Tired eh! Physical Cost of AGSM

Fatigue is the cost of correctly performed Anti-G straining manoeuvre (AGSM) to fight against the +Gz forces during air combat. Being an isometric exercise, akin to a 50 or 100 m race or weight lifting, the muscles maintain sustained contraction during AGSM to generate energy anaerobically. Thus time “to fatigue” and of “fatigue recovery” determine the ability of a combat aircrew to withstand high sustained G during air combat maneuvers [1].

Considering that the contracted muscles groups of the abdomen and the limbs are to be sustained during the exposure to the +Gz forces, the blood flow is significantly reduced or stops, particularly during the maximal voluntary contraction (MVC). Hence an assessment of blood lactate levels is considered an objective tool to assess the anaerobic metabolic cost of +Gz tolerance during such isometric contractions [2, 3].

An interesting fact to remember is that AGSM, being a sustained isometric muscle contraction, %MVC can be estimated from the AGSM required as per exposure to peak G. Burton et al. suggested that approximately 25% MVC is required for an increase in G levels beyond +5Gz, meaning thereby that at 7G, about 50% MVC is required [2], which increases to almost 100% MVC at +9Gz.

Another interesting fact to remember is recovery time. If considered “as a percentage of recovery of contraction time to fatigue”, time to recover is shortest at the highest %MVC [2]. Pilots flying high sustained G maneuvers (+ 8 or 9Gz) are advised to remember that 100% MVC, an effort required at those high peaks of G, can not be maintained for “longer than few seconds” – thus “a pilot capable of +9Gz early in an air-to-air engagement is capable of only 8G after 30 s and 7G after 45 s of sustained high-G effort” [2]. Therefore situations practiced latter in a combat sortie render him susceptible to GLOC.

Epperson et al. [4] and Tesch et al. [5] have proven that it is the resistance training that helps withstand G forces better, with aerobic training playing almost no role. In fact, resistance training helps improve both muscle strength and anaerobic capacity, giving advantage to combat pilots to perform AGSM [4]. The weight training requires the combat aircrew to train muscles for strength by repeated high intensity contractions. This helps him during combat to stay conscious with lower MVC, thus sustaining the contraction longer (delayed fatigue) and with faster recovery after the maneuver (early recovery from fatigue) [6, 7]. Hence dedicated weight training programmes, like the U.S. “Physical Fitness Program to enhance Aircrew Tolerance” are mandated to be followed by combat pilots for improving their anaerobic ability to perform AGSM optimally during combat manoeuevrs without getting unduly tired, with early recovery from fatigue [8, 9, 10].

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1. Harris RC, Sahlin K, Hultman E. Phosphagen and lactate contents of m. quadriceps femoris of man after exercise. J. Appl. Physiol. 1977; 43:852-7

2. Burton RR, Whinnery JE, Forster EM. Anaerobic energetics of the simulated aerial combat maneuver (SACM). Aviat. Space Environ. Med. 1987; 58:761-7

3. Burton RR. A conceptual model for predicting pilot group G tolerance for tactical fighter aircraft. Aviat. Space Environ. Med. 1986; 57:733-44.

4. Epperson WL, Burton RR, Bernauer EM. The influence of differential physical conditioning regimens on simulated aerial combat maneuvering tolerance. Aviat. Space Environ. Med. 1982; 53:1091-7

5. Tesch PA, Hjort H, Balldin UI. Effects of strength training on G tolerance. Aviat. Space Environ. Med. 1983; 54:591- 5

6. Astrand P, Rodahl K. Textbook of work physiology. New York: McGraw-Hill, I970

7. Funderburk CC, Hipskind SF, Welton RC, Lind AR. Development of and recovery from fatigue induced by static effort at various tensions. J. App1. Physiol. 1974; 37:392-6

8. Physical Fitness Program to enhance Aircrew Tolerance

9. Epperson WL, Burton RR, Bernauer EM. The effectiveness of specific weight training regimens on simulated aerial combat maneuvering G tolerance. Aviat. Space Environ. Med. 1985; 56:534- 9

10. Burton RR, Whinnery JE. Biodynamics: Sustained Acceleration. In Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002: 122-153

Acknowledgement  Image Courtesy Wikimedia Commons