View German pilot Norbert Biehler inside a human centrifuge training to withstand an accelerated centrifugal force

NARRATOR: At the German Air Force Institute of Aviation Medicine in Königsbrück, in Germany, scientists test the effects of acceleration on the human body. Here, the most powerful centrifuge in the world can generate a force of nine Gs - enough to instantly knock out anyone not accustomed to such forces. It even pushes experienced pros to their limits. Fighter pilots like Norbert Biehler train regularly to enable them to withstand such pressures. He is dressed in a G-suit to prevent blackout. It stops the blood rushing to his legs and depriving his brain of blood. Norbert Biehler has been flying the German Army's Eurofighter for seven years. When practicing combat manoeuvres he subjects his body to high levels of acceleration. The large human centrifuge where he now sits simulates the conditions of flight in a fighter jet. The pilot can set the magnitude of the G-force and the amount of time it's applied, allowing him to test his own limits. Flight surgeons, technicians and psychologists monitor the pilot's every reaction, recorded on camera. Four times the force of gravity is something your average rollercoaster fan has to tolerate. But withstanding this force for long is a tough challenge even for seasoned professionals. The heart races, the supply of blood to the brain is diminished. Dr. Heiko Welsch explains the physical effects:

DR. HEIKO WELSCH: "The blood pools in the lower part of the body. Look at it this way: At one G, the force of gravity which we are all normally subject to, the heart has to pump the blood 30 centimeters up. But at six G, for example, the heart has to pump it six times as high - 180 centimeters. That means that virtually no blood reaches the brain."

NARRATOR: With exposure to five Gs blood from the body's center is pressed into the legs. At seven G the pulse begins to rush. A trained pilot bears up to nine Gs for a couple of seconds. But the strain is visible, veins are appearing, the skin is distorted due to the immense pressure. Norbert Biehler is breathing even heavier to pump the blood into his head. And, indeed, 10 seconds are enough even for old hand Norbert Biehler.

NORBERT BIEHLER: "I feel okay now. I'm very sweaty but I feel fine. That was a like a very intense, 20-second interval training session."

NARRATOR: G-force, the power of acceleration. Not so much a thrill as a tremendous burden on the body.