The human body, like that of all terrestrial animals, evolved under the influence of Earth's gravity. In an evolutionary sense, physiological adaptation to gravity in vertebrate animals has been best exploited by birds. For humans and other ground-dwelling vertebrates, relatively modest deviations above or below normal gravitational field strength disturb physiological homeostasis. Life scientists do not yet agree on the principles that govern physiological effects in greater or lesser gravitational fields than that on Earth. What they do agree on is the importance of the hypothesis that a continuous relationship exists between the effects of extremes of gravity on the body.
The concept that hypergravity and microgravity are two extremes of a spectrum is known as the continuity principle. The issue of whether this principle reflects the true nature of the adaptive responses to high and low gravitational force is being thoroughly scrutinized because it may be critical to the success of longduration space flights. The operating assumption is that the adverse effects of microgravity can be counteracted by acceleration, or hypergravity. Despite a great deal of scientific research, there is no consensus about the utility of the continuity