Oh My Bones
The skeleton is in a constant state of flux, with osteoclasts breaking it down and osteoblasts making new bone. The force of a load --- which can just be your bodyweight -- confers different influences on this cyclical process depending on whether tension or compression forces dominate. IF there is no external load on the skeleton, such as when sitting, osteoclast activity predominates. So a sedentary life is particularly bad for bones.