space flight on the electrical activity of the heart before large scale EVA operations occur and careful attention must be paid to this during EVA operations. As well, the cardiovascular deconditioning could be a problem if a strenuous EVA or IVA task is required. Due to the reduction in blood reserve, the astronaut may not be able to supply the tissues with sufficient perfusion and performance would be affected. To counteract this deconditioning, an aerobic (high frequency, low resistance) exercise such as a treadmill or bicycle regime needs to be followed by the crew [Logan, 1992]. Musculo-Skeletal Effects In the absence of gravity, there is a decreased force required by skeletal muscle to perform certain tasks as compared to the same exercises on Earth. As a result, muscle has been seen to atrophy over the course of space flight without proper countermeasures. In particular, the most marked loss of muscle has been observed in muscles that have an antigravity function on Earth, for example the extensors of the leg such as the calf and quadriceps [Edgerton and Roy, 1992]. It is likely that the hip, back and neck extensor muscles also atrophy but this has yet not been proven. With inadequate use, muscles of the upper limbs will also atrophy in micro gravity, but to a lessor extent. Considering the importance of maintaining adequate strength to perform extravehicular activities (EVA) and construction for SPS, it is recommended that astronauts be placed on an intensive weight training regime prior to flight and follow a strength and intensity exercise regime (low frequency, high resistance) during their duration in micro gravity [Logan, 1992], Accompanying decreased muscle use in micro gravity is a decreased muscle load on parts of the skeletal system. It is believed that this may be the main cause of calcium depletion and bone resorption in micro gravity [Cann, 1992], Hormonal influences are likely the effectors of this calcium loss and changes in hormone balance in micro gravity may also be involved. It is proposed by Cann that a very specific exercise regime along with diet and pharmacological supplements be employed by astronauts in micro gravity to help offset this problem. This will be very important in order to avoid fractures induced by EVA construction activities. Decompression Sickness Decompression Sickness (DCS) is the foremost medical problem and health maintenance challenge facing EVA operations for orbital construction [Barratt, 1992a]. DCS is caused by evolution of nitrogen (N2) bubbles in the tissue, induced by a state of nitrogen supersaturation relative to ambient pressure. This most often occurs with a sudden decrease in ambient pressure, as seen with divers that resurface too quickly, or astronauts that move from a high pressure cabin to a lower pressure suit to perform EVA, or experience a sudden depressurization of their suit during EVA. The present mechanism for preventing DCS is to require astronauts to perform a “prebreathe” period in the airlock of the spacecraft (where the pressure is gradually reduced) and as well to breath 100% oxygen in this period just prior to EVA. The oxygen purges the circulatory system of nitrogen and thus lessens the risk of bubble formation. The clinical manifestations of DCS include localized joint and limb pain and can involve more serious pulmonary and neurologic damage. Space Adaptation Syndrome About one half of all astronauts have been affected by space adaptation syndrome and space motion sickness during the adaptation period to micro gravity (about three to four days). The characteristic symptoms are lethargy, anorexia, pallor, sweating, headache, general malaise, nausea, and vomiting The present theory on the origin of this problem is that motion sickness is the result of a sensory conflict in the brain resulting from discordant sensory inputs in micro gravity from the vestibular organs, the visual system, somatosensory organs, kinesthetic inputs etc.. EVA has thus been excluded during the first three or four days in space. At the very least, an astronaut may suffer from discomfort and decreased productivity during the adaptation period and at worst may have a life threatening encounter in the event of emesis while inside an EVA suit. Expelled stomach contents could be either aspirated, or could obstruct life support feeds to the suit producing an immediate threat to life. Current countermeasures against space motion sickness center around pharmacological agents, but undesired side-effects such as drowsiness often accompany the therapy. At present, there is no definitive way to counteract space motion sickness and this danger will likely necessitate a new crew to wait until the adaptation process is over before they can safely begin EVA. Proposed modifications to EVA suit design will be covered in a subsequent section.
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