G-force, or gravitational force equivalent, is a concept that moves beyond the simple pull of Earth’s gravity. This measure quantifies the sensation of weight that results from acceleration, placing a strain on the human body. While the feeling of being pressed into a seat during a fast car ride is normal, extreme G-forces encountered in high-performance situations, like military aviation or space travel, can pose significant health hazards.
Defining and Measuring G-Force
G-force is a measure of acceleration relative to Earth’s standard gravitational acceleration, where one G is equal to 9.8 meters per second squared. This unit quantifies the “proper acceleration” that causes the perception of weight. An object or person at rest on Earth’s surface constantly experiences 1 G.
G-force measurement uses a coordinate system based on the body’s axes to specify the direction of the force. The Z-axis (Gz) runs from head to foot and is the most consequential axis in aviation. Positive Gz (+Gz) forces push the body down into a seat, while negative Gz (-Gz) forces lift the body out. The X-axis (Gx) refers to forces from front to back, and the Y-axis (Gy) represents side-to-side forces.
Physiological Effects on the Body
The primary mechanical impact of sustained G-force is the disruption of the cardiovascular system’s ability to pump blood. Under positive Gz forces, blood is pulled toward the lower extremities due to a hydrostatic effect. This requires the heart to work against a greater force to pump blood upward to the brain and eyes.
As +Gz increases, the cerebral perfusion pressure driving blood flow to the head decreases significantly. The brain is highly sensitive to a lack of oxygenated blood (hypoxia) and has a limited metabolic energy reserve of about six seconds. If the heart cannot compensate quickly, the brain is starved of oxygen, leading to a progressive shutdown of function.
In contrast, negative Gz forces, directed from foot to head, cause blood to rush towards the head. This creates a dangerous increase in blood pressure in the capillaries of the head and eyes. Unlike the body’s natural reflex to increase heart rate under +Gz, the reflex response to -Gz is to decrease heart output, which happens much faster, within two to four seconds.
Acute Risks and Tolerance Thresholds
The danger of G-force is tied to its magnitude, duration, and direction of application. An average, untrained person can tolerate sustained +Gz forces in the range of 4 to 6 Gs before experiencing significant symptoms. As G-force increases beyond this threshold, the reduction in cerebral blood flow leads to a predictable sequence of visual impairment.
The first stage is peripheral vision loss, often described as tunnel vision, followed by a gray-out (loss of color vision). Next is a complete blackout, where vision is lost, although the person remains conscious. If the G-force is sustained, the final outcome is G-force induced Loss Of Consciousness (G-LOC), resulting from cerebral hypoxia. The minimum +Gz threshold for G-LOC in healthy individuals is around +4.7 Gz, though this varies widely.
G-LOC is dangerous because the period of absolute incapacitation averages about 12 seconds, followed by a period of relative incapacitation or confusion lasting around 16 seconds. While positive Gz causes G-LOC, negative Gz can cause “redout,” where the visual field appears reddish due to blood pooling in the head and eyes. Negative G-forces are tolerated less well, as the increased pressure can lead to outcomes like retinal or cerebral hemorrhaging, with tolerance limits often around -3 Gz.
Mitigation and Adaptation
Specialized equipment and training are employed to extend the human body’s tolerance to high G-forces. The anti-G suit, or G-suit, is a flight garment designed to counteract the pooling of blood in the lower body under +Gz acceleration. This suit contains inflatable bladders that automatically pressurize and compress the legs and abdomen as the G-load increases. The mechanical pressure applied by the suit slows the draining of blood away from the upper body, increasing the pilot’s tolerance by about 1 G.
In addition to the G-suit, trained pilots use the Anti-G Straining Maneuver (AGSM) to physically boost their G-tolerance. This maneuver involves forcefully tensing the muscles of the abdomen and legs combined with short, rapid breathing cycles. The muscular contraction acts like a second layer of external pressure, restricting blood pooling and raising the internal blood pressure. A pilot using AGSM and a G-suit can potentially sustain accelerations up to 9 Gs or more for short periods.