Board Thread:Theories and Speculation/@comment-1872728-20160724090700/@comment-25547449-20160726224342

Ashura simply inherited the ability to use rikudou senjutsu from his father(Just like how hagoromo inherited the same power from kaguya), seeing as Hagoromo said something along the lines of "He awakened bodily chakra that rivaled his brother's strength" or something like that.