A new treatment under study by NASA-funded doctors could reverse
bone loss experienced by astronauts in space.
By Patrick L. Barry
"Use it or lose it."
The familiar mantra of fitness buffs
applies as much in space as it does on Earth - perhaps more so.
The bones and muscles of astronauts, freed from the familiar strains
of gravity, can weaken alarmingly. Muscles atrophy relatively quickly,
while bones lose mass during prolonged exposures to weightlessness.
Reducing muscle atrophy requires exercise
- and lots of it. Astronauts in space spend about two hours each
day working out with the aid of exotic devices that rely on springs,
elastic, and harnesses to provide resistance and mimic body weight.
Unfortunately, such "countermeasures"
have not solved the problem of muscle or bone loss. It's an ongoing
problem for astronauts - and for researchers!
But now, perhaps, there could be a
solution - at least for bones: NASA-funded scientists suggest that
astronauts might prevent bone loss by standing on a lightly vibrating
plate for 10 to 20 minutes each day. Held down with the aid of elastic
straps, the astronauts could keep working on other tasks while they
The same therapy, they say, might eventually
be used to treat some of the millions of people who suffer from
bone loss, called osteoporosis, here on Earth.
"The vibrations are very slight," notes
Stefan Judex, assistant professor of biomedical engineering at the
State University of New York at Stony Brook, who worked on the research.
The plate vibrates at 90 Hz (1 Hz = 1 cycle per second), with each
brief oscillation imparting an acceleration equivalent to one-third
of Earth's gravity. "If you touch the plate with your finger, you
can feel a very slight vibration," he added. "If you watch the plate,
you cannot see any vibration at all."
Although the vibrations are subtle they
have had a profound effect on bone loss in laboratory animals such
as turkeys, sheep, and rats.
Photo credit: Cary Wolinsky. This image
originally appeared in a National Geographic feature article
such as this one were used for experiments on bone loss involving
turkeys, sheep, and rats.
In one study (published in the October
2001 issue of The FASEB Journal), only 10 minutes per day
of vibration therapy promoted near-normal rates of bone formation
in rats that were prevented from bearing weight on their hind limbs
during the rest of the day. Another group of rats that had their
hind legs suspended all day exhibited severely depressed bone formation
rates - down by 92% - while rats that spent 10 minutes per day
bearing weight, but without the vibration treatment, still had reduced
bone formation - 61% less.
These results show that the vibration treatment maintained normal
bone formation rates, while brief weight bearing did not.
Clinton Rubin, a professor of biomedical
engineering at SUNY Stony Brook and principal investigator for the
study, cautions that more experiments are required before scientists
can be sure that vibration therapy is effective for people. "Animals
are different than humans," he notes. And even among humans there
are important variables, like nutrition and genetic make-up. What
works for post-menopausal women (who often suffer from osteoporosis)
might not work for astronauts in space.
In a recent "Phase I/II" clinical trial
of vibration therapy, researchers applied the treatment to 60 post-menopausal
women. Studies using adolescent girls with very low bone density and
children with cerebral palsy are also underway.
Picture from 'Human Physiology in Space, a
curriculum supplement for secondary schools'. (Lujan and White)
weight-bearing bones - highlighted here in purple - are
also the ones most susceptible to weakening in space.
"The early results from the research
with post-menopausal women are very encouraging - but they are
preliminary. To determine efficacy, we will need a larger scale
clinical trial that runs for a longer period of time," Rubin says.
A broader "Phase III" clinical trial is currently being organised,
which will provide a strong indication of the treatment's effectiveness
for the general population of osteoporosis sufferers.
Whether astronauts would benefit from
a vibration-plate regimen is a question that can only be fully answered
by conducting experiments in space, Rubin says. Such tests have
been proposed, but none are scheduled yet.
Rubin hopes that future experiments
will reveal not only whether vibration therapy works, but
also why. It's a bit of a puzzle because the treatment doesn't
comfortably fit within the framework of conventional wisdom: Currently,
most bone researchers believe that the stresses placed on bones
by, e.g., bearing weight or strong physical exertion, signal
the bone-building cells through some unknown chemical trigger to
fortify bones. According to this thinking, the remedy for bone loss
in space should be exercises that duplicate stresses on our muscles
and skeletons experienced during a daily and active life on Earth.
Unfortunately, without the pull of gravity it is very difficult,
if not impossible, to duplicate loads routinely experienced by our
muscles and bones on Earth. The regimen of exercise that astronauts
perform in space has shown some promise as a countermeasure, but
not enough to protect long-voyaging astronauts from injury or bone
fracture when they are re-exposed to gravity - either here on Earth
or on some other planet.
Rubin suggests that perhaps it's not only
a few, large stresses placed on the skeleton that signal bone formation,
but also many smaller, high-frequency vibrations applied to bones
by flexing muscles during common activities such as standing or walking.
Image courtesy NASA
of bones isn't completely solid. Instead, it consists of a
web of mineral filaments - called "trabeculae" - and cells
(not shown in this micrograph). These trabeculae provide structural
rigidity while minimising weight, like the steel cross-members
in a crane or a highway sign.
Muscles may appear to pull steadily
and constantly when flexing - like the pull of a stretched spring.
But muscle contraction is more complex than that. Individual muscle
cells in most skeletal muscles can't provide a sustained pull -
they can only apply a quick "twitch." To create a constant pull,
the brain activates groups of muscle cells within a muscle (called
"motor units") in a rapid, repeating pattern.
You can feel these subtle patterns
by squatting and resting your hands on your thighs - the slight
trembling of your thigh muscles is the sequential contraction of
the muscles' motor units. The frequency of such contraction ranges
between 10 and 100 Hz. In comparison, the experiment with rats used
a 90 Hz vibration, and the experiments with humans are using 30
"Our hypothesis is that a key regulator of bone mass and morphology
are the mechanical stimuli that come out of muscle contractions,"
Rubin says. "So instead of these big, intensive deformations of
bone, it's basically lots and lots of little ones [that provide
a major stimulus for bone growth]."
"While exercise in space may generate
some of these signals, we believe that microgravity essentially extinguishes
these signals during the great majority of the day, as postural activity
is [markedly reduced compared to here on Earth]," he says. "The vibration
treatment generates a much larger signal in this frequency range,
and we believe that 10 minutes per day of this higher frequency signal
is sufficient to provide a maintenance signal to bone."
Painting by Pat
astronauts return to Earth after a long voyage to Mars and
back - all in reduced or zero gravity - they will need strong
bones to once again stride across their home planet. Vibration
therapy might be the key.
"This is a real departure from the
accepted theory of how mechanical signals control bone, and it is
certainly controversial," Rubin says.
Nevertheless, it might work. Good vibrations
- unexpected and controversial - could be the key to healthy bones
on Earth and beyond.