This is a scientific Americans 60 second science. I'm Karen Hopkins.

Exercise builds muscle, but it also strengthens bones.

Exactly how exercise boosts bone strength is a matter of debate and a subject of scientific

scrutiny. Now a new study in mice shows how a hormone secreted by active muscle cells

triggers bone remodeling.

Physical activity stimulates the release of multiple molecules from skeletal muscle.

Take irison, for example.

This hormone, named after the Greek messenger goddess Iris, is produced by running rodents,

and by us humans when we do high intensity aerobic training.

Once it's generated, Irisin improves bone density and strength.

To find out how, Bruce Spiegelman and his colleagues at the Dana-Farber Cancer Institute

and Harvard Medical School took cultured bone cells and located the receptor to which

Irisen binds.

This receptor protein is particularly abundant on cells called osteocytes.

These cells are the master regulator of bone remodeling and the most abundant cell type in bone.

What we found interesting in this paper is the magnitude of the effects we get in terms

of irison acting to stimulate osteocytes and preventing their death in culture.

Also their ability to make a key protein sclerosis under the stimulation of irison.

A sclerosisin is a protein that actually stimulates the breakdown of bone.

The irison, which builds bone, would promote the production of Sklerostin, which destroys it, may seem

counterintuitive. But Spiegelman says that it's this bit of bone breakdown that signals the body to engage in some

skeletal renovation.

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