Mechano growth factor (MGF), also designated IGF-1Ec, is a splice variant of the IGF-1 gene produced locally in skeletal muscle in response to mechanical loading and tissue microdamage, and is proposed to function as an autocrine/paracrine signal that activates muscle satellite cells and initiates the early repair response, a role distinct from the endocrine actions of systemic IGF-1. The unique C-terminal E-peptide domain of MGF is proposed to be the bioactive moiety responsible for satellite cell activation and progenitor cell proliferation, acting through mechanisms that are at least partially independent of IGF-1R and specific to the mechanical stress response rather than systemic growth signaling. Human muscle progenitor cell studies have demonstrated that the MGF E-peptide activates muscle progenitor cells and enhances their fusion potential across different age groups, suggesting a role in the age-related decline in muscle repair capacity. MGF is a research compound with no regulatory approval in any jurisdiction; exogenous administration is investigational, no human clinical pharmacokinetic or safety data has been established, and published evidence is limited to in vitro and preclinical contexts. MGF vs PEG-MGF: native MGF has a very short half-life in circulation, limiting its duration of action after exogenous administration. PEG-MGF (pegylated MGF) is a chemically stabilized version in which polyethylene glycol is conjugated to the peptide to extend its biological half-life — the same technology used to extend the half-life of therapeutic proteins such as PEG-EPO. Research compound suppliers often offer both forms; PEG-MGF is the more commonly studied variant in animal myopathy models for this pharmacokinetic reason. Neither form has human clinical data. MGF in muscle research: interest in MGF as a research compound centers on its proposed role in the lag phase of muscle repair following eccentric exercise-induced damage — the window when satellite cells are activated before proliferating and fusing into existing myofibers. In aged muscle, MGF expression following mechanical loading is reduced relative to young tissue, and this deficit has been proposed as a contributing factor to age-related sarcopenia in the research literature. Providers offering research peptides in the performance and recovery category are listed in the PeptideBase directory.