The present invention relates to a composition for promoting the differentiation of myoblasts comprising butylpyridinium, a derivative thereof, or a pharmaceutically acceptable salt thereof; a pharmaceutical composition for preventing or treating a disease associated with muscle weakness; a food composition for preventing or ameliorating a disease associated with muscle weakness; a composition for strengthening muscles; and a feed or feed additive for strengthening muscles. Further, the present invention relates to a method for promoting the differentiation of myoblasts using butylpyridinium, a derivative thereof, or a pharmaceutically acceptable salt thereof; a method for preparing differentiated myoblasts; and a method for treating a disease associated with muscle weakness.