It is well documented that both insulin-like growth factor-1 (IGF-1) and leucine stimulate skeletal muscle growth. Both IGF-1 and leucine stimulate mTOR signaling in skeletal muscle, which may be responsible for the enhanced muscle growth. There are two pathways contributing to skeletal muscle growth: (1) enhanced protein synthesis in existing muscle fibers; (2) activation of muscle satellite cells which fuse into muscle fibers. Researchers examined the effects of leucine and IGF-1 on the mTOR signaling pathway of satellite cells.  The research was performed in test tubes (in vitro study).  The researchers found that IGF-1 and leucine treatments stimulate mTOR signaling in myogenic satellite cells. Activation of mTOR signaling is necessary for the proliferation and protein synthesis in myogenic satellite cells stimulated by IGF-1 and leucine.

Han B, Tong J, Zhu MJ, Ma C, Du M. Insulin-like growth 
factor-1 (IGF-1) and leucine activate pig myogenic satellite
  cells through mammalian target of rapamycin (mTOR) 
pathway. Mol Reprod Dev. 2008 May;75(5):810-7.