The human body experiences acceleration forces during rapid changes in speed or direction. These forces can have profound physiological effects, including a temporary loss of consciousness known as “blacking out.” Understanding this phenomenon is important in fields like aviation, space exploration, and extreme sports.
What Are G-Forces
G-forces are a measure of acceleration, quantifying the force per unit mass exerted on an object or person. They are expressed in multiples of Earth’s standard gravity (g), where one g equals approximately 9.8 meters per second squared (m/s²). This unit allows for comparison of intense acceleration to everyday gravity.
These forces are not always gravitationally produced; they arise from any change in an object’s speed or direction. A person standing still on Earth experiences 1 g. When a car accelerates rapidly, brakes suddenly, or rounds a sharp corner, its occupants feel G-forces. Rollercoasters also subject riders to varying G-forces, causing feelings of being pushed into the seat or weightlessness.
How G-Forces Affect the Body
The human body’s response to G-forces depends on their magnitude and direction. With positive G-forces (+Gz, or head-to-foot acceleration, such as during a rapid ascent in an aircraft), blood is driven downwards. This pooling reduces the amount returning to the heart and, consequently, blood flow to the brain and eyes.
As blood supply to the brain diminishes, visual and cognitive symptoms occur. Initial effects include “greyout” (loss of color vision) and “tunnel vision” (narrowed peripheral vision). If G-forces persist, a complete loss of vision, or “blackout,” can happen even while consciousness is maintained. The most severe stage, G-force induced Loss Of Consciousness (G-LOC), results from a critical reduction of blood circulation to the brain, leading to unconsciousness.
This loss of consciousness is a temporary and reversible physiological response. G-LOC can last several seconds, typically followed by a period of confusion or disorientation, known as relative incapacitation. The entire incapacitation period, combining unconsciousness and confusion, can last around 28 seconds.
The Blackout Limit
For an untrained individual, the G-force threshold for experiencing G-LOC, or blacking out, generally falls within +4 to +6 Gz. This threshold reflects where the cardiovascular system struggles to maintain adequate blood flow to the brain. The exact limit can vary among individuals due to physiology, health, and other factors.
While positive G-forces are the primary cause of blackouts, negative G-forces (-Gz, or foot-to-head acceleration) also have significant effects. These forces push blood towards the head, causing a “redout” (reddish vision due to increased blood pressure). Redouts can cause retinal damage and hemorrhagic stroke, and human tolerance to negative G is generally much lower, around -2 to -3 G.
Modifying G-Force Tolerance
Individuals like fighter pilots and astronauts employ various methods to increase their G-tolerance. Specialized training, often in human centrifuges, simulates high G-environments. This training helps individuals cope with physiological stresses and practice countermeasures.
The Anti-G Straining Maneuver (AGSM) is a technique involving specific breathing patterns and muscle contractions. Pilots tense abdominal and leg muscles to resist blood pooling and maintain blood pressure to the brain. This maneuver, combined with controlled breathing, can increase G-tolerance by 1.5 to 2 G.
The anti-G suit features inflatable bladders that automatically compress the lower limbs and abdomen during high G-forces. This pressure prevents blood from pooling in the lower body, assisting the heart in circulating blood to the brain. Proper physical conditioning, including cardiovascular fitness and strength training, also enhances G-tolerance.