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00005768-199608000-0000600005768_1996_28_974_tipton_countermeasures_8miscellaneous-article< 33_0_3_1 >Medicine & Science in Sports & Exercise©1996The American College of Sports MedicineVolume 28(8)August 1996pp 974-976Physiological adaptations and countermeasures associated with long-duration spaceflights[Basic Sciences: Symposium: Physiological Adaptations and Countermeasures Associated with Long-Duration Spaceflights Restricted Physical Activity: an Update]TIPTON, CHARLES M.; HARGENS, ALANSection Editor(s): Tipton, Charles M.Department of Physiology, University of Arizona, Tucson, AZ 85721-0093; and Gravitational Research Branch, NASA Ames Research Center, Moffet Field, CA 94035Submitted for publication February 1995.Accepted for publication December 1995.Address for correspondence: Charles M. Tipton, Ph.D., Department of Physiology, Ina Gittings Building, Room 24A, University of Arizona, Tucson, AZ 85721-0093.ABSTRACTSince 1961, there have been more than 165 flights involving several hundred individuals who have remained in a space environment from 15 min to more than a year. In addition, plans exist for humans to explore, colonize, and remain in microgravity for 1000 d or more. This symposium will address the current state of knowledge in select aspects associated with the cardiovascular, fluid and electrolytes, musculoskeletal, and the neuroendocrine and immune systems. The authors will focus on responses, mechanisms, and the appropriate countermeasures to minimize or prevent the physiological and biochemical consequences of a microgravity environment. Since exercise is frequently cited as a generic countermeasure, this topic will be covered in greater detail. Models for simulated microgravity conditions will be discussed in subsequent manuscripts, as will future directions for ground-based research.More than a decade ago, we participated in the first ACSM symposium devoted to the physiology of space travel (4). Its purpose was to introduce the membership to select anatomical and physiological changes in conditions of microgravity and to speculate on the type of investigations that would be needed in the 1980s to resolve these issues. At that time, 90 Russian and American flights had occurred, and approximately 200 individuals had spent from 15 min to 211 d in space(2).However, since May 1982, more than 75 spaceflights have been completed, and almost 400 individuals have experienced conditions of microgravity, with several cosmonauts being in space for longer than a year. In addition, space exploration has become a national objective for the 13 member countries that form the European Space Agency with Finland as an associate member; for Japan and its National Space Developmental Agency; and for Canada, which in 1989 established the Canadian Space Agency (2).In recent years, a variety of scientific, economic, social, and political issues have caused NASA to redefine its goals and missions. Emerging from this process has been a template for the Mission from Planet Earth, which focuses on increased human activity in microgravity and culminates with a permanent presence on the moon and a landing on Mars. As noted by S. F. Shea, Chairman of the Space Systems and Technology Advisory Committee(1), “the question is not really whether we, either as a nation or a planet, will make the Journey. The question is when.”The planning for the exploration of space in the next century has been a complex and rigorous process that has required the participation of the personnel at NASA headquarters and its centers, the recommendations of eight established advisory committees, and the contribution of 12 separate panels(Discipline Working Groups) of scientists from NASA, School of Aerospace Medicine, and from various universities. One result of this planning process has been the publication of the NASA document entitled Strategic Considerations for Support of Humans in Space and Moon/Mars Exploration Missions (1). The significance of this document is that it provides an overview of the scope of the planned explorations, durations, and number of individuals involved with the tentative time periods for implementation purposes (Table 1).TABLE 1. Proposed space exploration time tables.*Although it is evident that humans can exist and function for a year in a microgravity environment, much remains to be learned at the system and cellular levels concerning the adaptive processes and their responsible mechanisms, despite the fact that Yuri Gargarin's historic flight occurred more than three decades ago (2). Equally uncertain are the countermeasures that should be used to avoid or to minimize the system and cellular changes that could be harmful to the health and safety of crew members in space and for their return from a microgravity environment.The content of this symposium will focus on what is currently known and expected to occur with exposure to conditions of microgravity by individuals who have or will spend from 14 to 1,000 d in space. In addition, the authors will mention select models of simulated microgravity currently in use and discuss their effectiveness in predicting the physiological and anatomical changes that occur with microgravity. Because changes associated with the cardiopulmonary, fluid, and musculoskeletal systems have been extensively characterized in humans and animals in short- and long-term flights, these topics will be discussed in depth by A. Hargens, K. M. Baldwin, and V. C. Schneider, while select interactions between the nervous, endocrine, hematological, vestibular, and immune systems will be discussed by C. M. Tipton. Although each author will emphasize various counter-measures that have or could be utilized to minimize the undersirable consequences of spaceflight(e.g., muscle and bone atrophy), V. A. Convertino will summarize the important features of exercise as a countermeasure because physical activity is frequently advocated for this purpose without concern for the principles of prescription and specificity or for the scientific advantages or disadvantages of its effects.Since the material presented by Schneider (3) at the symposium was unable to be included as a separate report, we have transcribed his speech and summarized his main points below. One key issue from his presentation was that bone loss in space was the result of bone atrophy and not bone demineralization, because the molar changes in hydroxyproline and hydroxylysine follow the molar changes in calcium. In microgravity, calcium is lost from the bones at a rapid rate before plateauing at a higher steady state. Moreover, bone loss is quite variable among individuals and site specific, with the highest rates occurring from the calcaneus, lumbar vertebrae, pelvis, femoral neck, and femoral trochanter. The lowest rates for bone loss were from the arms, ribs, and upper vertebrae. Interestingly, the cranium was associated with an increase. Analysis of existing data suggest that bone atrophy occurs at rates between 0.5 and 2.0%·month-1 depending on the site and gender being evaluated. A key mechanical factor that influences bone loss is the magnitude of the muscle force acting at insertion sites, especially in the lower spine. Interestingly, select regions of the lumbar vertebrae fail to show signs of postflight recovery even though calcium balance measurements indicate an equilibrium has been achieved. Limited studies on cosmonauts using newer technology to evaluate bone mass suggest that the recovery process for bone loss after prolonged flights is incomplete, despite the fact that muscle mass and strength have returned to preflight values. Since cosmonauts are instructed, 1 month before returning to Earth, to perform 4 h of daily exercise (treadmill with bungee cords, cycle ergometer, bungee cords), the bone changes suggest either the exercise prescription is inappropriate or that it is not being followed.Schneider also presented femoral neck bone data from bed rest and flight subjects for experiments lasting from 113 to 312 d to demonstrate that bed rest was a suitable model to simulate microgravity. He also presented data that indicated bone loss could be prevented by walking 12 miles·d-1 at 5.8 km·h-1 (3.5 mph). Other results indicated that the pharmacological use of bisphosphonates was effective in reducing the bone loss with bed rest. He concluded that more ground-based research was needed and that bed rest was an appropriate model to acquire such information. Schneider also felt the focus for future research should be on mechanisms that prevent bone loss.Lastly, we believe this symposium will provide new information and insights on physiologic adaptations to microgravity that will be useful in future deliberations concerned with the necessary countermeasures needed for long-duration spaceflight.REFERENCES1. NASA Advisory Council and Space Systems and Technology Advisory Committee. Strategic Considerations for Support of Humans in Space and Moon/Mars Exploration Missions, Vol. 1. Washington, DC: NASA, 1992, pp. 1-82. [Context Link]2. Nicogossian, A. E., S. L. Pool, and J. J. Uri. Historical perspectives. In: Space Physiology and Medicine, 3rd Ed., A. E. Nicogossian, C. L. Huntoon, and S. L. Pool (Eds.). Malvern, PA: Lea and Febiger, 1994, 3-49. [Context Link]3. Schneider, V. C. Bones and their responses. In:Physiology Adaptations and Countermeasures Associated with Long-Duration Space Flights. Parts 1 and 2 [audiocassette]. Mobiltape 94ACSMG4a. Valencia, CA: Mobiltape Co., 1994. [Context Link]4. Tipton, C. M. Preface and weightlessness and the 1980's.Med. Sci. Sports Exerc. 15:408-409, 1983. [CrossRef] [Full Text] [Medline Link] [Context Link]LONG-TERM SPACEFLIGHTS; COUNTERMEASURES; SPACE HISTORYovid.com:/bib/ovftdb/00005768-199608000-0000600005768_1983_15_408_tipton_weightlessness_|00005768-199608000-00006#xpointer(id(R4-6))|11065213||ovftdb|00005768-198315050-00011SL0000576819831540811065213P29[CrossRef]10.1249%2F00005768-198315050-00011ovid.com:/bib/ovftdb/00005768-199608000-0000600005768_1983_15_408_tipton_weightlessness_|00005768-199608000-00006#xpointer(id(R4-6))|11065404||ovftdb|00005768-198315050-00011SL0000576819831540811065404P29[Full Text]00005768-198315050-00011ovid.com:/bib/ovftdb/00005768-199608000-0000600005768_1983_15_408_tipton_weightlessness_|00005768-199608000-00006#xpointer(id(R4-6))|11065405||ovftdb|00005768-198315050-00011SL0000576819831540811065405P29[Medline Link]6645870Physiological adaptations and countermeasures associated with long-duration spaceflightsTIPTON, CHARLES M.; HARGENS, ALANBasic Sciences: Symposium: Physiological Adaptations and Countermeasures Associated with Long-Duration Spaceflights Restricted Physical Activity: an Update828