ROOSEVELT PARK, Mich. - Liam Hoekstra was hanging upside down by his feet when he performed an inverted sit-up, his shirt falling away to expose rippled abdominal muscles.

It was a display of raw power one might expect to see from an Olympic gymnast.

Liam is 19 months old.

But this precocious, 22-pound boy with coffee-colored skin, curly hair and washboard abs is far from a typical toddler.

"He could do the iron cross when he was 5 months old," said his adoptive mother, Dana Hoekstra of Roosevelt Park. She was referring to a difficult gymnastics move in which a male athlete suspends himself by his arms between two hanging rings, forming the shape of a cross.

"I would hold him up by his hands and he would lift himself into an iron cross. That's when we were like, 'Whoa, this is weird,'" Hoekstra said.

Liam has a rare genetic condition called myostatin-related muscle hypertrophy, or muscle enlargement. The condition promotes above-normal growth of the skeletal muscles; it doesn't affect the heart and has no known negative side effects, according to experts.

Liam has the kind of physical attributes that bodybuilders and other athletes dream about: 40 percent more muscle mass than normal, jaw-dropping strength, breathtaking quickness, a speedy metabolism and almost no body fat.

In fitness buffs' terms, the kid is ripped.

"We call him The Hulk, Hercules, the Terminator," his mother said.

Liam can run like the wind, has the agility of a cat, lifts pieces of furniture that most children his age couldn't push across a slick floor and eats like there is no tomorrow -- without gaining weight.

"He's hungry for a full meal about every hour because of his rapid metabolism," Dana Hoekstra said. "He's already eating me out of house and home."

Liam's condition is more than a medical rarity: It could help scientists unlock the secrets of muscle growth and muscle deterioration. Research on adults who share Liam's condition could lead to new treatments for debilitating ailments such as muscular dystrophy and osteoporosis.

If researchers can control how the body produces and uses myostatin, the protein could become a powerful weapon in the pharmaceutical arsenal. It also could become a hot commodity among athletes looking to gain an edge, perhaps illegally, on the competition, experts said.

For Liam, the condition has one potential drawback: Infants and toddlers need some body fat to feed brain growth and the development of the central nervous system.

Without adequate body fat, a child's growth can be stunted and the central nervous system can be impaired, said Dr. Erlund Larson, an internist at Hackley Hospital who is familiar with Liam's condition.

That Liam appears to be thriving, physically and mentally, is almost as amazing as his feats of strength. The product of a troubled mother who gave him up for adoption at birth, Liam was born with a suite of medical problems.

The fact that Liam was adopted by a physician assistant's family hundreds of miles from his birthplace -- a stable family with the knowledge and means to give him all the food, nurturing, horseplay and love he needs to thrive -- might be the most miraculous part of his story.

"God works in mysterious ways," said Neil Hoekstra, Liam's adoptive father.

Myostatin-related muscle hypertrophy was first documented in beef cattle and mice in the late 1990s, according to scientific literature.

In 1997, researchers at Johns Hopkins University Hospital in Baltimore determined that Belgian Blue cattle, an unusually muscular breed, had mutations in the gene that produces myostatin. Those scientists also produced muscular mice by deactivating the rodent version of the myostatin gene, according to scientific journals.

The first human case was documented in 2000, in a German boy, but wasn't reported in medical literature until 2004. The condition is so rare in humans that scientists don't know how many people have it, said Dr. Kathryn R. Wagner, a genetics expert at Johns Hopkins.

A genetic mutation prevents some people from producing myostatin. Those individuals can have twice the normal amount of muscle mass, according to medical literature.

In Liam's case, the myostatin his body produces is rejected by muscle cells. He and others with his condition can have up to 50 per cent more muscle mass than the average person, experts said.

The result of both types of myostatin-related muscle hypertrophy generally are the same: above average growth of skeletal muscles, incredible strength, a warp-speed metabolism and minimal body fat.

"Liam's never had any body fat," his mother said. "The only fat he has is in his cheeks."

The so-called myostatin blockade has generated tremendous interest in the bodybuilding community. Some nutritional supplements claim to block myostatin, but researchers have said the claims are not scientifically valid.

"If the myostatin protein is knocked out, muscles grow and rejuvenate much more quickly," Dr. Larson said. "It has potential for great abuse in the future as the new steroid."

For Liam's parents, the most pressing challenge is feeding the boy enough protein every day to fuel his body's high-performance motor. The wiry but muscular toddler eats six full meals per day and still struggles to gain weight.

Dr. Larson, the first physician to suspect Liam had myostatin-related muscle hypertrophy, said he was amazed by the toddler's strength.

"He was able to grab both of my hands and nearly do an iron cross," Dr. Larson said. "This is not something that happens for most men, ever, and here is this kid with this kind of power."

Larson said Liam's strength gives him a huge edge over other children, physically and in terms of self-confidence.

"When you've got that kind of power and that kind of strength, the world is open to you," Larson said. "He's agile because he's so strong -- when you've got that incredible power as a kid you're going to try a lot more things."

Liam's father, a die-hard University of Michigan fan, already is dreaming big things for his adopted son.

"I want him to be a football player. He could be the next Michael Hart," Neil Hoekstra said, referring to U-M's star running back.

Liam was born four weeks early and had a small hole in his heart. He also had eczema, enlarged kidneys, was lactose intolerant and had severe stomach reflux that made him vomit several times each day, his mother said.

No one knew then that the baby was among the few people known to have myostatin-related muscle hypertrophy.

Dana Hoekstra said her suspicion that Liam was physically different quickly intensified. Two days after he was born, Liam could stand up and support his weight if someone held his hands to provide balance, she said.

His heart and kidneys healed within a few months, but it took 18 months before he stopped throwing up daily.

Liam's muscular thighs at 5 months of age gave him the appearance of a miniature Lance Armstrong. By 8 months, Liam was doing pull-ups and, a month later, climbing up and down stairs, his mother said.

What really amazed his parents was the way Liam fell.

"When he fell backward, he would land on his butt, but he never hit his head on the ground," 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."

Liam has given his mother a black eye and once punched a hole in the plaster wall during a tantrum. "That's called attitude," his mother said.

After a series of stunning physical exploits, Dana Hoekstra's father -- retired Muskegon attorney Darryl Cochrane -- told Dr. Larson about the boy.

"Grandparents like to brag and Darryl was bragging about how powerful this kid was," Dr. Larson said. "I had to see for myself."

Dr. Larson said Liam exhibited phenomenal strength.

"When I saw him I knew he had some condition," said Dr. Larson, who considered it "a wild longshot" that Liam could have myostatin-related muscle hypertrophy.

After Dr. Larson observed Liam, the boy's pediatrician referred the toddler to the genetics clinic at Spectrum Health in Grand Rapids. Doctors there said Liam was well below average for height, weight and head circumference.

But they noted "significant hypertrophy (enlargement) by the Hoekstras. The diagnosis: Myostatin-related muscle hypertrophy.of his leg, calf and arm muscles as well as increased strength," according to medical records provided

The doctors at Spectrum said Liam likely inherited the condition from his biological father, who was reported to be unusually strong, according to medical records.

An ultrasound performed on Liam when he was 14 months old revealed he had 40 percent more muscle than average, Dana Hoekstra said.

Liam's condition also caught the attention of Johns Hopkins researchers who were studying myostatin-related muscle hypertrophy.

A blood test determined that Liam did not have the genetic mutation that blocks all production of myostatin. Rather, he has the myostatin blockade, his mother said. His is one of roughly 100 known cases in the world, according to experts and medical literature.

Researchers at Johns Hopkins wanted to include Liam in a study of people with the condition. When they found 100 adults to participate, Liam was no longer needed. That was a relief for his parents, who did not want to subject Liam to the painful muscle biopsy that would be required of everyone in the study.

Dana Hoekstra said she was prepared to allow Liam to be part of the Johns Hopkins study if it could have led to new treatments for muscular dystrophy patients.

For now, the Hoekstras are content to let Liam lead a normal life. They have no plans to take the advice of friends who have jokingly suggested they hire an agent for Liam to line up pro sports deals or modeling contracts.

"It's great that he's going to have some extra muscle mass, but I don't want him to be viewed as some kind of freak," his mother said.

Dr. Larson said Liam shouldn't be viewed or treated differently than other children.

"He's a normal kid. He's just got that lucky twist," Dr. Larson said. "It's going to be fun to watch him grow."