Wolff's Law

People usually think that only muscles grow, and it rarely occurs to them that bones grow too. Intuitively, it can feel as though bones are organs that simply deteriorate once you become an adult. But bones grow as well.

What is interesting about bone growth, though, is that the structure gets optimized too. When we talk about growth, we often think only of growth in quantity. For example, when we think of muscle growth, we mostly think of hypertrophy, an increase in the amount of muscle. When a bone is put under load, its structure changes to fit that load, and this is called Wolff's law.

Wolff's Law

Wolff's law is a theory formulated by the German surgeon Julius Wolff. It says that bone adapts to the loads placed on it. To put it colloquially, bone is "crispy on the outside, moist on the inside" (...). The outside is made of dense cortical bone, and the inside is made of cancellous (trabecular) bone, which is relatively less dense. 겉바속촉 구조는 같은 질량 대비 높은 굽힘 강성을 보유한 좋은 구조다.

So if a load that does not cause a fracture (microfractures included) is applied continuously, the cortical bone becomes thicker, and the cancellous bone changes its structure to support that load well. As you can see in the bone photo above, the cancellous bone is not connected randomly. It shows a pattern, such as crossing over in the central part. This process is called bone remodeling.

Having a lot of bone is not necessarily a good thing. For one, the energy cost of maintaining it is high. For humanity, surviving the ice age was a bigger problem than breaking a bone and getting hurt or dying. Also, extremely high bone density is itself a disease, called osteosclerosis. In this case, the bones can actually break more easily, because the cancellous bone can no longer absorb shock the way a sponge does. Like this, everything always comes with a trade-off.

For that reason, if no load is applied to a bone, its density decreases and its structure thins out. If it goes without load for a long time, the structure can even break apart, and because this is degenerative, it cannot be recovered. This is a problem seen in astronauts who have spent a long time away from the influence of gravity. ¹ The reason it is irreversible is that a broken structure cannot transmit load, so no matter what exercise you do afterward, it is impossible to restore that structure. This is also why osteoporosis is a frightening degenerative disease.

In this way, bone is a textbook example of an organ that follows "use it or lose it." That is why it is important to exercise appropriately from a young age and take good care of your bones.