The present invention relates to a composition for preventing and treating muscle diseases or improving muscular function, containing, as an active ingredient, at least one selected from the group consisting of morusin, kuwanon G, and a Mori Cortex Radicis extract. The Mori Cortex Radicis extract, morusin, or kuwanon G, according to the present invention, has an effect of remarkably enhancing muscular function by increasing the expression of p-mTOR protein involved in muscular protein synthesis, inhibiting the expression of mRNAs of MuRF-1 and atrogin-1 involved in muscular protein degradation, and increasing the expression of mRNAs of MyoD and myogenin involved in muscular differentiation. In addition, the present invention is a natural product so as to be used safely without side effects, thereby being usable in drugs, food, or cosmetics.
