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Biomechanics: The study of the mechanical laws relating to the movement or structure of living organisms (in this case, bone).
The human body has a magnificent capability to adapt to environmental stresses; stress being defined as pressure or tension exerted on a material object. This adaptation capability, also known as Wolff's Law, allows the body to become stronger or weaker respectively in presence or absence of stress. All biological tissues in the body, including bone and muscle, adapt to the demands placed on it, such as during physical activity. All physical activity, which includes exercise and physical fitness, places stress on bone to some degree. Bone has many physical properties, including but not limited to: bone mineral density (strength + stiffness) and elasticity. The strength of a bone is determined by its size, shape, cortical thickness, cross-sectional area, and trabecular architecture. These physical properties enable bone to overcome the physical forces of life, such as from the contraction of muscles and the force of gravity. The forces that are applied to bone include, compressive stress, tensile stress and shear stress. Compressive stress occurs when bone is pushed together, thus shortening bone. Tensile stress occurs when bone is pulled apart, thus lengthening bone. Shear stress occurs when bone is twisted or bent (Rubin & Ott, 2003). In theory, there is a local control system that influences the way that bone adapts to stresses; this theory is called Mechanostat theory. |
Image by Henry Vandyke Carter on Wikimedia Commons
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As noted previously, the bones, like the rest of the body, adapt to the demand of various forces. Mechanical forces and bone regeneration are intimately linked (Betts & Müller, 2014). Without the force of your body weight being placed on the bones, over long periods of time, atrophy will occur. This phenomenon can be explained via the Mechanostat theory and mechanotransduction. The Mechanostat theory states that there is a control system that influences bone adaptation. The control system that signals and permits bone adaptation occurs via a four-step process, known as mechanotransduction. The first step of mechanotransduction occurs when a force, such as compression or tension, is detected by sensor cells, known as osteocytes and bone-lining cells; this is called mechanocoupling. Following the detection of the force, the osteocytes and bone-lining cells must communicate that load to the cytoskeleton; this step is called Biochemical Coupling. The third step in mechanotransduction is the transmission of the biochemical signal. This occurs via the sensor cells, osteocytes and bone-lining cells, signaling the effector cells; in this case the effector cells are osteoblasts. Finally, the effector cells, osteoblasts, respond by producing bone; this step is also known as the Effector Response (Jaumard, Welch, & Winkelstein, 2011).
For example, disuse atrophy, a condition that is caused by not bearing weight on the bones during the recovery period, may occur from a prolonged period with absence of forces. Instead of osteoblasts producing bone, osteoclasts will remove bone due to the absence of forces. This is where a physical therapy program may be beneficial to progressively reintroduce forces on the bones, which in effect will cause osteoblasts will begin to build and repair bone. Research on animals suggests that physical activity strengthens bone via an increase in cortical thickness and cross-sectional area (Woo et al., 1981).
Bone repairs itself by modeling and remodeling. Modeling is the process of new bone being formed, by osteoblasts (cells that produce bone), over old bone; this can also be described as the deposition of new bone. Remodeling is the process by which old bone is removed, by osteoclasts (cells that remove bone), and is replaced by new bone; this can also be described as the resorption of old bone.
The common saying, " Use it or lose it" can be applied to many aspects of life, but it can definitely be applied when referring to the biomechanics of bone.
For example, disuse atrophy, a condition that is caused by not bearing weight on the bones during the recovery period, may occur from a prolonged period with absence of forces. Instead of osteoblasts producing bone, osteoclasts will remove bone due to the absence of forces. This is where a physical therapy program may be beneficial to progressively reintroduce forces on the bones, which in effect will cause osteoblasts will begin to build and repair bone. Research on animals suggests that physical activity strengthens bone via an increase in cortical thickness and cross-sectional area (Woo et al., 1981).
Bone repairs itself by modeling and remodeling. Modeling is the process of new bone being formed, by osteoblasts (cells that produce bone), over old bone; this can also be described as the deposition of new bone. Remodeling is the process by which old bone is removed, by osteoclasts (cells that remove bone), and is replaced by new bone; this can also be described as the resorption of old bone.
The common saying, " Use it or lose it" can be applied to many aspects of life, but it can definitely be applied when referring to the biomechanics of bone.
References
Betts, D. C., & Müller, R. (2014). Mechanical regulation of bone regeneration: Theories, models, and experiments. , 5, . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4261821/
Jaumard, N. V., Welch, W. C., & Winkelstein, B. A. (2011). Spinal facet joint Biomechanics and Mechanotransduction in normal, injury and degenerative conditions. , 133(7), . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3705911/
Rubin, C., & Ott, S. (2003). ASBMR educational materials. Retrieved November 29, 2016, from https://depts.washington.edu/bonebio/ASBMRed/mechanics.html
Woo, S. L., Kuei, S. C., Amiel, D., Gomez, M. A., Hayes, W. C., White, F. C., & Akeson, W. H. (1981). The effect of prolonged physical training on the properties of long bone: A study of Wolff’s law. Archive, 63(5), 780–787. Retrieved from http://jbjs.org/content/63/5/780.abstract
Jaumard, N. V., Welch, W. C., & Winkelstein, B. A. (2011). Spinal facet joint Biomechanics and Mechanotransduction in normal, injury and degenerative conditions. , 133(7), . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3705911/
Rubin, C., & Ott, S. (2003). ASBMR educational materials. Retrieved November 29, 2016, from https://depts.washington.edu/bonebio/ASBMRed/mechanics.html
Woo, S. L., Kuei, S. C., Amiel, D., Gomez, M. A., Hayes, W. C., White, F. C., & Akeson, W. H. (1981). The effect of prolonged physical training on the properties of long bone: A study of Wolff’s law. Archive, 63(5), 780–787. Retrieved from http://jbjs.org/content/63/5/780.abstract
