The therapeutic Benefits of Gravity in Space and on Earth

Abstract

The traditional scientific approach of investigating the role of a variable on a living organism is to remove it or the ability of the organism to sense it. Gravity is no exception. Access to space has made it possible for us to begin the exploration of how gravity has influenced our evolution, our genetic make-up and our physiology. Identifying the thresholds at which each body system perceives, how much, how often, how long the gravity stimulus is needed and in which direction should it be presented for maximum effectiveness, is fundamental knowledge required for using artificial gravity as a therapeutic or maintenance countermeasure treatment in exploration missions. Here on earth, although surrounded by gravity we are negligent in using gravity as it was intended, to maintain the level of health that is appropriate to living in 1G. These, changes in lifestyle or pathologies caused by various types of injury can benefit as well from artificial gravity in much the same way as we are now considering for astronauts in space.

Gravity is a physical force well-known, but not well understood. Its intensity and direction have been constant throughout evolutionary history on Earth, making it difficult to understand what role, may have on life. Gravitational loading is directional toward the centre of the Earth and acts on all masses at the Earth's surface defining the weight of each object. Weight drives many chemical, biological, and ecological processes on Earth. Altering weight changes these processes. If gravity causes changes to biology, then it must be a major physical environmental force shaping life on Earth¹.

Access to Space, has made it possible for us to begin the exploration of how gravity has influenced our evolution, our genetic make-up and our physiology. So what is the role of gravity in biological, chemical, and physical systems?

The expansion of human space flight to low earth orbit and beyond over the past forty years has provided a challenge to clinicians responsible for the health and safety of astronauts. Space medicine is currently entering an evolutionary phase of incorporating the understanding of the physiological changes associated with human space flight into the prevention, diagnosis and therapy of illness and injury in Space. Applying terrestrial diagnostic and therapeutic approaches to illness in Space may be problematic when not only the underlying physiology differs in microgravity; it is likely that the pathophysiology of certain diseases may differ as well. This led to a new discipline of growing importance, Bioprocessing in Space. It is closely related to understanding how cells function in gravity since many of these cells make useful products. Early experiments have focused on developing the apparatus and techniques for processing biological substances.

All the experiments performed in microgravity may have benefits on Earth especially for those people on the ground whose problems with balance, sleep, loss of muscle tone and loss of bone density are remarkably similar to those experienced by astronauts returning from their missions. But still even after all those years of Space research the physiologic challenges of spaceflight remain unchanged. Motion sickness remains a significant, but now partly treatable, problem in flight. Crewmembers return with difficulties in maintaining balance. Standing upright after spaceflight can be difficult due both to labile blood pressure and unstable posture. Muscle mass and strength are reduced. Astronauts tend to sleep poorly. Many of these symptoms reflect major underlying changes in the nervous system.

The primary physiological responses to spaceflight included fluid shifts, repositioning of certain organs or organelles, unloading of the cardiovascular system and support structures, and lack of stimulus to the gravity sensors. Even short term exposure to microgravity resulted in significant deconditioning of the musculoskeletal, cardiovascular, and vestibular systems^2–8. Initially the countermeasures applied to counteract this deconditioning included: cardiovascular training, lower body negative pressure, fluid volume loading, muscle and bone loading and vestibular challenges⁹.

Many questions rose from previous years. How has the information from gravity sensors (such as the inner ear) been reinterpreted? Has nervous control of the circulation been altered? How have circadian rhythms been affected? What neural plasticity is there and how does it work? Now days we have answers to most of those questions. The potential of artificial gravity as multisystem countermeasure is indeed worth pursuing, because present-day exercise-based countermeasures have failed to prevent adverse changes in cardiovascular and musculoskeletal systems^7,8,10–13.

Furthermore identifying the thresholds at which each body system perceives, how much, how often, how long the gravity stimulus is needed and in which direction should it be presented for maximum effectiveness is fundamental knowledge required for using artificial gravity as a therapeutic or maintenance countermeasure treatment in exploration missions.

Recent research is largely aimed at understanding how humans can live in Space for longer and longer periods, eventually perhaps travelling in other planets, but there is no evidence of how long a human being can live and function effectively in microgravity. The only known environment humans experience for long periods and the only way to prevent medical problems in Space, especially in longer missions is "gravity".

In the early years the use of short-arm centrifuge was also proposed as a countermeasure to microgravity¹⁰, but the effectiveness of passive or active short-arm centrifugation has not been evaluated so far. Current countermeasures focus only on certain organ systems and symptoms, require specially adapted therapeutic equipment, are time consuming, demand a high degree of individual discipline, and yet are only partially effective at slowing the crew's physical decline over a period of months^14,15. Artificial gravity has long been proposed as a means of holistically maintaining the entire human organism and avoiding all of micro gravity's adverse health effects. Yet it has not been implemented, in part due to concerns with system complexity and cost in mass and energy. Considering that half of all astronauts require one to three days to adapt to microgravity, a similar period of adaptation to artificial gravity is not unreasonable, especially since artificial gravity promises substantial health benefits with less demand on crew time and motivation. The principal application of artificial gravity is to preserve human health during multi-month space flights - especially during transits between Earth and Mars or other celestial bodies, in which long exposure to microgravity is a nuisance rather than a mission objective

Only recently research programs included the exploration of potential benefits of artificial gravity generated by a human-powered centrifuge offering many advantages and especially many physiological systems as skeletal, muscle, bone and cardiovascular could simultaneously benefit from it. The benefits of gravity and acceleration on musculoskeletal and cardiovascular system are obvious. John Cramer¹³ reported that the increased load on bone and muscle is a potent cue to growth. Within 8 weeks, strength increases by 30%. There is an associated increase in muscle mass and both aerobic and anaerobic endurance. Bone mass and density increases under the increased load for up to three months. Height decreases due to vertebral disk compression. For the Cardiovascular system the same author mentioned that the challenge is to maintain adequate cardiac output in the face of the increased tendency to venous pooling and the increased metabolic requirements of the muscles with loading. Cardiac output and blood pressure increase. Activation of fluid retention mechanisms will lead to ongoing volume expansion; effective volume returns to a 1g normal value within 12-72 hours, and ongoing expansion slowly occurs for one to two weeks thereafter. Red blood cell volume will increase over two weeks to a month, due to an increase in bone marrow activity mediated by erythropoietin, adapting to acceleration in the short term (seconds to hours)¹³.

There has been a small number of published studies so far^16,17 examining the physiological responses to shortarm centrifugation without clarifying whether passive or active centrifugation is more effective to prevent microgravity induced alterations. Currently it is not known what durations of activity on short arm will be required to prevent deconditioning induced by low gravity. Recent studies have shown that continuous exposure to gravity does not seem necessary and that, instead, intermittent exposure may suffice^17–22.

Caiozzo et al²³ as well as other researchers^24–27 proposed human-powered centrifuge as a countermeasure equipment. Different types have been developed by research groups as by Greenleaf et al., NASA-Ames Research Center^24–27, and Di Prampero et al^25,26.

Several ground-based human studies have provided data, suggesting benefits of Intermittent Artificial Gravity (I.A.G) in preventing deconditioning due to microgravity exposure^19,21,17. Sun Biao et al²⁸ demonstrated that daily gravitational loading by standing for as short as 1 h, is sufficient to prevent differential adaptation changes in function and structure of cerebral and hindquarter vessels during 28 days of simulated microgravity. The present work has provided data to support that I.A.G may be efficacious in preventing vascular adaptation changes and hence cardiovascular deconditioning due to microgravity.

Here on earth, although surrounded by gravity we are not using it efficiently in order to maintain the level of health that is appropriate to living in 1G. Hypergravity would prevent deconditioning due to long bed rest, hendling therapeutically neuromuscular diseases, osteoporosis, orthostatic intolerance and vestibular disorders. These, changes in lifestyle or pathologies caused by various types of injury can benefit as well from artificial gravity, equivalent to or in fact greater than 1G in much the same way as we are now considering for astronauts in Space. Adaptation to acceleration in the short term (seconds to hours) happens every day when we wake up in the morning and stand up. On standing, blood pools in the legs. A combination of vaso- and venoconstriction, increased heart rate, and activation of renal fluid retention mechanisms maintain cardiac output within tolerable limits. Above the level of the heart, the circulation dilates to guarantee an adequate blood flow to the brain. These changes take place within two seconds, but are insufficient to maintain brain blood flow for more than an hour or so. Walking helps keep venous pressures low in the legs ('muscle pump' action), preventing further falls in effective blood volume.

Enhancement of gravity cues on Earth, the state known as hypergravity, depends on the principle that the faster any mass moves, the heavier it becomes. Riding a bicycle, speeding in a fast car, riding a roller coaster, flying in an airplane, whizzing downhill on a sled or being strapped to a centrifuge can all provide the stimulation of hypergravity. Rocking chairs are objects we usually associate with the elderly. There may be good physiological reasons why human beings at either end of the age spectrum enjoy the motion of rocking. It might be a way to increase sensory perception and brain blood flow. The Experiments on a rotating chair on the Neurolab Shuttle mission in 1998 suggested that this may indeed be a feasible way of stimulating the organs of the inner ear to maintain the sharpness of their gravity sensors²⁹. Disruption of vestibular function also occurs during the ageing process accounting for up to 25% of falls in the elderly and significant health care costs. Enhancement of neural-adaptation will ultimately lead to improved quality of life for the elderly, improved therapies and rehabilitation for patients with spinal cord injuries and vestibular problems, and better training for pilots, athletes and divers.

According to Muriel Ross, neural plasticity, the ability of the brain to learn from experience and to adapt to new environments is recognized to be profound and exposure to altered gravity has an effect on communication sites between the sensory cells and the nerve fibers ending on them³⁰. This could lead to the assumption that similar exposure to a force greater than 1G on a centrifuge on Earth might one day prove useful in restoring a sense of balance to disabled people who have lost it. There are cases of children with medical disorders, such as autism, attention deficit disorders, fragile X e.t.c., which have sensory modulation problems, such as the inability of the nervous system to continually and accurately register sensory information which the causes are not well known. The problem in this case is that the child's nervous system is not modulating the sensory input successfully and is not responding to the sensory information appropriately³¹.

A brain damaged at birth may require hypergravity, a higher intensity of gravity stimulus, before a child's brain becomes programmed to respond to direction and acceleration and eventually learn to walk. Increasing gravity could modify or alter his perception of it prior to responding, by triggering gravity sensors and brain blood flow. This would mean that rehabilitation exercises in children with cerebral palsy should be more effective if done in the upright position in a way that the body may experience some load, even if the child had to be supported by a harness. Alternately, the movement therapy could be done on a centrifuge.

Furthermore identifying the thresholds at which each body system perceives, how much, how often, how long the gravity stimulus is needed and in which direction it should be presented for maximum effectiveness, using hypergravity might be proved to be a potential new therapeutic approach of children with disabilities, and elderly adding another terrestrial application of Space research.