Pulling G’s – The Effects of G-Forces on the Human Body
“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 in a stroke of remarkable genius, the mystery of the force of gravity was revealed to 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 went on to mathematically demonstrate) one of the four fundamental forces of nature. Later, the development of Newton’s three laws of motion further expounded on the general behavior of forces and more specifically the way in which the gravitational force behaves. The basis of these laws and our current understanding of Newtonian gravity are integral to many present-day scientific and technological disciplines. The field of aerospace is included in this group.
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.
Gravity is, in its simplest form, 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 may be considered 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.
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 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.
As you may expect, our physiological systems 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. Provided Newton’s First Law of Motion, however, during changes in direction and accelerative forces during flight, the occupant of the aircraft will attempt to remain in motion at a constant direction and velocity, but 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 as it applies to aviation can be found elsewhere, but for the purposes of this post, it is important to note that on Earth, we are always experiencing 1G and that in flight vertical accelerations can increase or decrease this value depending on the direction.
G-forces can act on the human body in different axes and directions. The various axes are annotated x, y, z. Each of these axes can act in 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. See attached diagram – Positive directions are as indicated, with the negative G’s experienced in opposite direction, but same axis.
Now that the stage has been appropriately set, it is finally time for a discussion of how these forces affect human physiology. The most relevant axis to consider is Gz. This is due to both the frequent experience of G’s being 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 and Gy accelerations are less relevant, but are gaining more attention due to newer generation fighter jets with multi-directional thrust engines. For simplicity, the remainder of this post will use the term ‘G’ to apply only to forces in the Gz axis.
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 and lowest intracerebrally (in the cranium) due to gravitational effects. Because our bodies have adapted in a 1G environment, however, we have built in mechanisms to compensate for this discrepancy. Experiencing higher magnitudes of gravity presents unique problems to circulatory homeostasis. At larger +G forces, the above physiologic phenomenon is magnified and a larger discrepancy of blood pressures between cranium and lower extremities 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.
In addition to these circulatory effects, increased +Gz also disrupt respiration through shifting blood to the lung bases causing collapse of alveoli and a general ventilation/perfusion mismatch as air remains in the upper lung where blood is less prevalent. As +Gz forces increase less blood flow combined with poorly oxygenated blood compound the problem of cerebral hypoxia described above. Other common, but less serious, effects of large G force insults are musculoskeletal pain (usually confined to the back and neck) and small punctate bruises called petechiae from burst capillaries in gravity dependent areas of the body. These petechiae are affectionately known as G-measles, or Geasles.
As stated above, the most significant physiologic effects from G forces are related to tissue ischemia (insufficient blood flow), specifically intracerebral ischemia. Because of the high level of sensitivity that the eye’s retina has to hypoxia, symptoms are usually first experienced visually. As the retinal blood pressure decreases below globe pressure (usually 10-21 mm Hg), blood flow begins to cease to the retinal, first affecting perfusion farthest from the optic disc and retinal artery with progression towards central vision. 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 produce a G-induced loss of consciouness (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. This period averages about 15 seconds. Upon regaining cerebral blood flow, the G-LOC victim usually experiences myoclonic convulsions (often called the ‘funky chicken’ – note the youtube video below) and oftentimes full amnesia of the event is experienced.
Many militaries train their aircrew about G-forces and the anti-G straining maneuver (AGSM) with centrifuges. 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. The AGSM technique and recommendations on how to improve G-tolerance are discussed in detail in another post. 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 for a 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. With these protective devices in use, 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
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 has focused almost exclusively on sustained G-forces, but during a crash or ejection sequence transient accelerative forces are acted upon the human body, usually with high risk for traumatic injury. The topic of transient forces of acceleration will be later covered in a separate post.
1. Stukeley, William. Memoirs of Sir Isaac Newton’s Life. 1752.
2. Fundamentals of Aerospace Medicine (Ed. 4) by Jeffrey R. Davis (Editor), Robert Johnson (Editor), Jan Stepanek (Editor), Jennifer A. Fogarty (Editor).