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


By Richard A. Leland
President of the NASTAR Center

Last month we introduced the concept of G and explored its general effects on the human body. This month we examine the specific effects of elevated G exposure along the various axes, Gx, Gy, and Gz, the directions of which are illustrated by the accompanying diagram.

+/- Gz (head to foot / foot to head) exposure can have the following effects:

Cardiovascular Effects: These effects range from blackout through loss of consciousness for +Gz and head congestion through incapacitation for -Gz.

Pulmonary Effects: As sustained +Gz forces are increased, people will experience difficulty breathing. This effect is due to the increased +Gz force pulling the diaphragm downward and the increased force that the muscles that expand the ribs must counteract.

Mobility Effects: Mobility is also impaired since every additional G increases body weight proportionately. For example, a 150 pound person experiencing 4 +Gz would effectively weigh 600 lbs.

Physical Effects: Another consideration is the potential for neck and extremity injury for a person who has not anticipated the increased +Gz. Since the head dramatically increases in effective weight under elevated G levels, as noted above, significant strain could be put on an unprepared person's neck. This could result in compressed vertebrae and muscle strains. If the onset rate was also high, people could experience limb flailing injuries, potentially consisting of bruising, dislocations, or broken bones. The normal human anticipatory response to application of increased G force is to flex the skeletal muscles. This lends rigidity to the skeletal structure and significantly reduces the chances for injury. For instance, trained pilots routinely tolerate up to +9 Gz during centrifuge training without injury.

+/- Gx (front to back / back to front) exposure can have the following effects:

People have the highest G tolerance to Gx exposure. Research studies (please do not attempt this at home) have shown that people can tolerate up to 15 +Gx and 8 -Gx so long as they are properly restrained and supported! Since most restraint and support systems consist of a forward facing seat with a rigid seat pan and seat back, and a 3- or 5-point restraint harness, most of the research data has been generated using these configurations. Generally, people tolerate +Gx better than they do -Gx because the G force is applied over a larger body area (i.e. the seat back).

+/- Gy [side to side] exposure can have the following effects:

Direction seems to have little to do with the effect of elevated Gy. Discomfort has been reported at 3 Gy due to the pressure of the restraint system on the person's clavicle. Discomfort due to pressure on the side of the body in the direction of the applied Gy was also reported. Swelling of the elbow with pain has been reported as low as +/-3 Gy. Increasing magnitude to +/-5 Gy has caused some external hemorrhaging and severe headache after only 14.5 seconds of exposure.

Next Month: Precautions and preparations that can be undertaken to mitigate G exposure.

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