Chapter 1: Space-physiology research, an introduction

Spaceflight exposes the human body to a totally different environment than it is used to, which imposes potential hazards on the people engaging in spaceflight. The two most hazardous conditions, besides technical failure or other catastrophes, are the near absence of gravity, called microgravity, and the increased exposure to radiation. Also playing a role in the complex physiological adaptations to spaceflight is the relatively inactive lifestyle in a small confined place (Newberg, 1994; Convertino, 1996), and a different circadian rhythm that disturbs the sleeping pattern (Stampi, 1994). The different environment in space elicits disturbances to the musculo-skeletal, cardiovascular, vestibular, pulmonary, thermoregulatory, immune, metabolic, and reproductive systems (Tipton, 1983a; Bachl et al., 1993; Lane et al., 1993; Tipton & Hargens, 1996).

In this study of literature, these disadvantageous physiological effects of spaceflight will be discussed. The main interest will be on the musculo-skeletal related problems, and how to minimize them, as these problems are expected to be the mission-length limiting factor of future spaceflights (Bachl et al., 1993). This first chapter acts as an introduction to the areas of spaceflight in general and space-physiology research. The most widely used countermeasure against spaceflight-related physiological problems, i.e. exercise, will be discussed and the main questions that will be addressed in this study of literature will be presented.

1.1 - A short history of spaceflight

In the 1950s, when space-exploration was just beginning, the main question was whether it would be possible to stay alive outside the earth’s atmosphere. In 1957 the orbital flight of Laika the dog, which was monitored for seven days by radio transmissions, showed it was. Hereafter, the primary concern shifted towards the unknown consequences of exposure to increased radiation (Berry, 1967; Busby, 1968). In space, different kinds of ionizing radiation occur, including cosmic radiation and high energy particles which have the potential of causing biological damage (Jennings & Santy, 1990; Knoche, 1991; Newberg, 1994). The earth’s atmosphere acts as a protective shield against this radiation, absorbing it and changing it, and thus making it harmless before it reaches sea level. The possible effects on humans of exposure to ionizing radiation without this protective shield were completely unknown. But safe returns from Yuri Gagarin and others in the 1960s showed that, at least in short-term flights, there were no detectable life threatening reasons not to continue with the spaceprogram.

An overview of the human spaceprograms is presented in table 1. While the number of flights increased, so did the knowledge of the effects of such flights on the human body.

Table 1: Overview of human spaceflight. Overview of spacecrafts (or orbiters) that have flown in space carrying humans. Sources: Convertino, 1990; Taylor, 1993; Bachl et al., 1996.

¹ Abbreviations: min = minutes; hrs = hours; d = days.
² Also used as a carrier to the Salyut and Mir space stations. Modified in 1980 and 1987.
³ Including six visits to the moon and used as a carrier to the Skylab space station.

After the moon had been thoroughly researched, the interest of spaceresearchers shifted towards long-duration stays in space, i.e. longer than three weeks. In the United States, the National Aeronautics Space Administration (NASA) proceeded with the Space Shuttle program, which was the first time a re-usable orbiter was used for spaceflight (Convertino, 1990). The Russians proceeded to specialize in long-term flights with the Salyut space stations and later the Mir space station. The current record of one continuous stay in space is set in the Mir and amounts 437 days (Bachl et al., 1996).

One of the reasons to develop space stations and the Space Shuttle was to conduct elaborate research in a relatively stable environment, as well-controlled scientifical research is difficult to conduct in space. The next paragraph will focus on these difficulties of space-physiology research.



1.2 - Scientifical problems related to space-physiology

Adequate scientifical research requires a well-controlled setting, with the availability of sufficient subjects and longitudinal research to detect any problems that might occur only after a distinct period of time. This kind of research is impossible in the area of space-physiology.

To start with the last requirement, space-physiology is a relatively young research area. The first human flight took place in 1961, and, consequently, potential effects of spaceflight on an ageing person are unknown. For example, it is imaginable that possible effects of increased exposure to radiation do not emerge for several decades.

Another difficulty is the low number of subjects that actually engage in spaceflight. Most flights have only a few crewmembers, which makes it difficult to detect any statistically significant results. Therefore, most results are reported as trends rather than true significant differences (Roy et al., 1991). This makes it very difficult to draw any cause-effect conclusions. Combining data of several flights is possible, but adds extra confounding elements since influential parameters such as dietary intake and activity patterns are not standardized.

Because of the tight schedule during a flight (especially during short-term flights), and because heavy equipments are difficult to launch, the possibilities of in-flight research are limited. In addition, although spacetravellers are mostly specialists in one particular area, accurate supervision of in-flight testing is often not possible. Thus, test results are often limited to pre- and post-flight data, making it virtually impossible to study the mechanisms behind certain changes.

A final issue confounding the possibility of well-controlled scientific research is the demand for quick results in order to keep the spaceflight-program going. The operational demands for health and safety and, in the early days of spaceflight, the race to the moon between Russia and the USA, with much more at stake than the moon alone, did not allow for carefully planned scientific studies. Any problem should immediately be counteracted, and several countermeasures are often taken simultaneously. Therefore, the effects of each countermeasure alone cannot be determined. All these problems make it impossible to conduct space-related research in the official scientific way.

To be able to do research with a sufficient number of subjects and with the possibility of monitoring changes during a protocol, weightlessness is often simulated. This is done by means of lower body positive pressure, immersion, bedrest, bedrest with head-down tilt, parabolic flights or by suspension of the whole body or of one or two limbs. Also, a lot of research is performed on animals, both during actual spaceflight and during simulations of weightlessness. The disadvantages of such simulations are obvious. Simulation studies have severe limitations as studies using animals can never be translated directly to humans and gravity is never reduced over the whole body or, in the case of parabolic flights, is eliminated for only twenty seconds or so (Davis & Cavanagh, 1993). But simulation studies remain necessary, since they are a relatively easy and cheap alternative and can raise and further explore hypotheses, that can ultimately be tested during actual spaceflight.

1.3 - The use of exercise protocols in space

As already mentioned, spaceflight induces several physiologic disturbances. To counteract these changes, dietary supplements, hormonal treatments and pharmacological aids have been used, as well as the application of electrical fields, electrostimulation, and the use of exercise protocols (Minaire, 1989; McLeod & Rubin, 1992; Lane et al., 1993; Rabin et al., 1993). As will be discussed in this paragraph, the latter is by all means the most important countermeasure during spaceflight in both the Russian and the American space programs (Grigoriev et al., 1992; Convertino, 1996; Davis et al., 1996).

Exercise has potential benefits on the cardiovascular, immune and musculo-skeletal systems, and is therefore also potentially beneficial to the vestibular and thermoregulatory problems (the relationship between the different physiological systems will be discussed in chapter 2). Consequently, exercise has been used since the early days of spaceflight. The first exercise testing was done in the Voskhod and Gemini IV flights (Convertino, 1990). In the light of the problems described in paragraph 1.2, it is not surprising that the past and present in-flight exercise protocols have not been totally effective in preserving normal physiological function, as they have not been based on adequately designed and controlled investigations (Convertino, 1996).

Recently, the effects of spaceflight on the musculo-skeletal system attract the most attention, especially in the light of planned long duration flights in the (near) future (Bachl et al., 1993). During spaceflight, there is atrophy of both muscle and bone, which, in long-duration flights, can become a potential hazard after return to the earth’s gravitational forces (1G) and can endanger the in-flight work performance, health and even the lives of spacetravellers (Bachl et al., 1993).

The effects of exercise on muscle tissue, has been studied extensively. The first question to be addressed in this study of literature will be:

What are the requirements of an in-flight exercise protocol during long-term spaceflights to effectively minimize the losses in muscle mass and muscle strength during such flights?

In contrast with the extensive studies of the effects of exercise on muscle tissue, the possible bone tissue enhancing effects of exercise are a relatively recent topic. Indirect evidence is continually building up to suggest that there is a relationship between exercise and a decreased degree of osteoporosis. This leads to the second question of this study of literature:

What are the requirements of an in-flight exercise protocol during long-term spaceflights to effectively minimize the losses in bone mineral during such flights?

Ultimately, one single effective exercise protocol will be sought to combine these effects. The unique environment of spaceflight imposes unique demands on exercise protocols in order to be effective to the uppermost maximum. The effectivity of a protocol will be determined by an adequate compromise among exercise efficacy, ease of performance, subject compliance and the operational demands of spacecraft, i.e. size and weight of equipment, operational time constraints and minimal environmental disruption.

1.4 - Concluding remarks

The recent landing of the unmanned spacecraft "Pathfinder" on Mars marks the beginning of a new era in the exploration of our solar system, which eventually should lead to human visits to Mars. NASA intends to visit Mars in 2014 (Tipton & Hargens, 1996), an effort which will put humans under microgravity-conditions for two to three years (Stroganova & Leonid, 1991; Newberg, 1994). The expected effects of such a journey on the physiological systems stated above will be discussed in chapter 2, based on the results of previous spaceflights and of simulation studies. The main interest will be on the effects on the musculo-skeletal system, as this system seems to be the most limiting factor during long-term missions (Bachl et al., 1993). In chapter 3, possible mechanisms of spaceflight-induced muscle atrophy will be discussed and the possibility of exercise as a countermeasure against muscle atrophy will be considered. Chapter 4 will have an identical purpose, now focusing on osteoporosis. Finally, in chapter 5, it is tried to formulate one single exercise protocol that is able to enhance the musculo-skeletal system during long-term spaceflights. In this study of literature, the terminology of flight-durations of Tipton (1983b) will be followed, which means that "short-term" will indicate a period of up to three weeks and "long-term" will indicate a period of up to three years.