Which factors reduce an aircrew's tolerance to high-G forces?

• A. Regular exercise, proper nutrition, sleep and hydration
• B. Hydration status, physique, fatigue, improper breathing technique
• C. Hydration only
• D. High altitude training

Answer: (B)

High-G tolerance depends on keeping enough blood flowing to the brain when the body is pressed by +G forces. Anything that reduces blood volume, makes it harder to pressurize the upper body, or fatigues the muscles used to resist G will shorten that tolerance. Hydration status matters because dehydration lowers plasma volume, which reduces venous return and stroke volume, leading to quicker cerebral underperfusion during high-G. Physique can influence how blood pools in the lower body and how much effort is required to perform the anti-G strain maneuver, so certain body types may have a harder time maintaining the necessary arterial pressure. Fatigue diminishes both the muscular endurance to sustain the anti-G maneuver and cognitive sharpness, making it harder to maintain effective pressure and breathing under high-G. Improper breathing technique undermines the anti-G maneuver: without proper diaphragmatic breathing and coordinated straining, thoracic pressure doesn’t rise enough to counteract the hydrostatic forces, causing blood to move away from the brain sooner.

Other factors listed would generally support tolerance rather than reduce it—regular exercise, good nutrition, sufficient sleep, and hydration help resilience. Hydration alone doesn’t capture all the elements needed for acute G tolerance, and high-altitude training isn’t a direct factor in immediate G-force tolerance.