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Muscle fiber is the basic unit of skeletal muscle. Muscle fiber types play a crucial role in improving meat quality. In the current study, Wistar rats were chronic injected with 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) for four weeks in order to define the 5′-AMP-activated protein kinase (AMPK) protein activity on muscle fiber composition, enzyme activity and the mRNA expression of muscle fiber type-related genes in gastrocnemius (GS) muscle. Food intake and body weight did not show a significant difference after AICAR treatment (P >.05). The AICAR treatment caused a significantly increased number and area proportion of type I muscle fiber and expression level MyHC I mRNA (P <.05), succinic dehydrogenase (SDH) and malate dehydrogenase (MDH) activities (P <.05), but decrease in the number proportion of type IIB muscle fiber and MyHC IIb and MyHC IIx mRNA level (P <.05). The Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) mRNA level (P <.05) was increased and glucose transporter 4 (GLUT-4) mRNA level tended to be higher (P =.052) in the AICAR group. These results demonstrated that chronic AICRA treatment induces oxidative muscle fiber switching, enhances oxidative enzyme activity and GLUT4 mRNA level, mediated by PGC-1α. [ABSTRACT FROM AUTHOR]