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A major concern of the US and Soviet (Russian) space programs is the health and safety of astronauts and cosmonauts. One of the areas receiving the most attention has been the effects of long duration space flight on the musculoskeletal system. During the Skylab period exercise programs and bone densitometry equipment were evolving. No treadmills were included on either Skylab-1 or Skylab-2, and only a teflon pad and elastic cords were available on Skylab-4. During the same period the Russians were beginning to experiment with longer duration space flight culminating in a milestone flight of 366 days in 1987-1988. Development of exercise protocols and equipment was an important part of their program. Early Skylab results were not considered encouraging. In spite of daily exercise, calcium balance studies measured significant increases in urinary calcium. And while calcaneal bone density adaptation on Skylab flights was not particularly high (+1% to -8%), the longest flight was only 84 days. Short duration shuttle flights have been the only other source of information on humans from the US space program. The fact that daily exercise protocols were not rigorously followed nor sufficiently intense likely contributed to their limited success. Interestingly, reduced muscle strength and bone loss were only detected in the lower limbs. According to published data and Joint US/USSR Working Group (JWG) reports, the health of cosmonauts returning from space is not related to the length of stay in microgravity but is directly related to the "intensity" with which they exercised in space. Pre- and post-flight bone density measurements of recent MIR crews have been taken with a Hologic QDR-1000/W dual energy x-ray absorptiometry (DXA) machine supplied by the US space program. DXA machines have a precision for repeated bone density measurements of 1-2%. These data are our best source of information on the effects of long duration space flight with exercise on regional changes in bone density. Regional lower limb bone density and muscle strength were reduced in most cosmonauts on their return. However, lumbar spine bone density measurements have been mixed. Vertebral body trabecular bone, measured by quantitative computed tomography (QCT), increased or remained unchanged in 6 of 7 cosmonauts; DXA data show a mean decrease in lumbar density (vertebra plus posterior elements) in a different group of cosmonauts.

It is well known that long-term exposure to microgravity causes a number of physiological and biochemical changes in humans; among the most significant are: 1) negative calcium balance resulting in the loss of bone; 2) atrophy of antigravity muscles; 3) fluid shifts and decreased plasma volume; and 4) cardiovascular deconditioning that leads to orthostatic intolerance. It is estimated that a mission to Mars may require up to 300 days in a microgravity environment; in the case of an aborted mission, the astronauts may have to remain in reduced gravity for up to three years. Although the Soviet Union has shown that exercise countermeasures appear to be adequate for exposures of up to one year in space, it is questionable whether astronauts could or should have to maintain such regimes for extremely prolonged missions. Therefore, the NASA Life Sciences Division has initiated a program designed to evaluate a number of methods for providing an artificial gravity environment.

The results of biomedical investigations carried out in the U.S.S.R. manned space missions are discussed. Their basic result is well-documented evidence that man can perform space flights of long duration. The investigations have demonstrated no direct correlation between inflight or postflight physiological reactions of crewmembers and flight duration. In all likelihood, this can be attributed to the fact that special exercises done inflight efficiently prevented adverse effects of weightlessness. However, human reactions to weightlessness need further study. They include negative calcium balance and anemia as well as vestibulo-autonomic disorders shown by crewmembers at early stages of weightlessness. Attention should be given to psychological, social-psychological and ethical problems that may also limit further increase in flight duration.