Acta Astronautica Vol. 33, pp. 217-220, 1994
Pergamon
Elsevier Science Ltd. Printed in Great Britain 0094-5765(94) 00113-8
THE ROLEOF ARTIFICIAL GRAVITY IN THE EXPLORATIONOF SPACE Russell R. Burton, DVM, PHD Chief Scientist, Crew Systems Directorate Armstrong Laboratory 2509 Kennedy Circle Brooks Air Force Base, Texas 78235-5118
L i t t l e is known concerning the physiologic mechanisms involved in the loss of adaptation to earth's gravity. Interestingly, terrestrial animals including the human have become adapted to the physiologic stresses of gravity over the millions of years since our distant (more primitive) ancestors emerged from a total water environment and began to live on land. For the f i r s t time, they l e f t the buoyancy of water and experienced the force of gravity. Even though that occurred eons ago -- with the beginning of physiologic adaptation to gravity -- many physiologic activities lose their normal function during stays in a weightless environment. This interesting fact is rather amazing because one might expect that adaptation to gravity would have developed a genetic basis and not require repeated stimulation to gravity to remain normal. But apparently developing a DNA basis of adaptation to gravity was unnecessary since gravitational forces are pervasive, stimulating all of our physiologic functions repeatedly every day. However there are occasions when certain physiologic activities are not regul a r l y affected by gravity; such as when we are lying horizontally in bed or swimming (excellent simulations for weightlessness). In fact, sleeping interrupts our gravitational stimulations for several hours every day. I t is apparent, therefore, that our physiologic functions normally do not require continuous gravitational stimulation to sustain homeostasis. What is unknown is the physiologic requirements for this periodic stimulation; i . e . , what are the optimum periodicities such as duration and frequency (Figure I)? Answers to these questions from controlled research are fundamental to our understanding of how animals and humans adapt to gravity. An operational spinoff from such studies is, of course, their application in preventing physiologic deconditioning in space from continuous exposure to weightlessness. Gravitational stimulation can be applied in space with a constant velocity centrifuge that develops sustained acceleration and produces an inertial force (G) that is identical to gravitational forces (g). Simply, we can take gravity with us whenever we go in space (2). With in-depth understanding of G stimulations as they affect all physiologic functions, space travelers will be able to maintain adaptation to gravity for all missions regardless of their duration.
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G stimuli that are regularly repeated will maintain physiologic function at the level found at I g on earth as phylogenetically developed through millions of years of evolution as animals l e f t the weightless effects of water to inhabit the rigors of gravity on land.
The practical process of applying periodic stimulations to maintain physiologic functions is not well known even though the physiologic response to a single stimulus has been studied by systemic physiologists regularly for many years. The generic pattern of a typical physiologic response to a proper stimulus is shown in Figure 2. This response has been called "the primordial pattern" by Grigg (3) who has identified this stimulus:response (x) profile in all biological and physical phenomena that he studied. This curve is defined mathematically with the following simple algebraic expression: x =~te "st . . . . . . . . . . . .
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This response curve, familiar to all physiologists, has this same characterist i c shape for all physiologic functions; e.g., the response period to the stimulus is followed by a prolonged period of decay. Obviously for our purposes, the longer the overshoot and decay periods, the less frequent the stimulus needs to be applied. This stimolus:physiologic response relationship can be described mathematically as follows: a < (b + c) . . . . . . . . . . . . . .
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where: a = J ' stimulus (dt) b = / stimulus overshoot (dt) c = J decay (dt) Of course, the more disproportionate "a" is relative to (b + c), the more effective wtll be the G stimulus. A physiologic response that continues for a brief period after the cessation of the stimulation (called an overshoot) must be considered probable and therefore is an important part of the response:decay pattern. A schemata of this response is shown in Figure 2. Clearly i f "a" is larger than (b + c), then the r e l a t i v e l y low effectiveness of period G stimulation wtll have been demonstrated and its operational use questionable. However considering its prevalence in physiologic functions, the nature of the stimolus response:time profile shown in Figure 2 for a single stimulus will probably prevail for all physiologic decondittons.
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Response of a physiologic system in a microgravity environment to a G stimulus (a). Once the stimulus is removed, the physiologic system continues to respond with an overshoot (b) before the decay (c) begins.
Although Grigg (3) was unable to find any time-related physical or physiological function that was not of this nature, this relationship should be validated and quantified for all physiologic parameters of interest in a periodic G exposures program to prevent deconditioning ( I ) . I t is well known that the acute G stimulus shown in Figure 2 generally has a direct G intensity duration relationship. Such a relationship when understood may be useful in reducing the duration of stimulus required to maintain adaptation to gravity in the weightless environment. The application of this knowledge in space travel w i l l , as stated earlier in this article, require the use of a human-use centrifuge. Since the centrifuge can be operated at any level of velocity, G can be produced at levels above those of I g. For operational application, the effectiveness of these G x time relationships will have to be determined -- probably using simulated weightless environmental conditions such as dry immersion or bedrest. Hypothesized G level:time of stimulus relationship to a physiologic response is shown in Figure 3.
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Predicted response of a physiologic system in a microgravitational environment to G stimuli. The 3-G stimulus produces the same response in a third of the time that is produced by a I-G stimulus.
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In summary, increased G stimuli applied regularly will probably be useful in the microgravity environment to prevent physiologic deconditioning. However, to validate this hypothesis and determine the optimum G exposure regimens for this application, numerous human studies will have to be conducted on earth using bedrest simulations and in the microgravity environment of space. If these hypotheses do prove valid, the physiologic problems of long-duration inhabitation of the microgravity environment will be readily solved with minimum hardware development and low space station energy costs (4) causing only brief periods of operational interruptions.
REFERENCES I. Burton, R.R. A Human-use centrifuge for space stations: Proposed groundbased studies. Aviat. Space Environ. Med. 59:579-582, 1988. 2. Burton, R.R. Periodic acceleration stimulation in space. 19th Intersoc. Conf. Environ. Sys., San Diego CA, 24-26 Jul 1989, Paper No. 891434. 3. Grigg, E.R.N. Biologic relativity. Amaranth Books, P.O. Box 50392, Chicago IL 60650, 1967, Lib. of Cong. Card No. 67-12430. 4. Meeker, L.J. and Isdahl, W.M. Parametric design study for a small radious centrifuge for space application, 10th IAA Man in Space Symposium, Nihon University, Tokyo, Japan, 19-23 Apr 1993.
ABSTRACT Terrestrial animals including the human require regular periodic gravitational (g) stimulation to maintain normal physiologic functions on earth or in space. Identical g stimulations can be produced in space with inertial forces (G) using a centrifuge. These stimulations may be made more efficient in preventing physiologic deconditioning by increasing G levels above I G. The effective operational use of the centrifuge in space to prevent physiologic deconditioning from microgravity exposures will require ground-based studies using weightless simulation such as bedrest or dry immersion with laboratories that have humanuse centrifuges. The use of periodic, increased-G exposures in space may offer a practical inexpensive solution in preventing physiologic deconditioning.