Figure 2.

Overview of how the extracellular matrix (ECM) and its cellular components influence muscle fiber adaptation to exercise and contribute to muscle growth. A: muscle fibers are surrounded by the ECM that contains several mononuclear cell types that influence the microenvironment. The ECM also relays mechanical signals that can trigger growth. B: in response to mechanical loading, the ECM and its cellular constituents produce and release growth factors that may affect intracellular muscle fiber hypertrophic processes. Fibro/adipogenic progenitors (FAPs) deposit collagen in the ECM and signal to satellite cells, thereby influencing their propensity to fuse and contribute myonuclei to the muscle fiber (10). Satellite cells also proliferate and communicate signals to fibrogenic cells (1, 11) and muscle fibers (12) via extracellular vesicles to control collagen deposition and remodeling during loading. The muscle fiber itself can release factors during loading that act on macrophages, specifically affecting the production of matrix metalloproteinases (MMPs) (13, 14). MMPs influence collagen turnover to facilitate a proper adaptive response. Globally, an upregulation in collagens and ECM turnover factors in skeletal muscle tissue is characteristic of a well-conditioned phenotype following various types of chronic exercise, including resistance training (15, 16). (Created with BioRender.com).