Scientists believe they may have discovered the secrets behind bigger muscles.
Myostatin controls the growth of muscles
Studies have suggested that a gene called myostatin controls the growth of muscles in animals.
But a team of international scientists has now found evidence to suggest it also affects humans.
The findings, published in the New England Journal of Medicine, could one day help people with muscle wasting diseases.
The discovery followed DNA tests on a German boy, born with unusually well developed muscles.
The boy's upper arm and upper leg muscles were roughly twice as big as other infants.
The scientists found that the boy had a mutation in the myostatin gene. This mutation meant he was unable to produce myostatin protein.
They believe that this was why his muscles were bigger than other boys of the same age.
The boy is now five years old. He is still much stronger than other children the same age.
So far, his lack of myostatin does not appear to be affecting his health.
However, doctors are keeping a close eye on him in case it causes key organs, like the heart, to increase in size.
A study published in 1997 found that knocking out the myostatin gene in mice enabled them to grow bigger muscles.
Studies on cattle also found that those that were naturally bulky produced less myostatin protein.
The scientists believe these latest findings show that the same process works in humans.
They also believe that techniques used to block this gene in mice could be used on humans, including those who have difficulty developing muscles.
"This is the first evidence that myostatin regulates muscle mass in people as it does in other animals," said Dr Se-Jin Lee, professor of molecular biology and genetics at Johns Hopkins University and one of those involved in the study.
"That gives us a great deal of hope that agents already known to block myostatin activity in mice may be able to increase muscle mass in humans too."
The UK's Muscular Dystrophy Campaign welcomed the study.
"This study furthers our information," Jenny Versnel, its head of research, told BBC News Online.
"However, we need more research before its full potential in humans can be evaluated. It is not a cure."
Products that claim to regulate myostatin, most of them untested, are already used by many athletes and bodybuilders.
However, John Brewer, director of the Lilleshall Human Performance Centre, warned people against using this latest study to justify taking them.
"Any athlete needs to be very careful with what they take," he told BBC News Online.
"We don't know if a lack of myostatin has an effect on key organs. Enlargement of the kidney, heart or other vital organs is potentially dangerous."