Why is bone density relevant? Consider this statistic: One in five people over the age of 65 who suffer a hip fracture will die within 12 months of the injury.*
How does bone strengthen itself? We now know that bone lays down new cells in response to applied forces through the bone that are sufficient to cause some bending of the bone. This in turn causes a squeezing action that mobilizes fluid from one part of the bone matrix to another. The fluid bathing the cells promotes a response to produce denser bone.
So which types of exercise causes bone bending? The authors of the study on bone bending theory suggest that fairly large forces in bursts are what is required. In practice, the best exercise for older women is BRISK WALKING. The authors further stipulate that running does not necessarily create the required forces.
Hop to it!
Lastly, for those who have sufficient bone strength (and presumably muscular strength), the optimal method for stimulating bone density is JUMPING ON THE SPOT!
*From information sourced from the American Medical Association in PhysEd: The Best Exercises for Healthy Bones, Gretchen Reynolds