11. Functional Stretch Reflex
    Exposure to the microgravity environment has been shown to cause disturbances. The Neurosciences Laboratory investigates a form of stretch reflex, referred to as the functional stretch reflex (FSR), located in the leg. This reflex appeared as a burst of activity in the electromyogram (EMG) after a force was applied. Weightlessness changes the amplitude of the FSR, making it a good candidate for proprioceptive assessment testing. In addition to changes in the leg muscles, there is also the possibility that kinesthetic and touch receptors, particularly from the bottom of the feet, are modified through lack of contact with the surface of the Earth, and there is a high probability that muscle fiber in the major postural muscles will show degeneration and atrophy from the lack of use during bed rest. All of these factors when taken together suggest a rearranged relationship between signals from visual, vestibular, and proprioceptive receptors that drive a unique bed rest adaptive state that could be similar to those changes in motor function that are observed as a function of spaceflight.

    The goal of this study is to test the performance of the major postural muscles before, during, and after head-down-tilt bed rest.

12. Functional Mobility Test
    The goal of this activity is to test subjects on the functional mobility test (FMT) before and after bed rest to further characterize functional changes that occur after bed rest and also to compare these data with those obtained before and after long-duration spaceflight.

    Following spaceflight, astronauts experience disturbances in balance and walking control during the postflight readaptation period due in part to changes in the way the central nervous system processes sensory information as a result of prolonged exposure to microgravity. Over the last several years this laboratory has investigated the effects of spaceflight on astronauts' postural and locomotor control.

    Image right: Platform used to test the functional mobility of subjects

    In summary, these data indicated that crew members experience disturbances in postural and locomotor control manifested by changes in head-trunk coordination, lower limb muscle activation patterning, kinematics and alterations in ability to coordinate effective landing strategies during jump tasks after spaceflight. These postflight changes in locomotor performance may affect the ability of long-duration crew members to egress their vehicle soon after landing on Earth or following a landing on the surface of Mars. Early Mars mission objectives may be compromised by significant postflight postural and gait dysfunction. These changes have implications for potential emergency egress scenarios particularly where support personnel will not be available to aid crew members.

    Presently no operational countermeasure is available to mitigate these balance and locomotor disturbances. Testing with the FMT procedure will allow the evaluation of countermeasures used in bed rest subjects to minimize these risks.

13. Gastrointestinal and Renal Function
    Gastrointestinal (GI) function plays a critical role in the absorption and bioavailability of orally administered drugs and nutrients. The therapeutic activity of a drug is dependent on its intrinsic pharmacologic activity at the site of action and its absorption, distribution, metabolism, and elimination. The absorption and bioavailability of ingested compounds depend on physiologic factors such as Gastrointestinal motility, blood flow, and acidity of the stomach.

    GI motility governs the time needed for food to travel from the mouth to the intestines. Motility, itself, is influenced by fluid intake, body position, gravity, food composition, the individual's activity, and a number of other conditions. Decreased GI motility affects drug absorption and bioavailability of oral medications and thus may compromise therapeutic effectiveness during spaceflights.

    Bed rest has been shown to induce loss of appetite, constipation, and indigestion, all of which can be attributed to changes in gastric function. Data from spaceflight studies have suggested that these symptoms are very similar to those seen by crew members.

    During these bed rest campaigns, studies were performed to:

    • Characterize changes in organ function
    • Evaluate changes in physiologic factors that influence pharmacodynamics
    • Data characterizing GI function and pharmacokinetic changes

    These pharmacological measurements provide information to characterize changes in the bioavailability and absorption of drugs, and their subsequent therapeutic activity during ground-based simulation of the microgravity environment of spaceflight.

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