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USAF Standard CSU-13B/P G-Suit
USAF Standard CSU-13B/P G-Suit

This installment will review the effect of G’s on the human body, the Anti-G Straining Maneuver (AGSM) and how you as the modern fighter pilot can decrease the likelihood of being the victim of a G-LOC (G-induced loss of consciousness).

With the introduction of the new full-coverage G-suit, we expect GLOC occurrences and loss of aircraft/pilot due to G’s to decrease significantly.

GLOC remains a significant threat to your life and the USAF mission.  The F-16 is uniquely at risk. The base I am at experienced a class-E GLOC over a two year period.  Fortunately, in this case, the pilot recovered consciousness and landed without incident.  The combined Class A and E G-LOC mishap rate in USAF F-16 for Fiscal Years 1993-2009 is 1.32 per 100,000 flying hours.



The Risk of Elevated G-Forces Reviewed:

Higher G’s make blood heavier and displace blood to gravity dependent areas (away from your nugget)

The brain is highly sensitive to cellular hypoxia (lack of oxygen or blood) and rapid loss of function follows, i.e. G-LOC.

After cessation of cranium blood flow, reserve time of 4-6 seconds exists before total loss of function.

  • Avoid maneuvers that sustain high G’s for longer than 4-6 seconds.  When required, the intensity of the AGSM must be maintained throughout the maneuver.  Consider and combat inevitable muscular fatigue.

Your body will reflexively make adjustments in heart output and blood pressure to increase blood flow to brain, but this takes 6-9 seconds for this reflex to initiate.  By this time, unconsciousness has likely occurred.

Negative G’s provoke the opposite response, with heart output and blood pressure decreasing to eliminate the excessive blood in the brain.  This reflex happens much quicker (2-4 seconds)

Rate of G-onset is just as important as absolute number of G’s.  Slower G-onset allow for increased cardiac compensation reflexes. Studies performed in the 1940?s and 1950?s by the U.S. DoD found that without any strain or G-suit, 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.

  • Focus on a high quality and more intense AGSM when performing maneuvers that may produce rapid G-onset.

Negative G’s followed by Positive G’s will place you at VERY HIGH RISK for GLOC as the reflex causing lower heart output and blood pressure will remain intact and will significantly diminish your G-tolerance.  This is called the ‘Push-Pull Effect’.

  • Avoid maneuvers that load high positive G’s following a negative G maneuver.  If required, be aware that an increased AGSM is necessary. 


G Tolerance & Protection:

G-tolerance is influenced by several variables:  heart-to-brain distance, muscle strength, rate of G onset, anti-G suit effectiveness, and positioning.

Studies demonstrate no differences in G-tolerance between males and females.

  • Decreased heart-to-brain distance
    • A shorter person theoretically will have a higher G-tolerance than a taller person, all things being equal.  (All things are never equal.)
    • The F-16 seat is reclined 30 degrees, shortening this distance, thus increasing G-tolerance.
  • G-duration
    • See above, increased risk of GLOC with sustained G’s 4-6+ seconds
  • The G-suit
    • The old G-suit thought to provide 1 to 1.5 G protection.
    • The new full body Advanced Tactical Anti-G Suit (ATAGS) reports increases G-tolerance by 0.5-1 G.
    • Pilot reported increases average 2.4 G’s of additional protection and 50% reduction of G-related fatigue
  • Findings from G-suit study comparison:  “Difficulties some pilots have reported with these “full-coverage” (FC) G-suits include:  perception of increased thermal burden, comfort, mobility, and cockpit functionality (see Advanced Technology Anti-Gravity Suit IOT&E Report, 2001 and McCarthy et al., 1994).  While some pilots reported these concerns, the data showed that core body temperatures were not significantly elevated in a small sample of pilots flying full coverage suit in RAF Hawk aircraft in a hot (30 deg C) climate, yet the G-protection afforded was better, with less fatigue resulting from a decreased muscle strain requirement (Sowood, et al., 1994 and Tong et al, 1998).”



The Anti-G Straining Maneuver (AGSM)

First developed during WWII (what modern military product, strategy, or weapon wasn’t?), the AGSM increases aortic blood pressure, maintaining brain perfusion and pilot consciousness.

Well trained, effective AGSM can increase G-tolerance by approximately 3 G’s

Two components

  1. Breathing:  Rapid (< 1 sec) exhalation/inspiration cycles every 3-4 seconds.  This maintains oxygen content and decreases carbon dioxide in blood, while also relieving increased pressure of chest, and allowing the heart to refill with blood.
  2. Isometric Contraction:  Flexion of skeletal muscles of legs and abdomen.  This step increases pressure in chest and displaces blood away from these contracted muscles into the arms, chest, and brain.

The Anti-G Straining Maneuver MUST be an instinctive habit requiring no conscious thought by professional fighter pilots



G-LOC Prevention:

New Technologies:  The new full coverage G-suits may prevent more GLOC events than any other preventive measure.  If you don’t have the new G-suit, get it!  Once you begin using the new G-suit, I recommend you NEVER return to the old G-suit.  Your habits will change, AGSM will deteriorate and G-tolerance may diminish.  This may increase your risk for GLOC.

Confirm that comfort zippers are closed, the correct G-suit is worn and is fit tightly to your body.  If the above do not apply, you will lack full protection.

  • Muscular endurance:  The AGSM is intrinsically fatiguing and is an anaerobic activity.  Anaerobic capacity and muscular strength can be increased with training.  Focus on the larger muscles of gluts, quads and hamstrings.  Conversely, full recovery of high-intensity weight training of these muscle groups may diminish your AGSM for 24-48 hours.  Avoid these types of workouts prior to high-G sorties
  • NO SMOKING:  One of the by-products of cigarette and cigar smoking is carbon monoxide.  This will diminish your blood’s oxygen-carrying capacity.  Having less overall available oxygen will also diminish your G-tolerance.
  • AGSM Technique:  Never hurts to continue to review, practice, and brief to what a good, effective AGSM looks like.  You are required to complete one annual AGSM HUD review, which may not be adequate.  Make it a habit!
  • Nutrition:  Just like preparing for a high intensity sport, ensuring that your glycogen (carbohydrate stores) are maximized should ensure that you have the available energy to delay fatigue for the longest time possible.


Full post on Supplements in Aircrew can be found here.

  • Creatine:  Although I do not think it has been directly studied, theoretically Creatine could assist with maximinzing AGSM effectiveness.  Creatine has demonstrate effectiveness in “Improving the athletic performance of young, healthy people during brief, high-intensity exercise such as sprinting.”1  The AGSM can be viewed as a brief, high-intensity exercise.
  • Other supplements:  Many supplements that contain nitric oxide or other chemicals will promote vasodilation (enlargement of blood vessels).  The advertised action is to increase blood flow to large skeletal muscle.  This is the OPPOSITE of what you want during an AGSM.  This action may BLOCK your AGSM.  In general, I do not recommend exercise supplements.  They are generally a waste of money, are not well studied, and have only modest if any proven effectiveness.



1. Natural Medicines Comprehensive Database, Consumer Edition. Accessed 27 Jan 2014.

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


  1. When we read about techniques to withstand for high-g maneuvers, most of the articles are about positive g’s. When we see this video showing Sean Tucker talking through his airshow sequence (https://youtu.be/CUlw8ZJdC94), we can see him doing several negative g’s maneuvers. He talks about his face getting deformed and about how much it hurts. He says: this hurts a lot, but it’s my job. How can he practice several times a day, flying high negative g’s maneuvers without injuring his retina or getting a stroke? Is there any technique for that?

    • Rafael, thanks for sharing that video! I loved the fully-narrated demonstration of a variety of acrobatic aviation maneuvers. Sean Tucker is a legend and it’s evident from the video that he absolutely loves what he does.

      Regarding your question, it is my understanding that there really is no ‘maneuver’ that a pilot can do to increase physiologic tolerance to negative G’s. This is why it is so uncomfortable to sustain negative G’s and experience the intravascular congestion that inevitably follows. Some aerobatic pilots claim that complete relaxation of the body during negative G’s can help. This makes sense physiologically as a pilot or anyone experiencing negative G’s should avoid simultaneously doing anything that simulates an AGSM (i.e. increases intrathoracic pressure or vasoconstriction of the lower portions of the body).

      An article written by the USAF at Wright-Patterson AFB in 1989 suggests that lower body negative pressure may be an effective counter measure for high sustained negative G’s.

      Another article published in the Journal of Aviation Medicine from 1950 determined physiologic findings of study participants at various negative-G levels. This paper quotes participants having discomfort at -1 Gz, intense facial congestion and reddening of vision at -2 to -3 Gz, and that some could tolerate -5 Gz without incident. A few aerobatic pilots have claimed to sustain -10 Gz or more.

      Last thing on Sean Tucker…I did a quick read of his bio and he apparently has bailed out of aerobatic aircraft twice in his career!

  2. See below to watch the video of Sean Tucker talking through negative-G’s as referenced by Rafael:

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