Myopic defocus
Myopic defocus is the primary stimulus that will reduce the axial length of the eyeball, according to clinical studies.
Reference: https://iovs.arvojournals.org/article.aspx?articleid=2189093 Eyes in Various Species Can Shorten to Compensate for Myopic Defocus Xiaoying Zhu; Neville A. McBrien; Earl L. Smith, III; David Troilo; Josh Wallman Anatomy and Pathology/Oncology April 2013