Double muscling in cattle

Myofibrillar hyperplasia
Muscular hypertrophy

Myostatin (also known as growth differentiation factor 8, abbreviated GDF8) 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. Myostatin is a secreted growth differentiation factor that is a member of the TGF beta protein family.

Myostatin is assembled and produced in skeletal muscle before it is released into the blood stream. Most of the data regarding the effects of myostatin comes from studies performed on mice.

Animals either lacking myostatin or treated with substances that block the activity of myostatin have significantly more muscle mass. Furthermore, individuals who have mutations in both copies of the myostatin gene (popularly, but inaccurately called the "Hercules gene") have significantly more muscle mass and are stronger than normal. There is hope that studies into myostatin may have therapeutic application in treating muscle wasting diseases such as muscular dystrophy.

Source: Wikipedia

(G) Very specific problems