32. G Force Tolerance

Full Scope Human Longevity

Summary
G Force is a measure of acceleration, or a change in velocity and/or direction. All of us experience 1Gz, while standing on earth. However, military fighter pilots may have to perform under 9Gz. Such large G Forces can lead to loss of consciousness (G LOC) and even death. G Force Tolerance is a measure of how many Gs a person can withstand prior to loosing consciousness. Many factors can affect a person’s tolerance for Gs.
Morbidity and Mortality
G Forces prevent blood from reaching the brain causing loss of consciousness. If you are flying a fighter jet this can lead to collisions or vulnerability to attack. If G Forces are sustained during G LOC, permanent brain damage, and then death will occur. Humans can handle incredible high G forces for short periods of time.

G Force Tolerance Thresholds for Severe Injury for Short Duration Exposures
| Axis | G Force
| +Gz | 25G
| -Gz | 15G
| +Gx | 50G
| -Gx | 45G
| +- Gy | 12G

Taken from Green’s, Handbook of Aviation and Space Medicine. 2019.

Story
Flight Surgeons and Aerospace Medicine researchers in the first half of the 20th century regularly used themselves as guinea pigs in physiologic tests. US Air Force Col. John Stapp, MD PhD took this idea to the extreme. He made rocket sleds at a New Mexico military base to test human tolerance for G Forces. On Dec. 10, 1954, after several runs (26 on himself), Stapp broke the land speed record traveling 662 mph, sustaining 46.2 Gx.

Key Points
1. The danger of any G Force depends on the direction of force relative to the body, magnitude of the force, rate of onset, and duration sustained.
2. Subscripts are used to identify the direction of force relative to the body. +Gz travels from head to toe; - Gz travels from toe to head; + Gx travels from chest to back; - Gx travels from back to front; + Gy travels from right to left; - Gy travels from left to right.
3. g is the acceleration due to gravity on earth, equal to 9.8 m/sec/sec. G = acceleration/g. Therefore 10Gs = 98 m/sec/sec. A person feeling 10Gs would feel 10x heavier and their heart would have to pump blood the equivalent of 10 time the normal distance in order to reach the brain.
4. +Gz Forces cause blood to pool in the legs. This prevents blood from entering the brain. If gradual in onset, G forces will eventually cause vision loss, followed by blackout, and then loss of consciousness (G LOC). Rapid onset G forces may lead straight to blackout.
5. G tolerance, refers to a human’s capacity to sustain G Forces without G LOC. G tolerance is different for everyone. For example, +Gz tolerance ranges from 2.7 to 7.8 (mean 4.7) for most people.
6. Several factors can improve G tolerance. Examples include anti-G straining maneuvers, G suits, reclining the pilots seat, proper hydration, proper nutrition, drugs, physical fitness, and proper training.
7. -G immediately preceding +G will lower G tolerance significant. For instance, push-pull aviation maneuvers (nose diving the plain and then trying to pull up quickly) is particularly dangerous for G-LOC.

References
- Green, Gaydos, Hutchison, Nicol. Handbook of Aviation and Space Medicine. CRC Press. 2019
- Mackowski. Testing the Limits, Aviation Medicine and the Origons of Manned Space Flight. 2006
- Wikipedia. G Force, G Suit, High G Training, AGSM
- Youtube. Multiple videos showing centrifuges, John Stapps Sled, and fighter pilots
- Tesch PA, Hjort H, Balldin UI. Effects of strength training on G tolerance. Aviat Space Environ Med. 1983 Aug;54(8):691-5. PMID: 6626076.