Bones, adaptation, and learning

For years, I fixated on how muscles worked, I cared which muscle was working in various positions, which muscle was a prime mover, which was a stabilizer. Muscles were deeply important, as though they themselves resulted in the complete picture of movement.

Like many things one spends time thinking about, I eventually realized this vantage point was incomplete. Muscles are, of course, one piece of the puzzle, just like the nervous system is another piece. If you step back and look at all of the pieces that allow us to move our bodies in space, the muscles are moving the bones. The suffix “skeletal” often takes a back seat when people are discussing the musculoskeletal system. It’s like the second child- just as important but the first, but doesn’t usually get as much attention. 

Bones transmit forces, from one joint during the next (1). During development, bones are more pliable. They have less of the inorganic materials, such as calcium, that make bones more rigid in adulthood. Areas like the clavicle remain largely cartilaginous until adulthood, meaning your skeleton isn’t fully mature until about the age of 24 (2). The bones, like the brain, aren’t fully developed, until after the teenage years. This may be partially why children in general seem less breakable than adults- their bones are weaker but more bendable, able to withstand greater amounts of energy before failure (3). Like their brains, which are extremely plastic in nature to accommodate new information and learning, their bones are a little more malleable, ready to adapt to the forces from the world.

Living bone is made of water, collagen, and calcium salts. Collagen enables the bone to be both pliable and strong, giving it tensile strength. When you hang from a pull-up bar, this tensile strength prevents your humerus from breaking apart under the pressure. It’s like when you take a rubber band and you pull the ends away from each other- the tensile strength is what prevents the rubber band from breaking. 

The calcium salts found in bone allow it maintain a sense of rigidity, providing compressive strength. It’s the reason when you land after jumping, for instance, the bone doesn’t snap. (A ceramic pot can withstand 1/2 as much compression as bone. A useful image when you are landing from a jump might be imagining your tibia made of ceramic material that you don’t want to shatter on landing. I bet that makes you feel lighter). 

This balance, between flexibility and rigidity in different positions, is what enables us to go about our everyday lives without being afraid of breaking. 

Bone is a living tissue through the duration of our lives, capable of changing its size and composition to tolerate the loads imposed upon it. However, If we move in the same manner repeatedly and increase the duration of the load significantly without allowing time for the musculoskeletal system to adapt, the force can be greater than what the bone can withstand (4). Runners who land with very little variation, step by step, mile upon mile, may eventually exceed the bone’s capacity to withstand load when they suddenly increase their mileage to twice as much as the week before. When the force is greater than what the system has adapted for, it breaks, resulting in a stress fracture. However, if we apply increased demands incrementally, with time for bone to adapt, the bone becomes stronger. It changes its constitution to withstand more and more force.

Perhaps this is why elite marathon runners from Kenya remain largely injury free (5). They run from a very young age. The bone adapts to meet the demands; as they develop, so too do their bones. They run on dirt, often times barefoot, which results in a foot strike that is consistent, but able to adapt to whatever surface they are encounter. They increase their mileage and intensity gradually, until they are eventually averaging 6-13 km/day They are, essentially, loading their bones in a variety of ways. Contrast this with the average recreational runner. She runs, increasing her mileage faster than she probably should, in shoes and on a hard surface. There is little variation in her foot strike, which means she applies a similar load to the bones in her lower extremity each time she goes for a run. When she increases her mileage in one fell swoop, ignoring the general advice that you shouldn’t do more than 10% this week than you did last week, instead of adapting, things begin to break down. The tissue adapts until it can’t, until the repetitive load exceeds the capacity for adaptation and she ends up, frustrated and broken. 

The muscles exert forces on the bone via tendons (6). They are a secondary stabilizer to the joints (the ligaments are the first). The force produced by the muscles provides this sense of stability. This is one of the reasons people with hypermobility tend to feel more stable when they begin gaining strength- the muscles are literally giving them more stability. Embedded inside the joint capsule are sensory receptors that provide feedback to the central nervous system (CNS) about joint position (7). The central nervous system uses this information to determine which movement options are available to optimize a balance between internal and external forces and which motor neurons to fire to make this happen. It’s a feedback loop, where the brain and the skeleton are in constant communication regarding what the best options are for the system to perform the desired movement efficiently and in a way that is non-threatening. The brain and the mechanoreceptors in the skin, muscles, joints, ligaments, and tendons are like helicopter parents deciding how the muscles should work- the muscles don’t have a lot of choice unless there is a conscious decision by the user to move in a different way.

You can decide to create more or less force in a muscular contraction through conscious thought. If you don’t think about a movement at all, the nervous system will carry out an action in the most efficient way possible, determined by the conversation between the sensory nervous system and the brain’s interpretation of that information. As most of us know by watching people move, sometimes movement patterns can benefit from an intervention to make things more efficient. 

This parallels our emotional responses. Our habits, both physical and emotional, don’t always serve us in the most effective way. 

One of the ways bones adapt to external and internal forces is, like muscles, they get bigger (1). They become a little bit heavier, increasing their mass, and more dense looking. As we age, bone loses mass. It becomes less responsive and more brittle if it isn’t used. 

Mindsets as we age can do this, too. We’ve all heard the phrase, “she’s set in her ways.” This implies a lack of flexibility, as though there are no longer options in how the day goes or what we experience. Like the grooves of a tire that are well worn, the outlook on life and our daily emotional habits become repetitive, with no change of veering off course.

We know, of course, that it is possible to learn new things, experience new adventures, stay engaged and curious about life until death. It requires using the brain in that way, reading the opposing point of view once in a while, and interacting with the world in a meaningful way regularly. 

The bones are the same. If we continue to apply varied forces, both internally and externally and we regularly use our bodies in ways that challenge us, we stay physically strong. There will always be changes that occur physiologically and emotionally as we grow older. That’s part of life, but we don’t have to be destined to frailty or a bent over shape.

When we learn a new language or dive into understanding a difficult subject, the neurons in our brain work to begin establishing new connections. We draw on past knowledge to give us a foundation for understanding. If it’s been a while since we’ve studied something, it takes a while for things to come back. Instead of being able to jump right into a complex algebra problem if you haven’t looked at math in 25 years, you would begin by practicing easier problems until you began to remember how things worked. We have to gain a grasp on the basics before we can really explore the deeper nuances of the subjects. 

If we are currently physically active and we try a new movement or skill with our body, we are likely to tolerate the movement well. It might be awkward at first, but we don’t have to worry about injuring ourselves because we know our skeleton can withstand a fair amount of force from previous activities. If, on the other hand, the person in front of me hasn’t moved in 30 years, save from walking to and from his desk to the parking lot, he can withstand less force. He doesn’t have a basic framework to draw from, his bones aren’t used to extra load. This simply means the coach or trainer has to spend a little more time easing the individual into exercise. Maybe programming 10 exercises for 3 sets of 10  isn’t the right choice the first session; instead, introducing one or two exercises and a lot of mobility and motor control work might be more appropriate. It’s not that you won’t ever get to a full exercise program; you are simply priming the body, getting it used to the increased load, getting the skeleton used to the higher levels of force. 

When you look at the person in front of you, instead of fixating on how the muscles are working, think about how the bones are receiving force. The change in perspective might help you see things a little bit differently. Neither is right or wrong, but changing the perspective can make certain things a little bit clearer. It can carve out a path when the practitioner is stuck because someone experiences discomfort or can’t quite seem to do an exercise in a fluid way. All parts of the person matter when you are coaching, including the bones.

References:

  1. Abernathy, B., Hanrahan, S., & Kippers, V., Pandy, M., McManus, A., & Mackinnon, L., (2013). Biophysical Foundations of Human Movement, 3rd Edition. Human Kinetics:
  2. McGraw, M.A., Mehlman, C.T., Lindsell, C.J., & Kirby, C.L., (2009). Postnatal growth of the clavicle: birth to eighteen years go age. Journal of Pediatric Orthopedics, 29(8), 937-943.
  3. Subrata, P., (2013). Design of Artificial Human Joints & Organs. Springer: New York.
  4. Warden, S.J., Burr, D.B., & Bruckner, P.D., (2006). Stress fractures: pathophysiology, epidemiology, and risk fractures. Current Osteoporosis Reports, 4, 103-109.
  5. Kong, P.W., & de Heer, H., (2008). Anthropometrics, gait, and strength characteristics of Kenyan distance runners. Journal of Sports Science and Medicine, 7, 499-504.
  6. Maas, H., & Sandercock, T.G., (2010). Force transmission between synergistic skeletal muscles through connective tissue linkages. Journal of Biomedicine and Biotechnology. https://www.hindawi.com/journals/bmri/2010/575672/
  7. Frontera, W.R., Herring, S.A., Micheli, L.J., & Silver, J.K., (2006). Clinical Sports Medicine. Saunders: Philadelphia.