Musculoskeletal adaptation to mechanical forces on Earth and in space.

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.