Introduction: Skeletal muscle consists of numerous fibers which differ in the contraction velocity and energy metabolism. Slow or type 1 fibers produce low mechanical force, they gain energy mainly through aerobic metabolism, are resistant to fatigue and capable of longer lasting activity. Fast fibers, which are devided to type 2a, 2x and 2b, whose contraction velocity increases in the same order, produce more force and power. They gain energy mostly from anaerobic metabolism from their own but limited glycogen stores, thus they are more fatiguable. Fiber types and their contraction velocities are mainly determined by myosin heavy chain isoforms. The changes of physical activity, which may occur also due to disease or injury, induce changed isoforms expression and consequently fiber type transitions either from slower types to faster ones or vice versa. Purpose: The aim of diploma work was to review the literature in Slovene and English language regarding the influence of various factors on the fiber type transitions and respectively on the plasticity of skeletal muscle. Methods: The data presented in diploma work were collected from different online databases: Cochrane Library, MEDLINE and BioMed Central. They were collected regardless the date of publication using keywords and phrases in Slovene and English. Results and discussion: The main factors influencing muscle fiber type transitions are: changes in the pattern of nerve and electrical stimulation, range of muscle load, type and duration of muscle activity, levels of different hormones, oxygen and nutrients availability and the aging process. The transitions towards faster fiber types were induced by high frequency electric stimulation, slow muscle denervation, spinal cord injury, unloading, immobilization in the shortened position, bed rest, microgravity, detraining, disuse, hyperthyreoidism, β - adrenergic agonists, glucocorticoids, fasting, high fat diet and hypoxia. On the contrary, the transitions towards slower fiber types were triggered by: chronic low frequency electric stimulation, fast muscle denervation, immobilization in the lengthened position, overloading, endurance training, resistance training, hypothyreoidism, androgenes and factors associated with aging. Conclusion: Skeletal muscle is one of the most dynamic and plastic tissues, it is able to respond to various factors by fiber type transitions and consequently by changes in their contractile and metabolic properties.
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