What would happen if long bones were solid?

Long bones are hollow and their cross sectional shape is a circle. Engineers will confirm that this structure is difficult to bend or twist. Under load walking, running and lifting, our bones do flex a little, but their basic shape helps to prevent them from grossly deforming or collapsing. If the central cavity of long bones were solid, bones would be considerably heavier, requiring much larger muscles to create movement. The added weight of extra muscle would compound the problem. If we could remodel long bones by using the same amount of bone to create a solid but thinner structure, the bone would have the same weight-bearing capacity, but its ability to resist twisting or bending would be reduced. Instead of flexing under load, thin bones would tend to fracture more easily. Another consequence would be that the fat store that currently exists in the central hollow area of long bones would need to be relocated. Blood production, however, would continue as now, since blood is produced in tiny cavities within the bony tissue itself at the ends of long bones, rather than in the central hollow marrow.

Reference: http://www.aphb.uwa.edu.au/research