When we’re pumping iron and getting all swole up in the mirror, most of us are focused on the mind/muscle connection. Visualizing the muscles working, getting filled with blood, and fatiguing as we put them through intense reps is what lifting is all about, but we rarely focus on the bones that support our hard-working muscle and tendons.

Scientists haven’t really thought much about what exercise does to bones either, but researchers at the University of North Carolina School of Medicine just released a study that found physical activity torches the fat in bone marrow, which leads to stronger and thicker bones in just a few weeks. The study, which was done on mice, discovered that—contrary to the previous opinion that bone marrow is not used to fuel energy when working out like other fat stores do—the fat in marrow plays an important role in bone health and may create more bone when burned during exercise.

Researchers gave a group of lean and obese mice a running wheel for exercise when they were four months old, and then analyzed their body composition along with marrow and bone quality throughout the study. At the beginning, the fat mice had bigger and more fat cells in their marrow than the skinny rodents, but after six weeks of exercising they both showed a drop in size and quantity of fat in the bones, with the obese mice showing the most significant changes.

"One of the main clinical implications of this research is that exercise is not just good, but amazing for bone health," said lead author Maya Styner, M.D., assistant professor of endocrinology and metabolism at the University of North Carolina at Chapel Hill. "Obesity appears to increase a fat depot in the bone, and this depot behaves very much like abdominal and other fat depots. Exercise is able to reduce the size of this fat depot and burn it for fuel and at the same time build stronger, larger bones.