6
ISSUES: Fitness & Health
Chapter 1: About fitness
What exercise does to your bones
An article from
The Conversation
.
By Alex Ireland, Postdoctoral Researcher in Neuromuscular
andSkeletalPhysiology,ManchesterMetropolitanUniversity
W
hen we think of bones, a
lifeless
skeleton
usually
comes to mind, but our
bones are a living organ that grows
and changes shape throughout our
life. Much of this shaping results from
forces which press, pull and twist the
skeleton as we move, and the biggest
of these forces is caused by our
muscles.
Bones experience huge forces during
movement. When a triple jumper’s
heel hits the ground, the force is
around 15 times their body weight
– or the weight of a small car. In fact,
because muscles normally attach
close to joints, muscular forces are
even greater than these impact forces
(in the same way that you have to push
harder to lift someone on a see-saw
the closer you get to the middle). As
a result bones also experience huge
impact and muscle force during daily
tasks, totalling more than five times
body weight even during walking.
These forces squash, twist and bend
bones. The shin bone briefly becomes
nearly a millimetre shorter as your
foot hits the ground when running.
The bone senses these small changes,
and can grow dramatically – in the
months after starting exercise – in
order to reduce the risk of breaking.
For example, the racket arm bones of
tennis players can be 20% wider and
contain 40% more bone mineral than
their other arm, while sprint runners
have up to a third more bone in their
shin bone than people who don’t
exercise.
But not all exercise gives us big, strong
bones. We seem to need high impacts
(hitting the floor from a jump, or
striking a tennis ball) to produce big
enough muscle and impact forces to
make our bones change. As a result, not
all exercise appears to be beneficial for
bone. Swimmers and cyclists may have
healthy hearts, lungs and muscles but
their bones are not much different
from people who do not exercise.
Bone’s response to these forces varies
along its length. Near the joints, bones
get bigger and more dense, whereas
bone shafts tend to get bigger and
thicker with little change in bone
density. Bones also change in shape.
The shin bone shaft starts as a circular
tube, but gets wider from front to back
as we grow and start to move until it
forms a tear-drop shape. But if we start
to load our bones less, they waste away
and these effects are no less dramatic.
Astronauts lose up to 1% of their leg
bone mass per month when in space,
while people who suffer a spinal cord
injury lose up to half of their shin bone
mass.
Stronger bones for life
This shaping of bones by forces
appears to occur throughout life.
Even at 15 months old, children who
started to walk early have up to 40%
more bone in their shin than children
who have yet to start walking; effects
that last until at least their late teens.
Bone seems to be most sensitive to
loading while we’re still growing.
Once we reach our final height, bone
appears less able to increase its width,
particularly near the joints. While some
of the benefits gradually disappear
once you stop exercising, exercised
bones remain wider even several
decades after exercise stops.
This suggests that exercise in
childhoodmay give us bigger, stronger
bones for life. This is important as
bigger, stronger bones are less likely
to break as we get older. Certainly,
exercise trials can be very effective in
making children’s bones stronger, and
also in reducing bone loss from bed
rest or even partly reversing bone loss
in spinal cord injury.
However, effects of exercise on bone in
elderly people have so far been much
smaller. This is a big problem as we
break our bones more often as we get
older. The lack of large improvements
in bone quality as a result of exercise
in older people might be because
we can’t produce as much force as
we get older, or that bones are less
sensitive to the forces we do produce.
Alternatively, it could be that changes
in our muscles and bones mean that
the amount of squashing, bending
and twisting our bones experiences
during movement also changes. New
techniques allow us to look at these
patterns for the first time, which
should allow us to plan more effective
exercises in people of all ages.
Forces acting on our bones during
everyday movements and exercise
have a strong influence on the size,
shape and strength of our bones.
If we move less this can make our
bones weak and more likely to break,
but being active and doing exercise
such as running, football or tennis
can help make our bones much
stronger. At the moment, exercise
trials seem most effective in children
and in stopping or slowing bone loss
in disuse. However, ongoing work is
giving us a much clearer picture of
how forces contort our bones during
different movements. This will allow
us to design more effective exercises
for bone in different groups, finally
allowing us to translate the dramatic
effects of exercise on bone seen in
athletes into benefits for the wider
population.
20 April 2016
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The above information is reprinted
with kind permission from
The
Conversation
. Please visit www.
theconversation.com for further
information.
© 2010–2017,
The Conversation Trust (UK)
