The Skeletal
System
O
n Earth, bones provide mechanical support so
the body can move against the force of gravity.
In space, the lessened gravity (microgravity) decreases
the amount of weight that bones support. Astronauts’
bones lose calcium and bone mass within days of a
space launch, and bone loss becomes most severe
within two to five months. Astronauts may lose as much
as 20% of their bone mass on an extended spaceflight.
Upon returning to Earth, they gain back most, but not
all, of it.
To counteract this loss, astronauts must exercise
for one to three hours each day that they are in flight,
as shown. They use special treadmills with resistance
straps against which the astronauts must generate force
in order to stand. The stress placed on the muscles
and bones is similar to the stress that would be pres-
ent under gravity on Earth. Another device, called
Vibrational Inhibition of Bone Erosion (VIBE), is a
plate on which astronauts stand and receive 10
minutes of high-frequency, low-magnitude me-
chanical acceleration. If VIBE works to inhibit
bone loss in the hips and legs, the astronauts
will be able to reduce the amount of time
spent exercising, freeing their time for oth-
er activities. Such countermeasures will
be essential if humans are to partici-
pate in more extensive space travel,
such as journeys to other planets
or even stars. Progress in this
area may also help the 60 million
patients in the United States each
year who suffer from an aging-
related form of bone loss called
osteoporosis.
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