(A–B) Ten- to eleven-weekold female wild type (WT) and muscle-specific TRB3 transgenic (TG) mice were used to determine TRB3 mRNA and protein expression from tibialis anterior and gastrocnemius, respectively. (C) Body weight was similar between WT and TG. (D) Weights of tibialis anterior (TA), extensor digitorum longus (EDL), soleus (SOL), and gastrocnemius (GAS) muscles were measured. (E–F) Muscle function was evaluated by measuring grip strength and voluntary wheel exercise capacity for 8 weeks. (G) Frozen TA muscles were sectioned in 10-µm-thickness by a cryostat microtome and stained with hematoxylin and eosin. Original magnification: ×400; scale bar: 50 µm. (H) Fibers were traced and analyzed by ImageJ in order to determine the mean CSA. (I) The frequency distribution of the fibers in wild type and TRB3TG mice was determined. Data are the means ± S.E.M (n = 5–6 per group). * P < 0.05, ** P < 0.01 vs. WT