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Bone 102: Bone Dynamics


Yes, you read that correctly, dynamics- not a word we often associate with bones. Close your eyes, and bring to mind one of the bones you are familiar with in the human skeleton. If you cannot think of one, use the image below.


Notice the shape and ask yourself based on the shape- Is this bone meant to allow movement? Is it used as an anchor point for many muscular attachments? Is it super strong?

Just by looking at the appearance of bone, you can infer meaning about its function in the body.


Next, reflect that the bone you are visualizing is a cartoon, plastic or if real, a dead bone. Even if the bone you are imagining in your mind is actual human bone from a course you attended or a museum artifact, that bone has been dead for a long time- treated and cleaned before your eyes ever fell on it.


If so, how do living bones differ, how do they behave?


I recall my time in the beautiful cadaver lab at UCSF with the fantastic PhD of anatomy and chair of the Physical Therapy Department. The entire lab loomed high on Parnassus over-looking Golden Gate Park. By the time I arrived on the scene as a volunteer, the lab was state of the art. While I circulated around the cadavers with the graduate and medical students, it was so clear how boney anatomy changed and varied by lifestyle.

Pointing to a body shape that had fused into a flexed posture, the chair lectured the students “You can tell this person spent much of their later life in a wheel chair”.

What a dynamic lesson in the adaptability of living tissue, to study the tissue of the gracious people who generously donate their bodies to science after they have passed away.


In these people whose final act was so selfless as to donate their bodies, we witnessed wear and tear on bone where implants had been places. We observed the effects of orthopedic surgeries. In every case, bone did its best attempt to remodel in order to fit the needs of that person.


Living bone is extremely dynamic. It is taking cues from our lives and our movements, to adapt to what we need them to do all of the time. If we need our skeleton to conform to a chair, it will. If we need our skeleton to carry us up a hill, we can create the changes needed for that.

As we have already looked at, movement drives the development of our bone. Today, let’s take an extremely simplified dive into the how this process of bone remodeling occurs.


First, I must be clear- what I am presenting is a highly reductive process of what is actually a highly complex dance.

For simplicity and brevity sake, as well as the limits of my own knowledge of bone physiology, we will take a gentle dip into this deep well.


Your Bone is Cracking and Repairing All of the Time


Bone, similar to muscle, is constantly breaking down and building back up. Again and again, and below the level of our conscious awareness, bone fatigues, cracks and is broken down. We can induce the cracks ourselves with heavy weight lifting and heavy plyometric exercise, and also we can naturally assume bone cracks and fissures quite naturally throughout our daily movements.


These cracks or tiny breaks, are no problem. Our body, in its infinite intelligence, has a repair mechanism that signals certain bone cells (the osteoclasts) to come in and dissolve the hydroxyapatite (the hard matrix of the bone) adjacent to the small crack. Then osteoblast cells signal the cessation of this reabsorption activity, and become activated to lay down new (and improved) bone material, as well as healthy matrix material. Voila- the crack or micro break is silently repaired.


All of the time, and without our awareness, this process is going on. Tearing down and building up. Like a silent internal cal trans system. The bones are so highly vascularized and innervated, that all of the cells of our skeletons turn over every 10 years.


Of course there are systemic processes (such as my Grave’s disease and other thyroid/ parathyroid conditions) that affect this process, as well as medications (such as corticosteroids).

But all in all, in a healthy system, this process proceeds through our lifetime.


When We Overdo and Under Rest, Or Underdo and Over Rest We Affect the Rate of Bone Turnover


As we noticed in our post from last week, bone (like cartilage, muscle and tendons) requires a Goldilocks condition to function optimally.

This happens when we fall, when the forces our bone encounters (usually a result of acceleration of our body into a hard surface) are greater than what it can sustain.

We can also create excessive force across bone if we lift too heavy a weight too soon, or add acceleration (plyometrics) before we are ready.


One the other hand, bone (similar to muscle), does not transform (become more tolerant to forces and turnover and repair readily) if we do not challenge it.


For healthy bone physiology to proceed, we need to work to find the right amount of challenge- not too much and not too little. Creating the right amount of challenge across bone, allows remodeling to occur to make the bone stronger, denser and more resistant to the forces you expose it to (even unintended forces such as in a fall).


Now that we understand (a simplified version of) the process through which bone remodeling occurs, we can explore movement concepts to help our bone become stronger and more resilient to the forces of our modern lives.


If you are interested in learning more, please consider joining us this Sunday for our workshop Building Good Bones, April 24, 2022, 9-10 am and until then, happy moving!


Trina


Look how this scapula is almost see through! It is wafer thin. You can tell by the shape and appearance, this bone does not need to contain a lot of compression to do its job well. Instead, its broad, flat surface serves as attachment points for many important muscles of the upper extremity (including the famously problematic rotator cuff).



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