Maintaining Strength in Space:
Bone, Muscle, and Metabolic Studies
Everyday activities like walking, lifting objects, and standing upright
are governed by skeletal muscles and bones. During space flight, support
muscles such as those in the calf and thigh decline in volume, strength,
and mass. To limit muscle weakness, astronauts regularly perform weight-loading
exercises that simulate the gravity of Earth. Despite this, crewmembers
continue to lose muscle strength and structure during long space flights.
Space flight may result in changes to muscle metabolism, the process of
building and breaking down muscle proteins, that can not be counteracted
with routine exercise. Abnormal hormone concentrations and other indicators
of altered metabolism have been identified during space flight, supporting
the concept that changes in metabolism contribute to muscle atrophy.
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This restrictive exercise
device measures the reaction speed and endurance of muscles, like
those in the calf, that are particularly affected by space flight.
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The skeleton provides a rigid support for the body in Earth’s gravity
and is similarly affected by microgravity. Bones lose calcium, the mineral
from which they derive their structure and strength, through the process
of demineralization. If enough calcium is lost, the skeletal system becomes
weaker and less capable of withstanding the stresses associated with daily
life on Earth. Once astronauts return to Earth, the gradual process of
returning calcium to skeletal bones begins; this recovery can last months
even years if an astronaut’s stay in space was of substantial length.
In addition to demineralization, bone marrow changes have also been linked
to bone weakness. One objective of these experiments is to define changes
in spinal bone marrow that may occur during and after space flight. Maintaining
bone and muscle integrity is critical to the welfare and performance of
astronauts. With increasingly longer missions and complex extravehicular
activities, crew functioning could be limited by muscular weakness and
bone demineralization. A balance between healthy nutrition, therapeutic
measures, and exercise is likely to be the most effective countermeasure
for changes in skeletal muscles and bones.
The Magnetic Resonance Imaging (MRI) and Bone Mineral Loss and Recovery
investigations will not require any actions during the mission itself.
Both pre- and postflight, MRI and dual energy x-ray absorptiometry (DEXA)
scans are taken of participating crewmembers. These scans measure the
volume of selected muscles, lean body mass, and spinal bone marrow composition.
The participating crewmembers will also be tested with a resistive exercise
device that measures the reaction speed and endurance of specific muscles
in the ankle, leg, knee, and back.
The Protein Turnover in Space Flight study will track the balance between
the two components of protein turnover that contribute to muscle atrophy:
protein building and breakdown. The building of new protein from amino
acids will be measured using a small amount of the amino acid alanine.
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Astronauts work together
to draw blood that will later be analysed for changes in protein
building and breakdown. |
The alanine contains a special tagging molecule that acts like a beacon;
when the tagged alanine is incorporated into newly built protein, it can
be measured to reveal metabolic changes. Similarly, breakdown of the body’s
protein will be studied with tagged histidine, another amino acid. The
simultaneous use of these tracers will provide a comprehensive view of
how protein levels change in response to space flight. The study requires
two preflight, two inflight, and two postflight data collection sessions.
Each three-day session begins shortly after awakening. After an initial
blood sample is taken, astronauts then ingest a capsule containing the
tagged alanine. Twelve hours later, another blood sample is taken and
the tagged histidine is given intravenously. Blood will be drawn at three
more intervals (24, 48, and 72 hours), centrifuged immediately and then
frozen for postflight analysis. Urine samples will also be returned to
Earth for measurement of the tagging molecules, as well as hormone indicators
of metabolism. All food eaten, exercise completed, and medications taken
will be logged for the entire 72-hour session. This Protein Turnover data
will be used with data from the MRI and Bone Mineral Loss and Recovery
studies to calculate changes in body protein.
These bone and muscle metabolism studies offer a unique opportunity to
study the physiology of healthy subjects as they are exposed to microgravity.
The information gained from this investigation may benefit the many people
here on Earth whose daily activities are affected by metabolic deficiencies,
weakened muscles, or loss of bone mass. Some metabolic diseases, for example,
result in debilitating muscular weakness, a condition that could be improved
by advances in protein turnover research. Likewise, muscle wasting is
problematic for senior citizens, patients confined to lengthy bedrest,
patients with spinal nerve damage, and even burn victims recovering from
traumatic accidents. Older people also commonly experience a loss of bone
mass, a condition often due to the age-related disease osteoporosis. By
exploring the interaction of aging and space flight, research on STS-95
will contribute to our knowledge of the aging process. A better understanding
of bone and muscle changes in space flight will also lead to treatments
for astronauts and Earth-bound patients alike.
Points of Contact:
MRI after Exposure to Microgravity
Principal Investigator
Adrian D. LeBlanc, Ph.D.
Baylor College of Medicine
Houston, TX
Bone Mineral Loss and Recovery
Principal Investigator
Linda C. Shackelford, M.D.
Johnson Space Center
Houston, TX
Protein Turnover During Space Flight
Principal Investigator
Arny Ferrando, Ph.D.
Shriners Burns Institute
Galveston, TX
FS-1998-09-009JSC (PDF Format)
Curator: Kim Dismukes
Responsible NASA Official: John Ira Petty
Updated: 24 October 1998
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