**Kent W. Jorgenson 1, Stuart M. Phillips <sup>2</sup> and Troy A. Hornberger 1,\***


Received: 23 June 2020; Accepted: 7 July 2020; Published: 9 July 2020

**Abstract:** The maintenance of skeletal muscle mass plays a critical role in health and quality of life. One of the most potent regulators of skeletal muscle mass is mechanical loading, and numerous studies have led to a reasonably clear understanding of the macroscopic and microscopic changes that occur when the mechanical environment is altered. For instance, an increase in mechanical loading induces a growth response that is mediated, at least in part, by an increase in the cross-sectional area of the myofibers (i.e., myofiber hypertrophy). However, very little is known about the ultrastructural adaptations that drive this response. Even the most basic questions, such as whether mechanical load-induced myofiber hypertrophy is mediated by an increase in the size of the pre-existing myofibrils and/or an increase in the number myofibrils, have not been resolved. In this review, we thoroughly summarize what is currently known about the macroscopic, microscopic and ultrastructural changes that drive mechanical load-induced growth and highlight the critical gaps in knowledge that need to be filled.

**Keywords:** fascicle; myofiber; myofibril; sarcomere; hypertrophy; hyperplasia; splitting; radial growth; longitudinal growth; exercise
