By Michelle Roberts
BBC News health reporter, in Washington DC
US scientists have made a "mighty mouse" whose big muscles could help find a way to prevent bones weakening.
Brittle bones can fracture easily
It has 70% more muscle mass than normal mice as it lacks the myostatin gene that ensures muscles do not overgrow.
Six years ago, a German boy was born with a myostatin mutation and had muscles twice the normal size.
A Medical College of Georgia team says bones may respond to the stress placed on them by extra muscle by forming more bone - helping to fight osteoporosis.
Dr Mark Hamrick, who is leading the work on the "mighty mouse", said: "We are interested in kids.
"We want to know how to maximise their bone during peaks of growth while they still can," he told the annual meeting of the American Association for the Advancement of Science (AAAS).
He said bone mass by the end of the pubescent growth spurt was a good predictor of a person's future risk of osteoporosis.
The myostatin gene is expressed at highest levels as babies and children grow, tapering off in adulthood. Its job is to make sure the muscles do not overgrow.
By knocking out myostatin, Dr Hamrick's team will be able to see what effect this has on the bones to which the muscles are attached.
"People have argued for many years that the way to increase bone density and strength is force."
He said bones responded to stress and strain placed on them by forming more bone.
"The best way to increase that force is muscles."
Fit not fat
Some studies have suggested that fat can have the same effect, but Dr Hamrick said the increasing number of obese children in the US tended to have lower bone masses which went against this theory.
In addition to their big muscles, mighty mice essentially have no fat because they lack myostatin.
Since the summer, Dr Hamrick's team has been comparing the mice with other ones that are overfed to become obese.
"We can look at whether muscle has more of an impact on bone mass than fat because our mighty mice don't gain fat," he said.
The other thing they want to find out is whether increasing muscle mass by exercise alone would have the same effect as knocking out myostatin.
Dr Hamrick said his early findings showed exercise did not benefit the bones of mighty mice as much as normal mice.
"There is no easy way out. The best way to make our kids strong, healthy and happy is a good diet and exercise," he said.
He is also examining mice with a disorder similar to Duchenne muscular dystrophy in humans.
These mice have big muscles but the muscles themselves are weak.
"This allows us to tease apart how muscle size affects bone versus muscle strength," he said.