Pulling G's - The Effects of G-Forces on the Human Body - Go Flight Medicine (2024)

Pulling G's - The Effects of G-Forces on the Human Body - Go Flight Medicine (1)

"After dinner, the weather being warm, we went into the garden and drank thea, under the shade of some apple trees...he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. It was occasion'd by the fall of an apple, as he sat in contemplative mood. Why should that apple always descend perpendicularly to the ground, thought he to himself..."1

...and so it was that the mysterious force of gravity was revealed to Isaac Newton and the human mind for the first time.From observing and contemplating that everyday occurrence of a ripened apple falling from a tree, the young Isaac Newton began to understand (and later to mathematically demonstrate) one of the four fundamental forces of nature. The development of Newton's three laws of motion further explained the general behavior of forces and more specifically the way in which gravity behaves. The basis of these laws and our current understanding of Newtonian gravity are integral to many technologies today. The field of aerospace is no exception.The fact that heavier-than-air aircraft can travel with intention to and from various points on earth, suspended in the Earth's atmosphere demonstrates our current understanding and manipulation of gravitational forces. The multitude of satellites currently orbiting our planet provide a constant reminder of the accuracy and applicability of Newton's Laws. Let us take a closer look at exactly what the force of gravity is and how it affects the human body, both on the ground and during flight.

THE FORCE OF GRAVITY

Pulling G's - The Effects of G-Forces on the Human Body - Go Flight Medicine (2)

Gravity is a force of acceleration. This means that it acts on objects to change their rates of velocity. All objects exert a gravitational force over one another and this force is unique because it can act over very large distances. On and near the planet Earth, the gravitational force of our planet is so great due to Earth's large mass that all other gravitational forces are essentially negligible. This force has been calculated to be approximately 9.82 m/s2, and is often called 'g', as you likely recall from high school physics class. It is important to note that in accordance with Newton's Second Law of Motion, F = ma, gravitational force is intimately tied to an object's mass and varies in direct proportion to this value. For example, gravity on the moon (a much less massive object than earth) is only 1.62 m/s2. Gravitational force is the reason why objects drop to the surface of the earth, and is also the force that an aircraft's airfoils must contend with to create lift. When the lift of an aircraft is greater than the force of gravity, controlled flight becomes possible as the Wright Brothers demonstrated to the world in 1903.

G-FORCES LINGO & NOTATION

Pulling G's - The Effects of G-Forces on the Human Body - Go Flight Medicine (3)

The human body, much like the rest of life on earth, has adapted to a terrestrial life in which we are always exposed to the gravitational force of Earth (g). For simplicity, let's call this standard gravitational force of earth (9.82 m/s2) 1G. During powered flight, however, it is possible to experience both more or less than this 1G constant. Magnitudes of this value are expressed numerically and therefore 'pulling 3 G's' is equivalent to experiencing 3 times the normal gravitational force. A person who weighs 150 lbs at 1 G will actually weigh 450 lbs at 3 G's - no kidding!As you may expect, our body's physiology will both be affected by and respond to this novel variability in G forces. When an aircraft is traveling towards the earth and exerting thrust in that path of motion, it is accelerating at that rate plus 1G (9.82 m/s2). When the same aircraft is accelerating away from the Earth's surface, the sum of accelerative forces will be the difference from the thrust and 1G. Newton's First Law of Motion explains why the occupant of the aircraft will attempt to remain in motion at a constant direction and velocity during changes in direction and accelerative forces during flight. Though they will be prevented from doing so by the seat restraints. These safety restraints will exert an equal (almost) and opposite force on the occupant's body as Newton's Third Law of Motion predicts. A detailed discussion on the physics behind G-forces and the gas laws of aerospace physiology can be found elsewhere. But it is important to note that on Earth we are always under 1G of force, but that in flight vertical accelerations increase or decrease this value depending on the direction.

Pulling G's - The Effects of G-Forces on the Human Body - Go Flight Medicine (4)

G-forces act on the human body in different axes (or directions). These are usually described as the x, y, z axes. Each has a positive (+) or negative (-) direction. When standing upright, the force of gravity acts along the longitudinal or Gz axis parallel to the spinal cord. +Gz acts downward in the same direction as Earth's gravity. Negative Gz's act in a direction opposite to gravity. Common notation identifies the axis acting through the front and back of the body as Gx and the axis acting laterally as Gy. These different axes correspond to yaw (Gz), roll (Gx), and pitch (Gy) of the aircraft.So, how do these forces affect the body's ability to function? The most relevant axis to consider is Gz. This is due to both the frequent experience of G's transmitted along this axis in flight and also it's significantly greater physiologic effects. Acceleration in the Gx axis is more commonly experienced by astronauts during shuttle launch. Gy accelerations are less relevant, but are gaining more attention due to newer generation fighter jets with multi-directional thrust engines like the F-22 and F-35. For simplicity, the term 'G' is often applied only to forces in the Gz axis.

HUMAN PHYSIOLOGY IN RESPONSE TO GRAVITY

The circulatory system is most significantly affected by increased G-forces during flight. Even at 1G, blood pressure in an upright person is highest in the lower extremities (the legs) and lowest intracerebrally (in the cranium) due to gravity. Because our bodies have adapted in a 1G environment, we have built in mechanisms to compensate for this discrepancy. Experiencing higher magnitudes of gravity presents unique problems to circulatory regulation. At larger +G forces, the above physiologic phenomenon is magnified and a larger discrepancy of blood pressures between cranium and the lower body occurs. At some point, intracranial perfusion cannot be maintained and significant cerebral hypoxia (no blood = no oxygen) follows. The end result is unconsciousness. In the world of aviation this is called a G-LOC, aka G-induced loss of consciousness, and remains a significant cause of loss of aircraft and pilot in both military fighter aviation and civilian acrobatic aviation. Throughout the 1990's, for example, the USAF lost approximately one aircraft per year due to G-LOC.

Pulling G's - The Effects of G-Forces on the Human Body - Go Flight Medicine (5)

In addition to circulatory effects, increased +Gz disrupts respiration by shifting blood to the lung bases, which collapses the small sacs of air (called alveoli) and creates a general ventilation/perfusion mismatch as air remains in the upper lung where there is little blood flow. As +Gz forces increase less blood flow combined with poorly oxygenated blood compound the cerebral (brain) hypoxia described above. Other less serious effects of large G forces are musculoskeletal pain (usually confined to the back and neck) and small punctate bruises called petechiae from overwhelmed capillaries that rupture. This usually occurs in gravity-dependent areas of the body and are affectionately known as G-measles, or Geasles.

THE G-LOC

As stated above, the most significant physiologic effect from G-forces are related to tissue ischemia (insufficient blood flow), specifically intracerebral (brain) ischemia. Because of the high sensitivity that the eye's retina has to hypoxia, symptoms are usually first experienced visually. As the retinal blood pressure decreases below the eye's globe pressure (usually 10-21 mm Hg), blood flow begins to cease to the light sensing receptors in the retina, first affecting perfusion (and therefore vision) farthest from the optic disc with progression of central vision loss. Therefore visual symptoms in response to increased G's usually progresses from increasing tunnel vision to 'graying out' to full 'black out' - a phenomenon in which a person retains consciousness, but full retinal ischemia causes absolute blindness.The final submission to G-forces produces a G-induced loss of consciousness (aka G-LOC), which is usually divided into a relative and absolute component. Absolute incapacitation is the period of time when the aircrew member is physically unconscious and averages about 12 seconds. Relative incapacitation is the period in which the consciousness has been regained, but the person is confused and remains unable to perform simple tasks (obviously cannot fly an aircraft). This period averages about 15 seconds. Upon regaining cerebral blood flow, the G-LOC victim usually experiences myoclonic convulsions (a seizure-like episode often called the 'funky chicken') and oftentimes full amnesia of the event is experienced.Many militaries train their aircrew about G-forces and the anti-G straining maneuver (AGSM) using a centrifuge. See the youtube video below for an anthology of centrifuge-induced G-LOC's. The AGSM has two components - isometric muscle contraction and a particular respiratory sequence, which attempts to maximize cerebral blood flow and cardiac output, while maintaining an adequate level of oxygenation. A separate post written for pilots describing the AGSM technique with recommendations on how to improve G-tolerance can be found here. When the AGSM is combined with an inflatable G-suit, one's tolerance to high G's increases markedly. Studies performed in the 1940's and 1950's by the U.S. DoD found that without any strain or G-suit, average G's prior to G-LOC was dependent on the rate of G onset. G-LOC occurred at an average of 5.4 G's at 1 G/sec rate and 4.5 G's at 2 G/sec rate. An effective AGSM is thought to increase one's G tolerance by 3 full G's. Most legacy G-suits like the CSU 13B/P used by the USAF and CSU 15 A/P used by the USN/USMC can increase G tolerance for an additional 1 to 1.5 G's. Newer G-suits such as the ATAGS provides even greater protection. When using this important life support equipment, a modern fighter pilot can be expected to remain conscious and continue to fly tactically at up to +9Gz. For a demonstration of G-LOC and AGSM (though in this case, inadequate AGSM), see the youtube video below.

PILOTS AGSM TRAINING IN THE CENTRIFUGE WITH G-LOC

TRANSIENT FORCES

It should also be noted that any discussion of gravitational and acceleration forces in the field of aviation would not be complete without mention of transient forces. The above discussion focused almost exclusively on sustained G-forces, but during a crash or ejection sequence transient accelerative forces act upon the human body, usually with high risk for blunt traumatic injury. The topic of transient forces of acceleration is in a separate post.

REFERENCES

  1. Stukeley, William. Memoirs of Sir Isaac Newton's Life. 1752.
  2. Fundamentals of Aerospace Medicine (4 Ed.)– April 16, 2008 by Jeffrey R. Davis MD MS, Robert Johnson MD MPH MBA, Jan Stepanek MD MPH, Jennifer A. Fogarty PhD
Pulling G's - The Effects of G-Forces on the Human Body - Go Flight Medicine (2024)

FAQs

Pulling G's - The Effects of G-Forces on the Human Body - Go Flight Medicine? ›

At larger +G forces, the above physiologic phenomenon is magnified and a larger discrepancy of blood pressures between cranium and the lower body occurs. At some point, intracranial perfusion cannot be maintained and significant cerebral hypoxia (no blood = no oxygen) follows. The end result is unconsciousness.

What does pulling GS do to your body? ›

When the body experiences too much G-force, a person might become lightheaded or even pass out for several seconds—also referred to as "G-LOC" or gravity-induced loss of consciousness.

What does it mean to pull G's? ›

G-forces refers to the force acting on an object that resists change to motion and is related to the normal pull of gravity on the body. Thus, 1 G equals the force one would feel on a constant trajectory without any change in speed; 2 Gs equals the feeling of double the force of gravity, and so on.

What does pulling GS feel like? ›

Modern fighters like the F-16 and F-35 pull 9G's, which translates to over 2,000 pounds on my body. Under 9G's, the world appears to shrink until it looks like you're viewing it through a toilet paper roll. Blood is being pulled out of your head towards your legs and arms, resulting in the loss of peripheral vision.

What are the effects of positive g-force on the human body? ›

Positive, or "upward" g-force, drives blood downward to the feet of a seated or standing person (more naturally, the feet and body may be seen as being driven by the upward force of the floor and seat, upward around the blood). Resistance to positive g-force varies.

What are the long term effects of pulling GS? ›

Is being subjected to G force repeatedly as a pilot, bad for you long term? Yes. Human bodies are designed to operate in Earth gravity, so rapid acceleration causes some real problems, such as damage to joints, concussions from the brain bouncing inside the skull, and eye damage.

What does G-force do to your brain? ›

For example, as significant downward g-forces are applied to the human body, blood is forced from the head to the lower extremities, quickly overwhelming the heart's capacity to pump it to the brain. On the other hand, extreme negative or upward g-forces force blood from the extremities up toward the head.

What is the most G-force a human has survived? ›

Indycar driver Kenny Bräck (SWE) survived a split-second deceleration of 214 g during a 220‑mph (354‑km/h) crash on lap 188 of the Chevy 500 at Texas Motor Speedway, USA, on 12 October 2003. This is according to data registered in Bräck's in-car “crash violence recording system”.

What does 1G feel like? ›

1G is the acceleration we feel due to the force of gravity. It's what keeps our feet firmly planted on the ground. Gravity is measured in metres per second squared, or m/s2. On Earth, the acceleration of gravity generally has a value of 9.806 m/s2 or 32.1740 f/s2.

What are the effects of G lock? ›

G-LOC typically starts with fixation of the gaze. Then the eyes roll up and to one side, and complete muscular collapse follows. There is a period of absolute incapacitation, and during the latter stages of this period the pilot may experience dreams and have convulsions.

What happens if you pull too many GS? ›

It depends on the direction. An upwards acceleration of about 5g is enough to overwhelm the ability of your heart to pump blood to your brain. This causes oxygen starvation and you will black out within a few seconds. Downward, or negative, g-force is even worse.

How do you breathe when pulling G's? ›

A pilot should take a big deep breath, holding the air in their glottis, which is located in the throat, before the G-onset or acceleration occurs and then they begin the "air exchange."

How much G-force can a human take? ›

Most of us can withstand up to 4-6G. Fighter pilots can manage up to about 9G for a second or two. But sustained G-forces of even 6G would be fatal. Astronauts endure around 3G on lift-off, one G of which is Earth's own pull.

What does pulling G's do to your body? ›

There are many standard flight maneuvers that require a pilot to pull multiple g's such as a 60-degree level banked turn. Anything that inhibits your heart's ability to supply blood and oxygen to your eyes and brain may result in temporary loss of vision or consciousness.

How does g-force make you feel? ›

You know that tingling feeling you get in your belly when you go down in a fast lift? We're sure you've felt it at some point – it's the g-forces in your body! It's the same when you ride a roller coaster or go down a ramp in your car.

What are the symptoms of high g-force? ›

Effects of g-forces

For higher intensity or longer duration, this can manifest progressively as: Tunnel vision – loss of peripheral vision, retaining only the center vision. Greyout – a loss of color vision. Blackout – a complete loss of vision but retaining consciousness.

How many Gs can an average person pull? ›

Most of us can withstand up to 4-6G. Fighter pilots can manage up to about 9G for a second or two. But sustained G-forces of even 6G would be fatal. Astronauts endure around 3G on lift-off, one G of which is Earth's own pull.

Does gravitational pull affect humans? ›

Because the force of gravity pulls the mass of our bodies toward earth, there is more loading on these discs when we are sitting or standing. That sends signals to the brain, allowing it to relay new signals to the spinal cord and muscles to keep us in an upright position so we don't fall.

What to do when pulling GS? ›

As far as I can tell there are basically three ways high-Gs are counteracted:
  1. Training and techniques (bracing with muscles to force blood towards the brain)
  2. G-suits which compress blood vessels to force blood towards the brain.
  3. Not pulling negative-G manoeuvres because those are way worse for your brain.
Jul 19, 2023

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