Ramon Boudreaux, M.S.
It is often joked that if exercise could be given as a pill it would be the most prescribed drug in the world. While some benefits of exercise are well known (e.g., the prevention of heart disease, high blood pressure, and obesity), some remain esoteric. Osteoporosis, a condition in which bones lose density by an increase in porosity, is one such example. Low density bones are at a much higher risk of fracture. If one
were to compare the architecture of a bone to that of a truss bridge, it is easy to see how a breakdown in truss supports (or trabecular struts in the case of bone, see Figure 1) could lead to a failure. Under the same loading conditions, a compromised support system can be catastrophic. In fact, one in four adults over the age of 50 dies within one year following a hip fracture. Currently, over ten million Americans are suffering with osteoporosis and an estimated 34 million are at risk. Fortunately, however, osteoporosis is a treatable disease. Adequate consumption of calcium, vitamin D, fruits and vegetables promote good bone health. On the other hand, smoking, excessive alcohol consumption and lack of physical activity and exercise can lead to poor bone health.
Bone, much like muscle, is a living and dynamic tissue that responds positively to mechanical forces placed upon it. Exercise, especially weight-bearing activities such as jogging, playing tennis, and weight lifting, induces small cracks in the 
skeleton. Damaged bone surrounding these microscopic cracks is quickly degraded and removed by bone resorbing cells. New bone is then laid down by bone-forming cells. The interaction of these cells, known as remodeling, dictates the overall health of bone. Exercise has been shown to increase the rates of bone forming cells, as well as increase bone density, strength and resistance to fracture. The beauty of exercise as a countermeasure to osteoporosis is that it is never too early or too late to take action. If started early in life, like during puberty or adolescence, bone acquisition can be built up and stored. As an adult,even after the onset of osteoporosis, exercise can help mitigate or even reverse the adverse effects of the condition. While physical activity and exercise help maintain healthy bone remodeling, an absence of weight-bearing, during periods of weightlessness or immobility for example, can lead to bone loss. Both astronaut and bed rest data have shown that disuse can lead to losses in bone mass as high as two percent per month (Lang TF et al., J Musculoskelet Interact., 2006). Regardless of the type of disuse, via an immobilized body part from a stroke or fracture, prolonged periods of bed rest after surgery or illness, or floating around in microgravity during spaceflight, exercise should be resumed as quickly as possible. In the case of bone, the old saying is true – If you don’t use it, you lose it!
For further reading on this topic, please visit:
- National Osteoporosis Foundation –www.nof.org
- NIH Osteoporosis and Related Bone Diseases National Resource Center – www.niams.nih.gov/Health_Info/Bone
Figure 1: The trabecular architecture of normal bone (A) is responsive to loads placed upon it and adapts to efficiently distribute the loads. Osteoporotic bone (B) lacks the efficient trabecular network of normal bone. Image obtain from Wiley et al., Clin Rev Bone Miner Metab. 2011.
Figure 2: Bone Remodeling Process - When bones experience loading, through weight-bearing exercise for example, remodeling is initiated via resorption of damaged bone and formation of new bone. Image adapted from the University of Michigan: http://www.ns.umich.edu/Releases/2005/Feb05/bone.html