PAYLOADS - Human Research
Maintaining Strength in Space: Bone, Muscle and Metabolic Studies
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 cannot 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
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.
restrictive exercise device measures the reaction speed
and endurance of muscles, like those in the calf, that are
particularly affected by space flight.
Resonance Imaging, or 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,
or 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
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. 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.
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.