G-force, or Gs, measures acceleration relative to Earth’s gravity. This sensation of increased weight arises from non-gravitational forces, such as those generated by a fast-moving vehicle. For instance, a person standing on Earth experiences 1 G. When an aircraft accelerates or turns sharply, the forces involved can multiply this effect, creating a feeling of being pushed into or pulled out of a seat. These forces can significantly impact the human body.
Understanding G-Forces and the Body’s Response
When the body experiences positive G-forces (+Gz), directed from head to foot, the cardiovascular system faces a challenge. Blood is pulled towards the lower extremities, specifically the legs and abdomen. This pooling reduces the amount returning to the heart and, consequently, the amount pumped to the brain and eyes. As blood flow to the head diminishes, the brain and eyes receive less oxygen.
Reduced blood flow leads to visual disturbances. Peripheral vision may narrow, creating a “tunnel vision” effect. This can progress to a “gray-out” (color vision loss), then a “black-out” (complete vision loss while consciousness may still be retained). If the G-force continues, the ultimate consequence is G-force induced Loss Of Consciousness (G-LOC), a temporary state of unconsciousness due to insufficient cerebral blood circulation.
The Threshold for Passing Out
An average, untrained individual typically experiences G-LOC from sustained positive G-forces ranging from +4 to +6 Gz. This threshold can vary based on how quickly the G-force is applied; a rapid onset can lower tolerance, sometimes leading to G-LOC at around +3 to +4 Gz. The brain’s oxygen reserves only last about 4 seconds without a fresh blood supply.
While positive G-forces push blood downwards, negative G-forces (-Gz) pull blood towards the head. The body generally has a lower tolerance for negative G-forces, often ranging from -2 to -3 Gz. This increased pressure in the head can cause discomfort, headaches, and “red-out,” where vision appears reddish due to blood congestion in the eyes.
Factors Influencing G-Tolerance
Several factors influence G-tolerance. The direction of the force plays a role; humans are more tolerant of G-forces acting perpendicular to the spine (e.g., chest-to-back, +Gx) than those parallel to it (+Gz or -Gz). The duration of exposure is also significant; brief G-forces are better tolerated than sustained ones. The rate at which G-forces increase also impacts tolerance; a gradual onset allows more adaptation than a rapid one. Personal health aspects, such as fitness, hydration, and fatigue, can also modify G-tolerance, and age can affect tolerance, with older individuals sometimes exhibiting a decreased ability to withstand G-forces.
Strategies for Increasing G-Tolerance
To enhance G-tolerance, especially for pilots, specific strategies and equipment are employed. The Anti-G suit, or G-suit, is a specialized garment with inflatable bladders around the legs and abdomen. When G-forces increase, these bladders inflate, compressing the lower body to prevent blood pooling in the extremities. This action helps maintain blood flow to the upper body and brain.
Another method is the Anti-G Straining Maneuver (AGSM), a physiological technique involving muscle tensing and controlled breathing. By contracting leg, abdomen, and arm muscles, and exhaling forcefully against a partially closed airway, individuals temporarily increase blood pressure, pushing blood back towards the brain. Training programs, often using human centrifuges, help individuals practice these techniques and condition their bodies to better withstand high G-forces.