No, this isn't regular pharmaceutical IGF-1 I'm talking about, it's an isoform (same gene, different protein).
Muscle has an intrinsic ability to change its mass and phenotype in response to activity. This process involves quantitative and qualitative changes by alternative splicing in gene expression, including that of the myosin heavy chain isogenes that encode different types of molecular motors.
This, and the differential expression of metabolic genes, results in altered fatigue resistance and power output. The regulation of muscle mass involves autocrine as well as systemic factors. We have cloned the cDNAs of local and systemic isoforms of insulin-like growth factor-I (IGF-I) from exercised muscle. Although different isoforms are derived from the IGF-I gene, the RNA transcript of one of them is only detectable following injury and/or mechanical activity. Thus this protein has been called mechano growth factor (MGF).
Because of a reading-frame shift, MGF has a different 3' sequence and a different mode of action compared
with systemic or liver IGF-I. Although MGF has been called a growth factor, it may be regulated as a local repair factor.
-G. Goldspink, Gene Expression In Skeletal Muscle, Biochemical Society Transactions (2002) Volume 30, part 2 285-290.