Flying and Health — Page 94, Lesson 145

I want to walk you through the factors that increase tolerance to long-duration G-forces, starting with the baseline. A normal, relaxed individual can usually withstand about +3.5g without serious effects like greying out, other visual disturbances, or unconsciousness. So +3.5g is the threshold where things start to go wrong for an untrained person. Now, several factors can push that tolerance higher. The first is body position. In the normal sitting position, you can get a small benefit by raising your knees and feet and bending your trunk forward from the hips. That helps delay blackout for two reasons. First, it decreases the vertical distance between your lower limbs and your heart — that reduces the hydrostatic column of blood that the acceleration has to work against. Second, it causes the acceleration force to drive blood in your thighs toward the heart instead of away from it. That's a small but real help. Far more effective is lying down. If you lie in the prone position — that's face down — or the supine position — face up — you go a long way toward eliminating the hydrostatic variation altogether. In fact, human beings have been known to tolerate up to 12G in this position before blacking out. That's a massive increase from the +3.5g baseline. The next factor is voluntary manoeuvres. You can raise your G tolerance by about 1 to 1.5g by taking a number of physical actions. The principle underlying the first three of these manoeuvres is to raise the pressure in the abdominal cavity, which maintains the level of the diaphragm and facilitates the return of venous blood back to the heart. Those three manoeuvres are: straining, shouting, and tensing leg muscles. But it's essential that these measures are kept short and intermittent — if you prolong them, they cause fatigue and will actually have a negative effect. That leads us to the Anti-g Straining Manoeuvre, or AGSM. This is now widely accepted as a means of combining the beneficial effects of those manoeuvres I just listed. It's a combination of muscle tensing and the Valsalva manoeuvre — that's a forced exhalation against a closed airway, which raises pressure in the chest and abdomen — performed rhythmically every 3 to 4 seconds. Then we have Anti-g Suits. The object of the suit is to provide more counter-pressure to the lower limbs and help maintain the level of the diaphragm. The operation is automatic — varying pressures are delivered to the suit according to the G level you're experiencing. When you combine the anti-g straining technique with an anti-g suit, the tolerance level can be increased by 1.5 to 2g. However, these techniques are normally confined to military flying. Now, let's contrast that with negative G-forces. Tolerance for negative G is much lower — only -3g, and that level can only be tolerated for a few seconds. Negative G pushes blood toward your head, and the body simply can't handle it for long. Finally, we have short duration positive (impact) G-forces. The body can tolerate at most 25g in the vertical axis and 45g in the fore and aft axis. Forces above these levels will cause serious injury and death. The simplest form of restraint is the lap strap, as worn by airline passengers — that's your baseline protection against impact forces. Let me show you a diagram of the gastro-intestinal tract, which relates to some of the gas-related issues we'll discuss later, but for now, here's the figure.