Myostatin Deficiency
Myostatin Deficiency causes an unusual growth of the muscles of the body. Myostatin Deficiency will cause the person to look like a bodybuilder and have the same strength too BUT Myostatin Deficiency also affects the growth of the heart. Myostatin Deficiency is very rare and you can only get Myostatin Deficiency in the womb before birth.
There is a Thread about a child with Myostatin Deficiency, check here: Source 7