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G-LOC, pronounced 'GEE-lock', is the abbreviation of G-force induced Loss Of Consciousness, a term generally used in aerospace physiology to describe a loss of consciousness occurring from excessive and sustained g-forces draining blood away from the brain causing cerebral hypoxia. The condition is most likely to affect pilots of high performance fighter and aerobatic aircraft or astronauts but is possible on some extreme amusement park rides. G-LOC incidents have caused fatal accidents in high performance aircraft capable of sustaining high g for extended periods. High-G training for pilots of high performance aircraft or spacecraft often includes ground training for G-LOC in special centrifuges, with some profiles exposing pilots to 9 gs for a sustained period. Such centrifuges are made by AMST Systemtechnik in Austria (Austria Metall SystemTechnik), Latacoere in France, Environmental Tectonics Corporation (ETC) and Wyle Laboratories in the USA.
Effects of g-forces 
Under increasing positive g-force, blood in the body will tend to move from the head toward the feet. For higher intensity or longer duration, this can manifest progressively as:
- Greyout - a loss of color vision
- Tunnel vision - loss of peripheral vision, retaining only the center vision
- Blackout - a complete loss of vision but retaining consciousness.
- G-LOC - where consciousness is lost.
(Under negative g, blood pressure will increase in the head, running the risk of the dangerous condition known as redout, with too much blood pressure in the head and eyes.)
The reason why vision is affected before other brain functions is because intraocular pressure within the eyes works against blood pressure in the head. Skilled pilots can use this loss of vision as their indicator that they are at maximum turn performance without losing consciousness. Recovery is usually prompt following removal of g-force but a period of several seconds of disorientation may occur. Brief but vivid dreams have been reported to follow G-LOC. If G-LOC occurs at low altitude, this momentary lapse can prove fatal and even highly experienced pilots can pull straight to a G-LOC condition without first perceiving the visual onset warnings that would normally be used as the sign to back off from pulling any more g's.
The human body is much more tolerant of g-force when it is applied laterally (across the body) than when applied longitudinally (along the length of the body). Unfortunately most sustained g-forces incurred by pilots is applied longitudinally. This has led to experimentation with prone pilot aircraft designs which lies the pilot face down or (more successfully) reclined positions for astronauts.
Another physiological effect that can happen from high levels of sustained gs is a temporary condition that pilots call the 'g-measles', where the skin around the buttocks or other extremities becomes mottled because blood capillaries have been broken from too much pressure.
The g thresholds at which these effects occur depend on the training, age and fitness of the individual. An un-trained individual not used to the G-straining maneuver can black out between 4 and 6 g, particularly if this is pulled suddenly. A trained, fit individual wearing a g suit and practicing the straining maneuver can, with some difficulty, sustain up to 9g without loss of consciousness. Prone position designs in aircraft have not proved successful and the problem has been addressed largely by the development of the G-suit.