Exercise for
bone strength
Griffith University’s Associate Professor Belinda Beck provides
a summary article that examines the effect of exercise on bone
strength, based on her recent presentation at the SMA Bone
Health in Sports symposium in February of this year.
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Why should we care about bone
What are the best kinds of loads for bone?
strength?
The development and maintenance of
strong bones throughout life is the key to
preventing osteoporotic fracture in later life.
Osteoporosis is a condition of low bone mass
and strength that results in an increased
risk of fracture. Bone mass and strength
is, to a very large extent, determined by
genetics. Certain lifestyle choices, however,
particularly diet and exercise, play an
important role in determining whether
a genetic predisposition for fracture
eventuates in reality.
What happens to bone when you
add or subtract exercise?
The loading of bone through exercise (or
other mechanical force) creates strain or
deformation of the bone structure. Since
the late 1960s when people were first
exposed to extended bouts of microgravity
(spaceflight), we have had strong evidence
that the consequence of unloading bone
is loss. It is now known that even in
normogravity bone loss will occur following
spinal cord injury, casting immobilisation,
bed rest and tooth loss. In fact, the loss
of bone experienced with ageing is also
likely to be strongly related to the decrease
in loading resulting from a reduction in
the high-impact types of exercise most
common in youth. That loss reflects the
direct reduction in strains applied to bone
from ground reaction impacts, as well as
reduced muscle loading required to resist
those impacts and to move the bones. By
contrast, activities producing bone strains
that exceed habitual patterns will stimulate
improvements in bone strength, either by
increasing bone density, or size, or both.
Strong evidence of these effects has been
derived from a multitude of animal studies
over the past 40 years, from which optimal
load parameters have been developed.
Animal studies have shown that site, magnitude, rate, number and bout timing are all loading
characteristics to which bone responds selectively. Specifically, only the bones that are loaded
will respond, large loads and fast rates of loading are most osteogenic, relatively few loads are
required to stimulate an adaptive response, and multiple short load bouts separated by breaks
are a more effective bone stimulus than a single continuous loading session.
Human exercise trials: a challenging research paradigm
The translation of osteogenic loading parameters from animal work into practical and
effective exercise recommendations for humans has been remarkably challenging. Under
strict animal experimental conditions, loading characteristics can be tightly controlled and
relatively predictable responses obtained. That is, when genes, diet, age, sleep, and physical
activity exposure are controlled, we can ascribe a bone response to a novel exercise stimulus
with a high degree of confidence. The inability to fully control the same highly confounding
variables in human exercise interventions has undermined our ability to draw conclusions
with a similar level of confidence. The nearest human surrogate for animal trials are interlimb comparison studies of racquet sport players that clearly show larger bones in the ‘hyper
loaded’ playing arm. Unfortunately, upper-limb loading is not a perfect model for weightbearing loading, thus extrapolations to whole-body bone exercise recommendations are
not entirely appropriate. Furthermore, in the absence of long-term follow-up data, it is not
possible to know if benefits from discrete human exercise interventions at one stage of life
will translate to a reduction in fractures later in life. Thus, while much is known, current
exercise recommendations to promote bone health should be considered a work in progress.
What is the human evidence?
Athletes in sports involving high-impact loads and heavy muscle resistance (such as gymnastics,
rugby codes, basketball and weightlifting) have been observed to have 15-20 per cent higher
bone mass than non-exercisers. By contrast, athletes who spend very long durations in weightsupported exercise (elite swimmers and cyclists) routinely exhibit bone mass that is actually
lower than sedentary individuals. Such observations should be treated with caution, however,
as there is a strong influence of selection bias in athlete observations. That is, there is a genetic
predisposition for athletes with stronger bones and muscles to demonstrate better performance
and resistance to injury in high-impact sports than weaker boned individuals—a predisposition
that is the reason they are in the sport in the first place. There is a similar argument that lighter
bones provide a performance advantage to swimmers and cyclists.
Yet, when high-impact exercise is introduced to a random selection of exercise-naïve
individuals (by way of a rigorously randomised controlled exercise intervention), observed
improvements in bone are typically only in the order of one to five per cent. The disparity
between the findings of observational athlete studies and randomised controlled exercise
trials is not entirely explained by selection bias. The culmination of a lifetime of exposure to
bone-stimulating loading in adult athletes will confer compounded benefit. Furthermore, it
is widely held that mechanical loading is a more potent stimulus to bones in childhood than
after growth has ceased, such that exercise started young will achieve the greatest returns for
the skeleton.
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Additional considerations
Exercise for
bone strength
Exercise recommendations: an educated guess
With those substantial caveats in place, the current consensus
around exercise for bone health can be summarised in the following
points. Children and healthy adults with average or higher bone
mass should be exposed to frequent bouts (at least twice a week)
of weight-bearing impact loading with a high level of variety
across the full course of their lifetime. The emphasis should be on
high magnitude (for example, jumps, landings, plyometrics) and
varied (for example, racquet ball, basketball) exercises that are not
normally encountered in the course of daily activities. An increased
risk of fracture to osteoporotic bone during high-impact loading
creates a catch 22 situation in terms of exercise recommendations
for those with severe osteoporosis. As 90 per cent of hip fractures
are a direct result of a fall, the object of exercise for adults with very
low bone mass moves from bone building to falls prevention. Lower
impact activities and neuromuscular training should be employed
in this cohort to maximise muscle strength and balance in order to
reduce the risk of falling, thereby indirectly preventing osteoporotic
fracture.
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A number of additional factors should be taken into consideration
when prescribing exercise for bone health. The first is that those
who have moderately low bone mass (including some elite swimmers
and cyclists) stand to gain the most from a bone-targeted exercise
program. At the opposite end of the spectrum, athletes who have
participated in long-term, high-intensity weight-bearing sports are
unlikely to achieve notable bone gains from additional loading.
Secondly, our recent work has shown that high-impact weightbearing training in minimalist shoes may be more osteogenic than
training in regular athletic shoes. This novel and intriguing finding
requires further investigation. Thirdly, gains in bone strength will
be lost if exercise initiated in adulthood is discontinued. There is,
however, some evidence to suggest that exercise-related benefits to
bone achieved in childhood may persist throughout life. Finally, the
benefits of exercise will only be fully realised in an environment of
adequate nutrition. As a general rule, throughout life roughly 1000
mg of calcium is required per day, and is best obtained through the
diet (as opposed to supplements). Vitamin D is vitally important
for the absorption of calcium from the gut. It is most efficiently
obtained through sun exposure, but in the absence of opportunity
or inclination for the latter, between 600 and 800 IU should be
consumed in the diet each day. Sun exposure guidelines have been
developed collaboratively by the Australian and New Zealand Bone
and Mineral Society and the Australian Cancer Council and differ
according to latitude and time of year. (cancersa.org.au/information/
a-z-index/how-much-sun-is-enough)
In summary
• Exercise effects are specific to the loaded bones.
• Exercise must be dynamic, varied, and exceed normal loading
patterns.
• Multiple bouts of high-load, low-repetition exercises with breaks
between bouts are most osteogenic.
• Childhood is a window of opportunity to build bone before the
cessation of growth.
• Individuals with low bone mass stand to benefit most from bonetargeted exercise.
• Older adults with severe osteoporosis should focus primarily on
anti-falls exercise programs.
• Adequate calcium and vitamin D are required for exercise benefits
to be fully achieved.
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Belinda Beck is an Associate Professor in the School of Allied
Health Sciences at Griffith University, Gold Coast campus. She
holds a degree in human movement studies, a master’s in sports
medicine and a PhD (Exercise Physiology). She completed
a postdoctoral research fellowship at Stanford University
(USA). Her research focus is prevention and management of bone
stress injuries, and exercise interventions for the prevention of
osteoporosis and fracture.