Bone Density: Myths and Truths These days, people are not only interested in longevity – they wish to stay active and healthy as long as possible.  I have written before about how maintaining a physically active lifestyle as we age delays the onset of age-related disease and disability by a remarkable 15-20 years.  Of course, everyone dies eventually, and modern medicine in many cases allows individuals to live for years with serious health problems that would have killed swiftly in decades past.  The crucial issue here is not number of years lived – it is the quality of life one maintains through the aging process.  We all know someone (or many people!) who develop a myriad of health problems in their 50s and 60s, and although they may live another 20 years it is with great pain and disability.  On the other hand, most of us can name an individual we know who manages to stay active, independent and relatively healthy until their 80s.  My grandfather comes to mind – although now at the age of 96 he is unwell, he maintained an active and full life until past his 90th birthday.  He exercised every morning of his adult life until recently, including a daily jaunt on the stationary bicycle in his basement in addition to a daily walk. An aging, health-oriented population has resulted in widespread interest in osteoporosis prevention.  Most older adults (60+) know very well that weakened bone structures can turn a minor fall into major disability.  Osteoporosis, although striking women more frequently, affects men as well.  Many people are familiar with the fact that regular exercise, especially weight training, can have a positive effect on bone tissue formation.  Bone renews itself through a process called bone remodeling, which involves two phases – formation and resorption.  Bone tissue formation, also called ossification, is the process by which soft cartilage transforms into hard bone.  Bone resorption is the process by which osteoclasts (specialized cells that secrete enzymes that dissolve bone) break down bone and transfer calcium, magnesium, and phosphate products from bone fluid to the bloodstream – ultimately weakening bone structure.  Bone resorption can be the result of disuse and lack of stimulus for bone maintenance – that is to say, lack of physical activity.  During childhood, bone formation exceeds resorption, but as people enter their 40s and 50s, resorption exceeds formation.  Preventing bone resorption is one of the central arguments for promoting physical activity. The great question the above information elicits is: what type of exercise most favourably counteracts bone resorption?  More specifically, is it the anti-gravity nature of weight bearing exercise modalities – such as running or walking – or it is muscle contractions themselves, which have a positive effect on the maintenance of bone density?  The difficulty answering this question lies in the fact that researchers do not fully understand the series of signals that occur in skeleton to promote bone growth.  Futhermore, it is difficult to separate out these two stimuli as many exercise movements incorporate both gravitational forces, as well as forces produced by muscle alone.  For example, when we walk the lead leg hits the ground and absorbs the force of gravity; at the same time, the other leg rises on accord of muscle force alone.  It appears that bone growth is very sensitive and responsive to different patterns of muscle force recruitment, in addition to various speeds of force and contraction.

The bottom line is that there is highly convincing evidence that being physically active dramatically reduces the risk of hip fractures – the most debilitating type of fracture caused by osteoporosis.  Moderate to vigorous physical activity has been associated with a reduced hip fracture risk of 45% in men, and 38% in women.  The risk of hip fractures is related to loss of bone strength, as well as deficits in balance that increase the risk of falling.  A hip fracture is more than a broken bone – 25% of people over the age of 50 who sustain a hip fracture will die within one year of the injury. Reference Kravitz, L. (2010).  What Makes Bone Adapt: Gravitational Forces of Muscle Loading?  IDEA Fitness Journal, March pp.16-19. Exercise Specialist Recommendations:

• Follow the American College of Sports Medicine (ACSM) guidelines for physical activity: 30 minutes of moderate intensity cardiovascular activity 5 days per week.  “Moderate” is 6-7 out of 10 on what is called the Scale of Perceived Exertion, where a 10 is maximal effort that cannot be sustained for longer than 90 seconds for the average individual, 3 minutes for a highly trained athlete.
• Include a mixture of different activities that are both weight- and non weight-bearing, gravitational and non-gravitational.  For example, include 3 brisk walks, one bike ride, and one swim.  The elliptical trainer, which loosely mimics the movement pattern of running but uses muscles in a different way, is also a good option – but bear in mind that although the user is standing up while exercising, this modality is non-gravitational.
• Include swimming if desired – but make sure that your routine also involves exercise modalities with a gravitational component as well.
• Incorporate a well-designed resistance training program in order to capitalize on the muscle force production contribution to bone health.  This program should follow a protocol of progressive resistance, whereby the resistance/weight is gradually progressed.  This allows the participant to increase strength – and stress on the skeletal system – in a manner that minimizes the risk of injury.  Based on the body of evidence on the beneficial effects of force production on bone health, high repetitions with very light weights (ie, 1-3 lb weights for most women) will not produce desired results.
• Engage in daily exercises that challenge balance and ambulatory stabilization.  As mentioned above, fractures are the result of both weak bones as well as poor balance.  Always do your balance exercises first, as other forms of exercise compromise the ability to balance.  As well, include a cognitive component as well – for example, while standing on one foot try to count backwards from 30, or name all the United States.  You can also add a vestibular challenge by closing one eye at a time, or squinting.
• Make a concerted attempt to adopt a routine of regular exercise throughout the lifespan, even during periods when external demands appear to leave little time for self-care.  While it is never too late to begin an exercise program, it is always better to start at a younger age when most people’s bodies are more receptive to higher intensity exercise regimens that are ideal for circumventing bone loss in the first place.