Why do we stiffen up as we age after not moving for a long time?

Excerpt from:
Q & A: Still, Then Stiff
Social Good Through Social Media: Together We Can Move Mountains.
Why do we stiffen up as we age after not moving for a long time?

Excerpt from:
Q & A: Still, Then Stiff
Previous post: Personal Health: Risks, as Well as Hope, for Very Tiny Infants
Next post: Monday Morning!