Experiments with human beings on large centrifuges have shown that relatively high levels of acceleration can be tolerated by some people for brief periods of time without permanent damage. For example, as indicated in Figure 4, accelerations of the order of 5 g (five times the

Figure 4. Human time tolerance: positive g.

normal gravity level at the surface of the Earth) can be tolerated by a seated man not wearing a "g-suit" for about 2 minutes without a blackout (loss of vision caused by an inadequate supply of blood at the eye level). An acceleration of 4 g can be tolerated for about 8 minutes, while 3 g have been tolerated for as long as an hour by some subjects in several experimental runs (Miller et al., 1958). The subjects were seated and immobilized, however; they were not walking around or otherwise functioning in an everyday manner. At the conclusion of the 3-g experiments, the subjects reported quite pronounced muscular fatigue. Other experiments conducted in 1947 at the Mayo Clinic give at least an idea of the limitations imposed by increased gravitational fields (Code et al.,

1947). In these experiments, five human subjects were timed to see how rapidly they could scramble, creep, or crawl across the end of the centrifuge gondola, a distance of 7.5 feet, under various imposed accelerations. The results are shown in Table 1. The subjects were also timed to see how

Table 1. Time in Seconds Required To Move 7.5 Feet under Various Imposed Accelerations

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