High-G training is done by aviators and astronauts who are subject to high levels of acceleration ('G'). It is created to avoid a g-induced loss of consciousness (shorten G-LOC), a situation when G-forces circulate the blood away from the brain to the extent that consciousness is lost. Incidents of acceleration-induced deprivation of consciousness have caused fatal accidents in aircraft capable of sustaining high-g for considerable periods.
Human body tolerances of G-force process depend on the value of the G-force, the length of time it is applied, the direction it acts, the location of application, and the posture of the body. The human body is flexible and deformable, particularly the softer tissues. A hard slap on the face may briefly impose hundreds of G locally but not produce any real damage. A constant 16 G for a minute, however, may be deadly. When vibration is experienced, relatively low peak G levels can be severely damaging if they are at the resonance frequency of organs and connective tissues. To some degree, G-tolerance can be trainable, and there is also considerable variation in innate ability between individuals. In addition, some illnesses, particularly cardiovascular problems, reduce G-tolerance.
Aircraft pilots (in particular) sustain G-forces along the axis aligned with the spine. This causes significant variation in blood pressure along the length of the subject's body, which limits the maximum G-forces that can be tolerated. Positive, or "upward" G, drives blood downward to the feet of a seated or standing human. Resistance to positive G varies. A typical person can handle about 5 G (49 m/s2) (meaning some people might pass out when riding a higher-g roller coaster, which in some cases exceeds this point) before losing consciousness, but through the combination of special g-suits and efforts to strain muscles—both of which act to force blood back into the brain—modern pilots can typically handle a sustained 9 G (88 m/s2).
In aircraft particularly, vertical g-forces are often positive (force blood towards the feet and away from the head) - this causes problems with the eyes and brain in particular. As positive vertical G-force is progressively increased, the following symptoms may be experienced: grey-out (where the vision loses hue, easily reversible on levelling out), tunnel vision (where peripheral vision is progressively lost), blackout (a loss of vision while consciousness is maintained, caused by a lack of blood to the head), G-LOC, death (if G-forces are not quickly reduced, death can occur).
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