvious changes; at 15 minutes after exercise, 7.4% and 46.% de-dyed cells positive for desmin were found respectively in rabbit extensor digitorum longus and tibialis anterior muscle; the de-dyed percentages were peaked to 30% and 15% respectively in rabbit extensor digitorum longus and tibialis anterior muscle at 72 hours after exercise[44]. Decrease in the muscle strength of rabbit extensor digitorum longus and tibialis anterior muscle was different, which was correlated with the different expression of desmin. When Desmin expression decreased, Vimentin and Titin expression began to increase. After that, these scholars used immunohistochemistry and electron microscopy techniques to study the extensor digitorum longus of New Zealand rabbits undergoing eccentric exercise and found that the loss of Desmin immunohistochemistry staining was the earliest change reflecting the injury[45]. Muscle fibers negative for Desmin was positive for anti-fibronectin, indicating when the cytoskeleton is destructed, the integrity of cell membranes is also damaged. In the past years, there are few studies on Vimentin expression changes during exercise-induced skeletal muscle microdamage. Lieber et al [44] found that after 30 minutes of eccentric exercise, the expression of Desmin in rabbit extensor digitorum longus and tibialis anterior muscle was reduced, but Vimentin and Titin expression begun to increase. It is also found that two or more of the intermediate filament proteins can expressed simultaneously in many tissues[46], and Vimentin is usually one of them, indicating Vimentin may play an important role in cell growth. Thus, in adult tissues, Vimentin co-expressed with other intermediate filament proteins is often related to the active proliferation of cells.
At the same time, we found in the same specimens, either desmin or vimentin had different degree of staining at different positive parts. This phenomenon may have a great relationship with the animal intermittent downhill running, because in the late stage of intermittent downhill running, muscle injury and regeneration were alternated at different parts, both of which existed independently of each other. Therefore, these pathological changes during the intermittent downhill running are actually the result of the superposition of injury and regeneration.
So, one possible mechanism of skeletal muscle injury and repair after eccentric exercise may be mechanical factors, the “mechanical damage theory”, and the direct evidences are as follows: (1) changes in the fine structure of skeletal muscle; (2) changes in skeletal muscle ultrastructure; (3) changes in cytoskeletal proteins.
One-time eccentric exercise can cause delayed skeletal muscle damage and delayed onset muscle soreness; after eccentric exercise, histomorphology and ultrastructure of the skeletal muscle have been changed to different extent, and mechanical factor may be the important reason for delayed onset muscle soreness.
中国组织工程研究杂志出版内容重点：组织构建；
骨细胞；软骨细胞；细胞培养；成纤维细胞；血管内皮细胞；
骨质疏松；
组织工程
