"These results suggest that coffee increases skeletal muscle function and hypertrophy by regulating the TGF-β/myostatin - Akt - mTORC1."
BACKGROUND ON MYOSTATIN
"Myostatin is a protein that in humans is encoded by the MSTN gene. Myostatin is a myokine that is produced and released by myocytes and acts on muscle cells to inhibit muscle growth." Wikipedia
Therefore myostatin is a negative regulator of muscle growth or muscle mass acting on skeletal muscle to determin both muscle fiber number and size.
When this gene is compromised or mutated and is therefore not functioning properly there is a pronounced increase in muscle mass in both animals and humans.
Therefore researching ways to inhibit myostatin release through diet or medical intervention has been shown to promote increases in size and strength of skeletal muscles.