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The undergraduate orthodontic courses at Toronto and Liverpool are compared. Each course comprises more than 250 hours of teaching and within that, more than 100 hours involve clinical tuition. Both courses contain laboratory modules for the teaching of removable and fixed appliance technique. Undergraduates treat their own patients with both simple and complex appliances, within their clinical training period which extends over at least 2 years. Liverpool undergraduates treat more patients per student than their counterparts in Toronto (P

Straining gradual G onset rate (GOR) tolerances are considered by physiologists as a measure of G-level tolerance. Using recently developed G-level and G-duration mathematical models, it was found that straining GOR tolerances may well be a measure of tolerance to G-duration. G-duration tolerance was determined to be limited with the onset of fatigue and not cardiovascular insufficiency. G-level tolerances that were predicted using a mathematical model were higher than determined using straining GOR tolerance measurements of subjects on a centrifuge. Also the G-duration tolerance mathematical model showed that those centrifuge subjects had not expended all of their "energy reserve" during their sustained G exposure most probably because of the onset of fatigue. Even if they were able to use all of their potential energy reserve, their G-duration tolerance would not have allowed them to reach the maximum G-level predicted with the G-level tolerance model. It is therefore concluded that the straining GOR tolerance profile, with G onset rates of 0.1G/s, is not a measure of G-level tolerance, as has been assumed, but is a measure of G-duration tolerance. These findings have significant safety implications world-wide since this straining GOR profile is commonly used as a G-level tolerance fighter-pilot-selection determination; i.e. pilot selection standards for G-level tolerance are not a measure of G-level tolerance. In testing equipment design changes, the proper G tolerance profiles must be used to correctly measure its impact on G tolerance.

Mathematical models that predict straining G-level tolerances of subjects reclined55 degrees are described. Straining G-level tolerances are defined as those G-levels of exposures that require the use of the anti-G straining maneuver (AGSM). A subject reclined55 degrees has a reduced inspiratory volume that limits the maximum intrathoracic pressure that can be developed that is the basis for the AGSM. Therefore the inability to perform a maximum AGSM reduces the maximum G-level that can be tolerated. G-level tolerances of subjects at seat-back angles between 75 degrees and 55 degrees are modeled. The use of positive pressure breathing (PBG) to increase G-level tolerance is considered. Maximum G-level tolerances are dependent upon anti-G suit pressures (psi) used and are predicted as follows: 55 degrees = 11.7 and 12.7 G with 7 and 10 psi; 65 dergrees = 12.1 and 12.8 G with 6 and 9 psi; and 75 degrees = 13.4 and 14.2 G with 5 and 8 psi anti-G suit pressure.

Mathematical models that predict fatigue-based G-duration tolerances for relaxed and straining subjects are developed and validated using published data. These models are based on regression analysis calculations using published G-duration tolerance data of relaxed subjects exposed to 3-5 G and subjects exposed to 6-9 G using an anti-G suit and performing the anti-G straining maneuver. These G-duration models are derived from published G-level tolerance models based on intravascular hydrostatic pressures and physiologic responses to maximum voluntary contractions (MVC%). Included in the validation of these models are the baroreceptor and muscle contraction cardiovascular reflexes that support arterial BP. A basic energy pool that supports a G-duration of 140 s for G exposures5 G is theorized. Because of the long duration of sustained G exposures in these models, the physiologic dynamics involved in predicting straining G-duration tolerances, are identified and validated using different time periods, i.e., Phases I and II. These models, based on sustained G exposures to a constant G level are also applicable to exposures of variable G levels known as simulated aerial combat maneuver (SACM) G-profile tolerances. G-duration tolerances9 G are predicted using these models for subjects using reclined-seat backs and positive pressure breathing.

The mathematical models developed in this article predict the following human G-level tolerances: 1) rapid onset relaxed (ROR); 2) gradual onset relaxed (GOR); and, 3) straining-rapid onset. Included in the model are specific functions of: 1) anti-G suit; 2) positive pressure breathing (PBG); 3) baroreceptor reflex; 4) handgrip reflex; 5) anti-G straining maneuver (AGSM) increasing intrathoracic pressures (Pi); 6) leg elevation; and, 7) reclining seatback anglesor = 55 degrees. These functions are based on sound physiologic principles. Also discussed in the development of this model, but not included in the models, were: 1) isometric muscle contraction reflex; 2) Qigong (Q-G) maneuver; and, 3) straining GOR tolerances. The straining GOR tolerance profile was calculated to be a measure of G-duration tolerance and not G-level tolerance. A maximum P of 125 mm Hg from the AGSM was used in these models that could be augmented with PBG to 185 mm Hg. G-level tolerance predictions using this model were validated with published data.

The use of a personal centrifuge as a countermeasure to weightlessness during space flight is presented. The authors argue that it is more effective than exercise techniques now used. Recent research in intermittent gravity exposure is reviewed and future opportunities and requirements for research are examined.

The relationships of anaerobic power, blood lactate levels, and selected anthropometric measurements to +Gz tolerance were examined in 10 adult males. Upper and lower body anaerobic indices were determined by Wingate anaerobic tests (WT). Acceleration tolerance was measured as duration time for a simulated aerial combat maneuver (SACM) centrifuge profile with alternating 4.5 and 7 +Gz 15-s plateaus until exhaustion. Group mean (+/- SD) for SACM duration was 250 +/- 97 s. Peak blood lactate concentration was 4.9 +/- 1.5 mmol/L and overall rating of perceived exertion was 7.4 +/- 2.1 using the Borg Category-Ratio Scale. Group mean for WT lower body 30-s mean power (MP, index of anaerobic performance) was 620 +/- 128 W; peak power (PP, highest 5-s power output) was 851 +/- 169 W. Upper body MP and PP were 380 +/- 68 W and 497 +/- 81 W, respectively. SACM duration time was positively correlated (p0.05) with lower body MP and PP, upper body PP, various body circumferences, weight, fat-free body weight, and height; but did not correlate with WT power outputs relative to body weight, or with other SACM variables. Results suggest that anaerobic power is an important physiologic component in SACM tolerance.

This study was conducted to determine if preinflation of a standard five-bladder anti-g suit 10 minutes before exposure to a centrifuge simulation of a Space Shuttle reentry would provide significantly better protection against orthostasis than the standard symptomatic inflation regimen. This study differed significantly from prior studies: The rate of g onset was slower, peak g was lower, duration of exposure was longer, and the subjects were dehydrated to mimic conditions observed in astronauts immediately postflight. Preinflation demonstrated physiological advantages as determined by arterial blood pressure and heart rate changes in seven healthy male, experienced centrifuge subjects.

Clearly, physiologic adaptation to terrestrial life for all animals is assured only by frequent encounters with gravity. Indeed, upon exposure to weightlessness in space flight, losses of physiologic functions quickly begin. Some physiologic parameters change more rapidly than others, but the deconditioning process starts rapidly. The rates of functional losses for all affected parameters are interesting in that they appear to approach a limit; i.e., losses of these functions may not continue until indefinitely. The regulation of this functional asymptotic response to space is not known, but probably based on functional requirements of the body to life itself and perhaps genetic expression. The latter controlling mechanism (DNA) functions only on aquatic (weightless) animals on Earth--land animals must stimulate these physiologic functions as they relate to gravity on a regular frequent basis. This loss of regulation upon entering the weightless environment is fascinating since land-based animals including the humans have evolved from millions (perhaps billions) of years of terrestrially adapted ancestors. One would expect some DNA involvement in the regulation of its physiology, but it appears to be absent. Therefore, if the functional debilitation of space is to be denied, we must begin to understand the adaptation process of the sole basis for the control of our physiologic processes on land; i.e., how gravity regulates our biologic functions. To learn about this regulatory mechanism, some inquiry into how aquatic animals first adapted to living on land might be helpful.

The Armstrong Laboratory and its predecessors have conducted responsible animal research in support of USAF operations for nearly three decades. The use of animal models, is essential in research that requires a complex living system, but would be too hazardous to humans. The Laboratory also has aggressively pursued alternatives to the use of animals and improved methods in conducting animal research. Thus far, fewer animals are used currently in some areas of research. Since the human is the focus for Armstrong Laboratory research, human test subjects are used as frequently as possible. As improved noninvasive physiologic monitoring methods become available, humans will be used more extensively.

Pyridostigmine Bromide (PB) is used as a pre-exposure antidote for the prevention of potentially lethal effects of certain chemical warfare nerve agents by reversibly inhibiting acetylcholinesterase (AChE). This study was designed to determine whether PB has any deleterious effects on acceleration tolerance (+Gz) or performance. Double-blind placebo trials were conducted to evaluate the effects of PB (90 mg) per day on +Gz tolerances and performance. Three types of exposures were used: 1) gradual onset rate (GOR) exposures of 0.1 G/s; 2) a series of rapid onset rate (ROR) exposures of 6.0 G/s; and 3) a simulated aerial combat maneuver (SACM) of 4.5 to 9.0 +Gz. Performance tasks included the Unified Tri-Service Cognitive Performance Assessment Battery (UTC-PAB). The subjects were not able to correlate their symptoms with PB, placebo, or the acceleration exposure itself. Plasma PB individual levels ranged between 6 and 31 ng/ml and AChE levels of inhibition had a range of 12 to 45%. There were no significant effects on +Gz tolerance or performance related to PB. Based on the results of this study, PB does not significantly alter +Gz tolerance or performance. Therefore, we do not expect aircrew taking prophylactic doses of PB to be adversely affected during aerial combat operations.

This article is a contribution to the workshop on "Operational Requirements in the Prevention of G-Induced Loss of Consciousness (G-LOC) in High Performance Aircraft"; it focuses on the "operational" side of the requirements to prevent G-LOC. There are two types of requirements for prevention of G-LOC; a) pre-G-LOC detection devices that monitor physiologic changes of the pilot before G-LOC occurs, and b) the more generic personal G protection systems that increase G tolerance. Recently there have been major advances in G-protection research and development (RD) systems (soon to become operational) that significantly improve G-level and G-duration tolerances. We do not know the extent of the impact of these new systems on G-LOC, but it could be substantial. These near-term operational anti-G systems are: 1. Positive pressure breathing (PPB) systems assisted by chest counterpressure that are activated by an increase in G levels; i.e., the higher G level, the greater the pressures applied to the aircrew member (Morgan et al., 1992). This PPB system is known as PBG. PBG has been flight tested with the standard operational anti-G suit. This test program known as Combat Edge significantly reduces pilot fatigue and extends G-duration tolerance. In limited operational testing of F-16 and F-15 aircraft, PBG has received substantial pilot acceptance. 2. An improved anti-G suit that provides uniform pressure of the lower body that includes the operational anti-G suit abdominal bladder. This new anti-G suit concept increases both G-level and G-duration tolerances (Krutz et al., 1990; Morgan et al., 1992). Flight tested as the Advanced Tactical Anti-G Suit (ATAGS), it is comfortable and preferred by pilots over the standard operational anti-G suit. 3. The combination of Combat Edge and ATAGS, the most advanced anti-G system in the final stages of development (Morgan et al., 1992). This anti-G equipment has been tested in the laboratory on the centrifuge. Experimental subjects are able to tolerate 8 to 9G "relaxed" or with a minimal anti-G straining maneuver (AGSM) (Morgan, 1992). Accleration scientists believe that once these systems become operational on high performance aircraft that the incidence of G-LOC will be significantly reduced, particularly G-LOC associated with fatigue.

A short-arm centrifuge (SAC) of 5-6 ft (1.5-1.8 m) radius may be useful in space to measure tolerances to acceleration (G) and to stimulate the cardiovascular system, thereby reducing cardiovascular decompensation that occurs in weightlessness. Relaxed rapid (1G/s onset rate, ROR) and gradual (0.1 G/s onset rate, GOR) G tolerances were measured on seven men using a 5-ft (1.5 m) radius centrifuge and compared with their G tolerances obtained on the 20-ft (6.1 m) radius human-use centrifuge at the Armstrong Laboratory, Brooks AFB, TX. Since the subjects were required to flex their legs to assume a squatting position on the SAC, a similar position was used on the 20 ft (6.1 m) centrifuge called feet up (FU), and compared with normal-seated +Gz tolerances (controls). The subjects tolerated the SAC exposures without any problems. ROR and GOR tolerances were as follows: control, 3.6 G and 4.2 G; FU, 4.5 G and 5.6 G; and SAC, 4.6 G and 6.4 G. We concluded that a 5-ft radius centrifuge can be used to measure G tolerances. The increases in the SAC GOR tolerances over ROR tolerances indicate that the baroreceptors were stimulated by the G, and the SAC exposure would be useful in preventing cardiovascular decompensation in microgravity.

Over the last decade, G-induced loss of consciousness (G-LOC) has been recognized as a significant operational problem for pilots of high performance aircraft in both the U.S. Air Force and U.S. Navy. Consequently, government laboratories have initiated research studies to learn more about the G-LOC phenomenon in an attempt to reduce its hazards. Many of these studies require the occurrence of LOC during their conduct. For this reason, animal models have been developed to use in these studies. However, even though sophisticated animal models such as baboons can be taught to perform tasks before and after G-LOC has occurred, these models have deficiencies that can be overcome only by the use of human volunteers who willingly submit to G-LOC studies on the centrifuge. Such human G-LOC studies began in World War II and continue today. These studies are presently conducted without guidelines regarding subject selection, numbers of G-LOCs allowed per subject per unit time, restraint systems, or specific medical examinations required before, during, and after each episode of LOC, or after each completed study. This panel will discuss what is known about the pathophysiology of G-LOC, the limitations of animal models in these types of studies, the possible medical and psychological sequelae, and the legal implications of conducting deliberate G-LOC research. We hope that the information developed by this panel will be useful to laboratory human-use review committees in determining the requirements and the nature of guidelines for conducting such studies.

A new uniform-pressure pneumatic anti-G suit (UPS) was compared with the standard CSU-13B/P anti-G suit, using measurements of: blood lactate, heart rate changes, and segmented lower-body blood pooling (by impedance plethysmography). Subjects were exposed to a series of gradual-onset-rate (GOR) runs (0.1 G.s-1), rapid-onset-rate (ROR) runs (6 G.s-1), and simulated aerial combat maneuvers (SACM) on the USAF School of Aerospace Medicine human-use centrifuge. All measured parameters and subjective reports indicated that increased protection was afforded by the UPS. The impedance plethysmography measurements indicated that prevention of blood pooling in all lower-body segments is the predominant mechanism whereby uniform pressure permits significantly longer times-to-fatigue during SACMs.

An optoelectronic ear oximeter (Hewlett-Packard, model 47201A) was evaluated as a noninvasive method for determining arterial oxygen saturation (SaO2) in human subjects during exposure to various levels of accelerative forces. This physiological calibration involved exposing five subjects, while breathing air and wearing the ear oximeter for 60 s to each of three levels of accelerative forces (3, 5, and 7 G); arterial blood samples were withdrawn concurrently. SaO2 was calculated indirectly from the oxygen tensions (PaO2) measured from the sampled arterial blood with a blood gas analyzer and corrected for pH and base excess. These data were compared, as were similar data taken from the same subjects breathing three different hypoxic gas mixtures while resitng at earth's gravity (1 G). Regression analyses of these data for both experimental groups (a, G exposure, or b, hypoxic exposure), comparing the ear-oximeter SaO2 with the calculated SaO2, showed the ear oximeter to be accurate with correlation coefficients of 0.95 and 0.98, respectively.

SummaryOrgan and body mass changes in restrained adult, male domestic fowl were compared with tissue from control and fasted animals. Body mass loss rate in the restrained animals was 63.5% (p < .001) greater than in the fasted group, yet the two groups recovered at the same rate. Compared with controls and fasted animals, restrained fowl exhibited an increase in size of heart, lungs, kidneys, liver, and adrenals but a greater loss of spleen, superficial pectoral muscle, small intestine, and pancreas masses. Relative water content of organs generally increased in restrained and decreased in fasted animals. The data suggest that restraint—although accompanied by inanition—affects organ and body mass in the domestic fowl in a different way than does fasting.

Aircrew incapacitation resulting from very high onset sustained +Gz stress has resulted in significant losses of aircraft and aircrew. Enhanced protection and training toward prevention of +Gz-induced loss of consciousness (G-LOC) will continue to be vital. Techniques for reduction of the time of incapacitation, should G-LOC occur, must also be explored and developed. Current capability of aircraft autorecovery as demonstrated by the Advanced Fighter Technology Integration F-16 (AFTI/F-16) promises to enhance safety from the acute incapacitation resulting from G-LOC (and spatial disorientation). Physiologic monitoring for determining G-LOC has certain advantages especially in the aerial combat arena. The optimum physiologic monitoring technique would be direct determination of failure of brain cell function at the cellular or subcellular level. Complete investigation of G-LOC is necessary to understand the phenomenon and to develop methods for enhancing recognition and recovery. This paper discusses aircraft autorecovery technology and potential methods for physiologic monitoring of G-LOC. Integration of physiologic monitoring techniques into aircraft autorecovery systems requires a broad approach for optimal development.

Two studies were conducted to evaluate physiological responses and possible tolerance to high sustained +Gz forces at the USAF School of Aerospace Medicine, using the 20-ft (6.10 m) radius Human Centrifuge. In the first study (A), 14 human volunteer subjects--fully protected with an anti-G suit and performing the M-1 or L-1 straining maneuver--were exposed to +Gz forces. These forces began at +6.5 Gz, and increased weekly in 0.5 G increments to +9.0 Gz, for a duration of 45 sec at each level. Physiological performance was based mostly on the subjects' ability to maintain clear vision during the 45 sec exposure. Of the 14 subjects, 9 were able to maintain vision and remain at the +9 Gz level for 45 sec. In the second study (B), 12 human volunteer subjects--also protected with anti-G suits and using the straining maneuvers--were exposed to levels of +1, 3, 6, and 8 Gz, for 60 sec at each level. Extensive physiologic parameters were recorded in order to detect objective criteria which could be used as determinants for a medical tolerance endpoint to the particular level of +Gz stress. In both studies, the reasons for stopping the runs at levels exceeding +7.5 Gz included: fatigue, cardiac arrhythmias, blackout, and one occurrence of ventricular tachycardia. Other physiological responses to the sustained +Gz stress (such as heart rate, arterial blood pressure, central venous blood pressure, arterial oxygen saturation, gastric pressure, and ankle venous pressure) did not furnish specific information that could be used as "physiological tolerance limit" criteria.

Acceleration atelectasis is the absorptional collapse of alveoli in the dependent lung due to increased accelerative forces. It is exacerbated by breathing 100% oxygen and, during +Gz exposure, by the use of an anti-G suit. Experiments were conducted on 12 subjects using simulated aerial combat maneuvers (SACM) with G profiles having peak exposures of either 4.5 G or 9 G. Decreases in vital capacity (VC) measurements were used as quantification of atelectasis, two types of reduction being identified and described. Labile reductions in VC were readily restored by a deep breath or cough. Such reduction approximated 28% following the 4.5-G SACM and 25% following the 9-G SACM. More persistent (so called) stable reductions were of lesser degree, values of -20% being seen following both 9 G and 4.5 G maneuvers. Acceleration atelectasis causes symptoms of chest pain, coughing, and shortness of breath. Subjective ratings of the severity of these symptoms were obtained from the subjects, and these were much greater following the 4.5-G SACM exposures than after the 9-G runs. Acceleration atelectasis was reduced by dilution of the inspired oxygen concentration by argon and nitrogen (evaluated at 95, 82.5, 70, 50, and 20% oxygen); the addition of unassisted positive pressure at 30 mm Hg (4 kPa) to the breathing mask; or the performance of the anti-G straining maneuver (AGSM).

A static model based on eye-heart vertical distance has been developed which predicts group mean G tolerances relative to the application of any of the following anti-G methods and/or physiologic responses: a) anti-G suit, b) reclined seat, c) anti-G straining maneuver (AGSM), d) positive pressure breathing (PPB), e) gradual onset of G, f) isometric muscular contraction, and g) leg elevation. This model was validated with published data. A variation of this model (derived equation) predicts the amount of AGSM (in mm Hg) required, in combination with any of the anti-G methods/responses at any G level. This calculated effort of AGSM can be equated to level of fatigue and performance decrements. A level of 50 mm Hg or an increase of 2 G in the upright seat was the maximum AGSM recommended for routine use as an anti-G method for operational fighter pilots.

The effect of both assisted and unassisted positive pressure (PPB) breathing on +Gz tolerance was evaluated. A GOR (gradual onset rate = 0.1 G/s) acceleration profile was used to evaluate five different experimental conditions: 1) G-suit-only, 2) G suit + unassisted PPB, 3) G suit + PPB assisted by the English jerkin, 4) G suit + PPB assisted by the Canadian waistcoat, and 5) the Swedish counterpressure garment G suit combination. Acceleration tolerances for these experimental conditions (excluding the Swedish garment) with the subjects performing M-1 maneuvers were measured using an exhaustive ACM (Aerial Combat Maneuvers) acceleration profile. GOR tolerances were similar for both assisted and unassisted PPB with the pressure breathing experimental conditions providing a significant GOR tolerance increase over the G suit only condition. Assisted PPB increased ACM tolerances over both unassisted PPB and G suit only conditions. It appears that assisted PPB may offer a practical method for reducing the fatigue associated with exposure to high G.

Eight experimental subjects from the USAF Sschool of Aerospace Medicine (SAM) and four YF-16/17 test pilots were exposed to a simulated aerial combat maneuver (SACM) which included a maximum G exposure of 6 s at 8 G. The following physiologic parameters were examined relative to seatback angles of 23degrees, 28degrees, and 40degrees; heart rate and rhythm; arterial oxygen saturation; performance; intrathoracic (esophageal) pressure; arterial pressure; and subject comfort, effort, and fatigue. Relaxed and straining high sustained G (HSG) tolerances (6 G for 60 s) were also determined using only SAM subjects. The advantages of the 40 degree setback angle during the SACM included increased subject comfort, less fatigue and effort, greater pilot acceptance and a statisically significant reduction in the increased mean heart rate associated with G exposure. On the other hand, a statistically significant reduction in arterial oxygen saturation was obtained during the SACM at 40 degrees compared with the 23 degree back angle. An increase in relaxed G tolerance was found with the 40degree seatback angle--statistically significant only compared with the 28 degree seatback angle.

+Gz-induced loss of consciousness (G-LOC) results in incapacitation which can be characterized by the unconscious period (absolute incapacitation) and a subsequent period of confusion/disorientation (relative incapacitation). The sum of the absolute and relative incapacitation periods represents the total incapacitation period and may be equated to the overall length of time a pilot would be in uncontrolled flight should G-LOC occur. Reviewing the centrifuge induced G-LOC episodes in 55 subjects allowed a detailed description of G-LOC. The absolute incapacitation period was 16.6 s with the subsequent relative incapacitation being 14.5 s resulting in an overall total incapacitation of 31 s. The G-LOC incapacitation was dependent on the rate of onset of the +Gz-stress and the +Gz level where G-LOC occurred. G-LOC episodes could be subdivided into 2 separate types: Type I being shorter unconsciousness episodes without convulsive movements, and Type II being longer unconsciousness with more frequently associated dream states and convulsive type movements. This detailed description of G-LOC allows a more complete understanding of the phenomenon and establishes the basis for research toward decreasing the resulting incapacitation from G-LOC. Minimizing G-LOC incapacitation and enhancing recovery should enhance safety in the aviation environment of the high-performance-fighter aircraft pilot.

Five subjects wearing standard USAF anti-G suits and seated at a 65 degrees back angle were exposed to a simulated aerial combat maneuver (SACM) which was repeated 5 times with 4-min 1-G rests between each SACM exposure. The SACM was 122 s in duration with 10 s acceleration peaks of 10, 8, and 6 G; between these peaks, the subject was at 4 G for 15 s. This series of repeated SACM exposure fatigued four of the five subjects. Heart rate and rhythm, arterial oxygen saturation, expired gases, lactate, pyruvate, glucose, CPK enzymes and isoenzymes, blood volume, subjective fatigue measurements, and subject performance were examined relative to the development of fatigue, the energy cost of the M-1, and tolerance to the SACM. All physiologic-metabolic parameters were significantly affected by the repeated SACMs. However, only heart rate changes appeared to be correlated with developing fatigue. A significant amount of energy is required to perform the M-1. Subjects whose energy-metabolic and cardiovascular states are least disturbed by high G exposure are those persons who will perform best and become least fatigued during repeated aerial combat maneuvers.

G-induced loss of consciousness (G-LOC) is defined as "a state of altered perception wherein (one's) awareness of reality is absent as a result of sudden, critical reduction of cerebral blood circulation caused by increased G force." This phenomenon was first identified in Great Britain in World War I (circa 1918-1919) as "fainting in the air." In the United States G-LOC was first encountered in 1922 during the Pulitzer Trophy Air Race. Although recognized during World War II as an operational hazard for fighter aircraft, the invention of the pneumatic anti-G suit reduced concern about G-LOC at that time. A 1984 survey of pilots of high performance aircraft has shown G-LOC to be an operational problem--probably one that has caused aircraft mishaps for several years. The concern of this panel was to focus on various approaches in reducing the G-LOC hazard.

+Gz-induced loss of consciousness (G-LOC) continues to be a threat to aircrew flying high-performance fighter aircraft. All avenues to prevent G-LOC, and to reduce the resulting incapacitation should G-LOC occur, must be explored. Research has begun to accurately quantify all aspects of the G-LOC phenomenon. The emerging pattern from this research indicates that, theoretically, G-LOC incapacitation could be significantly reduced. Comparison of G-LOC with LOC induced by acute arrest of cerebral circulation reveals that the G-LOC incapacitation could be reduced by as much as 17 s. Results also indicate that the relative incapacitation period (confusion and disorientation) following unconsciousness could be reduced by at least 9 s for an individual who has previously experienced G-LOC. This suggests that exposure to G-LOC during centrifuge training could provide this orientation to G-LOC and potentially reduce the incapacitation period should it occur inflight. This exposure may be likened to the current altitude-hypoxia training requirement for aircrew. Experience to date supports the contention that such training may be accomplished with an acceptable safety margin.

Loss of consciousness (LOC) during exposure to +Gz occurs in aircrew flying high-performance aircraft. This phenomenon is responsible for several USAF aircraft losses with accompanying loss of life. It has been recognized as a potential flying problem since 1938. Acceleration-induced LOC results in 15-to 20-s periods of aircrew total incapacitation, amnesia, and clonic spasms. It can occur at relatively low G levels and without symptoms of loss of light (blackout). Operational anti-G equipment and methods are not completely effective in preventing LOC. Considering the present status of G protection, a deficiency in the anti-G straining maneuver is considered to be the primary cause of G-induced LOC. This deficiency usually can be corrected with proper centrifuge training. Our present understanding of G protection suggests that the elimination of LOC as a serious problem for USAF high-performance aircraft operations must include the supination of aircrew to a minimum seat back angle of 60 degrees to 65 degrees.

A summary of human +Gz tolerance and physical conditioning to simulated aerial combat maneuvering is reviewed as a response to the preceding Science News Note by Dr. Bulbulian.

The influence of physical conditioning on tolerance to a centrifugation profile called the Simulated Aerial Combat Maneuvering (SACM)--was determined using 24 young men as subjects. These subjects were assigned to groups as controls (no physical training, C), runners (R), and weight trainers (W). They followed a 12-week protocol of specified physical training. During this study, tolerance to the SACM, maximum oxygen consumption, muscle strength, and body composition were periodically determined. SACM tolerance was defined as the total time that a subject could withstand continuous exposure to a 4.5 and 7.0 +Gx centrifugation profile as determined by his voluntary endpoint of fatigue. The +Gx tolerance of the runners and controls increased at an average rate of 4 s/week during the course of the experiment. On the other hand, the weight trainers increased their G tolerance at an average rate of 15 s/week. The difference between group W compared with groups C and R was statistically significant at the 5% level. Fatigue scores indicate that group W subjects take longer to reach a given level of fatigue than did the subjects of the other groups. It appears therefore that a physical conditioning program of weight training will improve human tolerance to aerial combat maneuvers.

The USAF is developing an On Board Oxygen Generation System (OBOGS) for use in fighter aircraft. This study was conducted to determine the inert gas dilution requirements of the OBOGS necessary to prevent acceleration atelectasis. Human subjects were exposed to either 21, 50, 70, 82.5, 95 or 100% inspired oxygen, along with an increased +Gz Simulated Aerial Combat Maneuver (SACM) profile using the USAF School of Aerospace Medicine human centrifuge. The SACM profile utilized four +4.5z peaks (136 s at this +Gz) superimposed on a +3 Gz baseline, representing a total ride time of 276 s. A significant reduction in vital capacity (VC) occurred at inspired oxygen concentrations of 70% and greater. The addition of 5% argon (a natural product of OBOGS) to pure oxygen did not reduce the magnitude of acceleration atelectasis observed, nor the severity of breathing symptoms. A 30-s exposure to positive pressure breathing at 30 mm Hg during the end of the SACM reduced the level of VC reduction caused by subjects breathing 100% oxygen during +Gz.

An acceleration profile was developed on the USAF School of Aerospace Medicine's 6.1-m radius centrifuge to measure human tolerance to the Aerial Combat Maneuver (ACM). The ACM profile is a continuous repetitive, 4.5 G for 15 s to 7 G for 15 s, cyclic G exposure which is terminated by the subject at his fatigue endpoint. ACM tolerances using this type of G profile were determined for seven subjects at four different seatback angles; i.e. 13 degrees, 30 degrees, 55 degrees, and 65 degrees from the vertical. Group (mean +/- S.E.) tolerances for the ACM were 170 +/- 17 s at 13 degrees and 541 +/- 48 s at 65 degrees. These tolerances were not usually correlated with relaxed gradual onset G tolerances. The subjective fatigue endpoint was physiologically verified using heart rate, heart rhythm, and performance criteria at the four seatback angles. The potential value of the ACM profile is considered as a measure of the effectiveness of anti-G equipment and methods in the aerial combat environment.

The role of anaerobic metabolism in +Gz duration tolerance was measured using venous blood lactate concentrations before G exposure and after subjects had been fatigued from exposure to one of several levels of G: low (4.5 G) sustained G (LSG); high (7-9 G) sustained G (HSG); and simulated aerial combat maneuver (SACM) of 4.5/7 G levels. A mean +/- S.E. blood lactate of six subjects fatigued from LSG was 29.8 +/- 4.0 mg%. Four subjects fatigued from HSG had blood lactates of 42.4 +/- 3.2 mg%, and six had blood lactates of 46.7 +/- 7.2 mg% from the SACM. Blood lactates appeared to correlate directly (on a group basis) with maximum heart rates found during G exposures. Six subjects exposed to 8 or 9 G for 10 s or less demonstrated an anaerobic alactate capacity. These G findings were related to fatigue produced with isometric muscle contraction in physiologic studies conducted at 1 G. We concluded that anaerobic metabolism and isometric exercise physiology are directly related to duration tolerances of fatigue at all levels of +Gz.

To assess the effectiveness of muscle-strength (weight training) on simulated aerial combat maneuvering (SACM) G tolerance, seven young men were exposed to a 12-week program of whole-body weight training in which were measured, strengths of various muscle groups, body circumferences, body mass, and the percentage of body fat. The magnitudes of the weights used in training were used to measure muscle strength and were compared and correlated with each subject's SACM tolerance--defined as the total time that a subject could withstand continuous exposure to a 4.5 and 7.0 + Gz centrifuge profile using fatigue as his voluntary endpoint. Chest and biceps circumferences increased 4.2% and 3.1%, respectively; abdomen and thigh circumferences did not significantly change; body fat decreased 16.8%; and body mass increased 2.3%. Abdominal (sit ups) and biceps (arm curl) strengths increased 99% and 26.2%, respectively, and were highly correlated with SACM tolerance time (p less than 0.01); leg (leg press) and chest strengths (bench press) made less significant contributions to the SACM tolerance time. A net increase in SACM tolerance times of 53% resulted from weight-training. Multiple regression analysis of all four muscle groups between weeks 1 and 12 with the SACM tolerance had a correlation of determination of 0.61.

In aerial combat maneuvers (ACMs), at Luke AFB, Az, eight pilots flew their two F-15 aircraft against nine pilots in three F-106 aircraft. A total of nine flights, consisting of 23 ACMs, were accomplished in 5 successive days. The degrees of fatigue, stress, and sympathetic activity were quantified using both subjective analyses and the biochemical constituents in the urine of the pilots of the F-15 or F-106. Biochemical indicators, reported per 100 mg creatinine, included: epinephrine, norepinephrine, 17-OHCS, urea, inorganic phosphate, sodium, potassium, and sodium/potassium ratio. The F-106 pilots exerted more relative effort than did the F-15 pilots--effort which appeared to be associated with high-G experience. Both groups of pilots were equally fatigued following ACMs; however, only the fatigue of the F-106 pilots was directly correlated with the length of the ACM. Sympathetic and stress responses during the ACM--similar for both groups of pilots--showed postflight increases of 54% in epinephrine, 19% in norepinephrine, and 20% in 17-OHCS over preflight values, thus suggesting a moderate stress response. Resting levels of these same indicators, for days the pilots did not fly and for pre-ACM values, were similar but higher than control values previously reported for other stressful activities. By late afternoon, postflight values for these indicators had returned to near-preflight levels.

Six male subjects wearing anti-G suits were exposed to +4.5 Gz and +4.5-7 Gz simulated aerial combat maneuvers (SACM), sustained until terminated because of fatigue. Before and after each G exposure, venous blood was withdrawn (using the finger-prick method) and analyzed for lactate concentration. Five samples were taken serially, at 1, 3, 6, 10, and 20 min after each G exposure to determine the maximum (peak) level of blood lactate. Individual lactate levels varied, with peaks at 1, 3, and 6 min; however, group levels were maximum at 3 min after the G exposure. Maximum lactate levels (mean +/- S.D.) of 27.8 +/- 11.3 mg% and 42.7 +/- 19.4 mg% were found for the 4.5-G and SACM exposures, respectively. Lactate recoveries were a simple exponential function, with a half life of approximately 10 min. The relationship between these data, following G exposure, and those measured after aerobic physical activities, is discussed.

Discrete, finite, Fourier transforms of input +Gz stress and output arterial oxygen saturation (Sao2) were used to obtain transfer functions between G and Sao2 in 12 subjects. Ensemble averaging of transfer functions based on responses to variable G stress provided reasonable capability for predicting Sao2 responses to sustained 6-G stress. Prediction of responses to variable G stress from transfer functions based on responses to sustained G stress was unsuccessful, however. A synthetic transfer function with physiologic explicability and substantial predictive ability was obtained from an impulse response fabricated from simple mathematical functions.