Mechanisms of IGF 1 Mediated Regulation of Skeletal Muscle Hypertrophy and Atrophy If your priority is building muscle, IGF 1 is what really matters. Increased testicular insulin like growth factor 1 is associated with gonadal activation by recombinant growth hormone in immature rats Reproductive Biology and Endocrinology Springer Nature Link Recovery responses of testosterone, growth hormone, and IGF 1 after resistance exercise Journal of Applied Physiology American Physiological Society IGF 1 LR3 Huma Peptide