G Wisdom: The Physiological Effects of G Forces, Part I


Richard A. Leland
President, NASTAR Center

Getting aboard a sub-orbital craft is not the same as getting on a commercial jetliner. The physiological effects a private space traveler will encounter are similar to those experienced by professional astronauts. Accordingly, prospective space travelers will need to prepare, so that both body and mind are ready to cope with and enjoy the experience. Just like any other sport or strenuous activity, it is possible to prepare and condition yourself for a space launch, but success takes effort, commitment, and a well-designed training program.

We begin our series by examining what G forces are. Let's look first at the physiological effects of sustained and transient linear accelerations, or "G". Initially we will isolate and examine only the physical and cardiovascular effects.

The term "G" describes the force exerted due to the linear (or straight line) acceleration of a body that has mass (Force = Mass x Acceleration). The force that is felt is equal, but opposite to, the acceleration. For example, if you speed away from a stop sign, your automobile will be accelerating forward, but you will experience the equal and opposite inertial force of being pressed back into your seat (courtesy of Newton’s third law of motion).

A useful way to describe this inertial force is by referring to it as "G". A “G” is a dimensionless unit that describes the magnitude of the resultant force generated by accelerating an object expressed in multiples of the earth's gravity. For example, standing on earth, we all experience the pull of the Earth as 32ft/sec² acceleration, which equates to one G. If we were accelerated at a rate of 64ft/sec² we would experience a force of 2 Gs (64ft/sec²/32ft/sec²) . Any increase in G effectively multiplies an object’s weight. So, a 160 lb person being accelerated at 2 Gs would effectively weigh 320 lbs.

Gs are described by type and as either positive or negative with respect to X, Y, and Z axes plotted on a coordinate system with the human at the center of the coordinates. The image at the beginning of this article illustrates these coordinates.

Exposure to elevated G forces affects humans in several ways. These effects include musculo-skeletal, cardiovascular, pulmonary, mobility, visual and vestibular (or organs of balance) effects. The type and significance of these effects is dependent on several aspects of the applied G. These include magnitude, direction, rate of application, duration of exposure and surface area of the body that is exposed.

Next Month: The physiological effects of increased Gs along the various G axes (Gx, Gy, Gz)

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