Muscle histochemistry of flight and control animals were analyzed in the Medical Image Analysis Facility at the Jet Propulsion Laboratory, which includes a microscope-mounted television camera capable of converting the specimen image into numerical form for computer processing. The computer program isolates the individual fibers, and measures the area, perimeter, and average optical density of each. A scatter plot is then produced showing how the fibers are distributed in diameter and optical density. Individual and mean fiber area were also printed with the program.

Results:

Both fast-twitch and slow-twitch fibers showed a significant reduction in fiber area. With only two exceptions, the proportion of slow fibers was reduced by space flight. The ratio of slow fiber area to fast fiber area was lower in the flight groups, indicating that slow fibers suffer size loss more than do fast fibers. These results appear to give a snapshot of how muscle physiology adapts to the space flight environment. Hypogravity produces an insufficient loading mechanism, leading to hypokinesia and hypodynamia. This, in turn, produces trophic changes, particularly in antigravity muscles (or slow twitch oxidative fibers), decreased protein metabolism, negative nitrogen balance, etc., producing muscle atrophy as the final result. Slow fibers, important in maintaining posture against gravity, are of little use in space, and their size, and even proportion, were reduced by the adaptation process. Fast fibers also suffered a disuse atrophy, but to a lesser extent since they are used for locomotion.