Superheroes, a genetic possibility?
"The Hoekstras quickly began to notice Liam being able to do things out of the ordinary. Two days after birth, he was able to fully stand-up and support his own weight, given someone held his hands. Months later he began developing ripped abs, naturally doing pull-ups, inverted sit-ups, Olympic styled iron cross, thigh muscles compared to that of Lance Armstrong and even punching holes into walls during tantrums (accidentally gave his Mom a black eye once as well).
What really amazed his family was what happened whenever he fell.
“When he fell backward, he would land on his butt, but he never hit his head on the ground,” , his adoptive mother, Dana Hoekstra said. “His stomach would tense up and he would catch himself before his head hit the ground. You could see his stomach muscles. He had a little six-pack,” said Dana, in an interview.
After further investigation into Liam’s ability, doctor’s found he was genetically unique as he has a condition called myostatin-related muscle hypertrophy.
In Liam’s case, his body’s muscles reject myostatin, allowing them to undergo quick growth and repair."-end snip
It's being researched as a cure for people who suffer from muscular defects/problems, it could also potentially be used to make people "double-muscled" and it would be difficult to detect in athletes.