STS-95 PAYLOADS - Human Research
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 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 muscle atrophy.

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.

A restrictive exercise device measures the reaction speed and endurance of muscles, like those in the calf, that are particularly affected by space flight.

The Magnetic 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 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. 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.