Here's What It Looks Like When G-Force Knocks You Out

Soaring sideways over the Nevada desert in a small propeller airplane, a young boy laughs with awe at the tilted world below him. Then, a few seconds later, the boy's muscles droop. He passes out. And now it's his instructor's turn to laugh.

Don't worry: The boy in the plane — the star of a viral GIF titled "G-forces: first you feel silly, then you pass out," posted to Reddit on Feb. 5  — wakes up fine but groggy after a few seconds. Still, the question remains: Why did he pass out, but the pilot sitting behind him did not?

The answer, according to the National Aerospace Training and Research Center (NASTAR), ain't nuthin' but a G thang, baby. [6 Weird Facts About Gravity]

"This seems like a typical case of G-LOC, or G-Induced Loss of Consciousness," Dr. Swee Weng Fan, a former flight surgeon and current managing director of training at NASTAR, told Live Science in an email. G-LOC is "one of the hazards that pilots flying high-performance aircraft must be well trained and prepared for," Fan said.

In physics, G-force is used to describe the acceleration of an object relative to Earth's gravity. Assuming you are sitting at a desk or standing on solid ground right now, you are in a 1 G environment; Earth's standard force of gravity (G) is pushing against you as it normally does.

However, when a plane makes a rapid, near-90 degree turn as it does in the GIF, it creates a radial acceleration that can generate more than 6 G's of force — or six times the force of Earth's gravity — Fan said. (According to the YouTube video from which the GIF was taken, the plane was actually hit with closer to 8 G's of force as it turned.)

Because humans adapted to survive in a 1 G environment, any rapid increases or decreases in G-force can have immediate adverse effects, Fan said. Part of this has to do with how blood circulates throughout your body.

"In a 1 G environment, the heart generates enough blood pressure to deliver the blood above the heart and to the organs above the chest (like the brain and the eyes)," Fan said. "But in a rapidly building G [environment], the acceleration force is strong enough to force the blood down the legs, making it difficult or almost impossible to flow back to the heart for re-circulation."

"On top of that, now the heart has to fight the force to push the blood to the brain with much higher pressure," Fan said. "And no blood in the brain means no oxygen in the brain."

Your brain cells hold a small oxygen reserve that can keep them functioning for about 4 seconds, Fan said. After that reserve is depleted, the brain will "shut down," causing you to lose consciousness as the boy in the GIF did. This is G-LOC.

"It is like going to sleep," Fan said. "Some report vivid dreams or nightmares after waking up. It can be followed by convulsions and uncontrolled muscle movements. All in all, this can take perhaps 20 – 30 seconds, though it can vary widely."

In an untrained adult, as few as 3 G's can be enough to deprive the brain of oxygen, Fan said. For this reason, all military and acrobatic pilots train in Anti-G Straining Maneuvers (AGSM), which include various breathing and muscle-tensing techniques to minimize downward blood flow and keep the brain as oxygenated as possible.

Under 8 G's of force, though, even 15 seconds of AGSM can be incredibly hard to sustain, Fan said. Pilots must maintain strong physical fitness, wellness and nutrition to have any hope of staying alert in high-G environments. Protective devices like Anti-G Suits, which automatically inflate to restrict blood flow to the legs, can help, too. A combination of these factors is likely why the boy passed out, but the pilot behind him did not.

Luckily, Fan said, brain cells are resilient, and it would take around 4 to 6 minutes of oxygen deprivation for any cells to die completely. In other words, the boy in this GIF almost certainly walked away unscathed — and, hopefully, unashamed.

Originally published on Live Science.