I like sprinkling a little Crow Pose into my classes. Of all the arm balancing poses, it is the most accessible. It’s the gateway to the other addictive arm balances, I like to say. During one recent class, I noticed a woman, probably in her 60s, who didn’t want to try the pose. Fair enough. I always remind people in class that they are the authorities of their body and I am just there to guide and encourage them. And if I see someone reluctant to try something, I like to ask why.
So, I approached this lovely lady after class, asking her why she didn’t try Crow Pose. I like to know what holds a person back, whether that’s an old injury, fear, or something else. Her response was, “I guess I’m fearful that I might break my arms.”
“I guess I’m fearful that I might break my arms.”
This is where knowledge of how our body works can be incredibly useful — as a yoga teacher or just a yoga practitioner. While I cannot guarantee that someone will not injure themselves, I can share information about how our bodies adapt. I told her how bones —including those of the hands, wrists and arms — grow stronger by stressing them. Our bone mineral density can increase based on the demands we place on our bodies!
To her, this information was new. Surprised, she said, “I thought bones just got stronger by eating eggs and drinking milk.” While proper nutrition is necessary for the remodeling of bones, so too is the mechanical stress of loading the body.
I’m happy to report this lady has since begun to actively try balancing in Crow. While she has yet to balance in the pose — which is its own challenge — at least she has committed to trying!
The excerpt below is from my book The Physiology of Yoga and goes into the science of how bones adapt.
Being rigid structures, bones maintain the form of the body and protect internal organs. But they also create the framework for movement. All voluntary movement, including all movement performed in a yoga practice, happens at joints, which is where two bones articulate. And the loading that occurs during asana practice and other weight-bearing activities is very important to the health of bone.
Without your realizing it, your bones are adapting every day. While you might not give much thought to gravity, which is pulling you toward the center of the earth at a rate of 9.8 meters per second squared, your skeleton is constantly adapting to it. In the absence of gravity, as happens with astronauts in space flight, significant losses in bone mass occur. In fact, astronauts lose an average of 1 to 2 percent bone mass per month in space in a phenomenon known as spaceflight osteopenia (Kelly and Lazarus Dean 2017; NASA 2001). Most of the loss occurs in the lower limbs and lumbar spine, with the proximal part of the femur losing roughly 10 percent of its bone density for every six months in space, even though the astronauts exercise 2-1.2 hours per day, six days a week, using springs and vacuum canisters for resistance (NASA 2001).
To maintain bone density and strength, our body requires an adequate supply of calcium and other minerals as well as vitamin D, and the endocrine system must also produce the proper amounts of several hormones, such as parathyroid hormone, growth hormone, calcitonin, estrogen, and testosterone. Another requirement is adequate mechanical loading to induce remodeling.
Well before the advent of space flight, Julius Wolff first proposed in 1892 that bone adapts its architecture to the stresses put on it, a concept that came to be known as Wolff’s law (Wolff  1986). In 1964, Harold Frost refined this observation to reflect the knowledge that bones are not straight but slightly curved structures, and the mechanostat model was born (Frost 1964). As loads are applied to the body, this mechanical stimulus is converted into electrochemical activity in a process known as mechanotransduction. These signals are sent to the central nervous system, which responds by instructing the bone to build a stronger and denser framework to support the new demands. Osteoblasts are bone-making cells that initially live on the outside of the bone, turn into osteocytes, and become embedded within the bone, causing new bone to be laid down (Turner and Pavalko 1998). Meanwhile, osteoclasts break down older, damaged, or unhealthy bone tissue so the materials can be reabsorbed for new bone (Robling, Castillo, and Turner 2006).
In short, our nervous system senses how much bone strength is required and adapts accordingly. This process is happening so frequently that every bone in the body is completely reformed about every 10 years (Manolagas 2000). With physical activity and adequate amounts of hormones, vitamins, and minerals, trabecular bone develops into a complex lattice structure that is lightweight yet strong. In addition to the external force provided by gravity, the internal force provided by muscular contraction can also provide enough stimulus to elicit bone adaptation. As muscles contract, they pull on bones, and that tugging can create enough stimulus to increase the strength of bone (Russo 2009). But the best adaptations of bone happen through high impact and/or high load activities like plyometrics or resistance training.
Want to learn more?
Frost, H.M. 1964. The Laws of Bone Structure. Springfield, IL: Thomas.
Wolff, J., trans. 1986. The Law of Bone Remodeling (translated from the 1892 original, Das Gesetz der Transformation der Knochen, by P. Maquet and R. Furlong). Berlin: Springer Verlag.
Kelly, S., with M. Lazarus Dean. 2017. Endurance: A Year in Space, a Lifetime of Discovery. New York: Alfred A. Knopf.
Manolagas, S.C. 2000. “Birth and Death of Bone Cells: Basic Regulatory Mechanisms and Implications for the Pathogenesis and Treatment of Osteoporosis.” Endocrine Reviews 21 (2): 115-137. https://doi.org/10.1210/edrv.21.2.0395.
NASA. 2001. “Space Bones.” NASA Science. https://science.nasa.gov/science-news/science-at-nasa/2001/ast01oct_1.
Robling, A.G., A.B. Castillo, and C.H. Turner. 2006. “Biomechanical and Molecular Regulation of Bone Remodeling. Annual Review of Biomedical Engineering 8:455-498. https://doi.org/10.1146/annurev.bioeng.8.061505.095721.
Russo, C.R. 2009. “The Effects of Exercise on Bone. Basic Concepts and Implications for the Prevention of Fractures.” Clinical Cases in Mineral Bone Metabolism 6 (3): 223-228.
Turner, C.H., and F.M. Pavalko. 1998. “Mechanotransduction and Functional Response of the Skeleton to Physical Stress: The Mechanisms and Mechanics of Bone Adaptation.” Journal of Orthopaedic Science: Official Journal of the Japanese Orthopaedic Association 3 (6): 346-355. https://doi.org/10.1007/s007760050064.