Dr Luke Perraton physiotherapist and physiotherapy educator/researcher
I find the human body fascinating. Unlike a machine that is built from parts that need maintenance and replacement, the human body is a living, adaptable independent organism. One of the most adaptable tissues in the human body is bone.
Bones are living tissue that hold up our bodies against the forces of gravity and allow us to move.
Bones act as levers to transfer forces from our muscles to the ground or the objects that we interact with. They protect our organs from damage; for example, our ribs protect our lungs and our skulls protect our brains. They store important minerals like calcium and they buffer the levels of these minerals in our blood stream. Bones protect the marrow that powers our immune systems and makes red blood cells to transport oxygen to our cells.
The dead bones you see in movies or in a museum are very different from the bones in the living body. Dead bones are what is left of the minerals within the bone structure. They are light and weak. The bones within our body are strong, heavy and very much alive.
Here are some incredible facts I want to share with you about bones and how they develop:
Bones are stronger than steel on a kilogram for kilogram basis (pound for pound for our North American friends). Bones are many times lighter than steel and they grow strong in specific places where strength is needed most; for example, at the end where they bear weight and in the middle where bending forces are greatest.
Bones can bend. It is normal for your bones to bend slightly as you put weight onto them. This helps the bone to spread the force throughout its structure and makes it less likely to crack or break. Materials that are less bendy and more brittle are more likely to break; for example, concrete. We need our bones to be very strong, relatively light and slightly bendy.
Bones that have more load placed through them during daily life are slightly curved. The long bones in your legs are noticeable curved to help them bend slightly and absorb/spread ground contact forces. the long bones in your arms are not as curved.
Bones are made from a hard-outer layer and a spongy inner layer. Both hard and spongy bone are living tissue with a rich blood and nerve supply. Bone is constantly remodelling in response to the forces placed through it. When you walk, run, jump, land and lift weight you produce force in your muscles which adds to the force of gravity and this force is transferred through your bones. Every time this happens, specialised cells within the bone remodel the bone and make it stronger.
As bone remodels, the harder bone on the outside gets thicker and stronger and the spongy bone on the inside develops stress lines that align with the forces put through the bone. On cross sections of spongy bone you can see these lines of stress where the bone has adapted to the loads placed on it.
You are not the same person you were ten years ago. Well, from the point of view of your bones that is. Every ten years the bones within your skeleton have turned themselves over and created new bone. As you get older, the process of bone remodelling slows down. This is one of the reasons why physical activity and strength training is so important to maintain as you get older.
All bones develop in utero from mesenchyme, an embryonic connective tissue. By the time you are born, most of your bones have started to ossify – the process in which cartilage turns to bone. A baby’s body has over 300 bones at birth and many of them are yet to ossify. This is why babies are very flexible – an advantage for childbirth. As you grow, these bones fuse together, ossify and become harder so you can walk, run, jump, play and interact with the world.
There are so many more amazing facts about bones – I am going to have to save them for future blogs. In an upcoming blog, I will share what I know about bone health in adults, why bones break and common bone injuries to watch out for in adults and kids.
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About Luke
Luke is a physiotherapy educator and researcher from Monash University, Australia. He teaches into the first year of the Bachelor of Physiotherapy course in the Department of Physiotherapy and is the co-director of the Monash Musculoskeletal Research unit. Luke is the co-founder of Perraton.Physio and he runs online CORE & FITNESS classes with Zuzana twice a week. Connect with Luke @LukePerraton.