Introduction

With increasing importance of flight missions to military operations, more high performance aircrafts have been introduced to the military force. Aviators of high performance aircrafts are frequently exposed to high gravitational force (G-force) that results from quick turns or movements. High G-force can cause cerebral hypoxia and even loss of consciousness in humans by disturbing blood supply to the brain [1–3]. G-force–induced loss of consciousness is simply referred to as “G-LOC.” Aircraft accidents due to G-LOC occurred in the First World War [1]but have only been formally reported and studied since the 1980s [4]. When an aviator experiences G-LOC, he or she may experience a lack of purposeful movement for at least 30 seconds even when the high G-force is removed immediately [5]. Therefore, even short-term G-LOC can cause fatal aircraft accidents. To address those concerns, there have been many studies on G-LOC, and much effort has been made to prevent it. Only the G-force applied on the z-axis (head-to-foot direction) is counted and referred to as “G-force” throughout this paper.

An individual’s tolerance to G-force (G-tolerance) depends on factors such as the degree of G-force and the duration of exposure. G-tolerance varies among individuals and can be improved by measures such as anti-G suits, anti-G straining maneuvers (AGSM), positive pressure breathing systems, and reclined seats [6,7]. Among them, AGSM is known to be the most effective method [8,9]. AGSM is broadly divided into 2 stages: the first stage is muscle tensing, and the second stage is the L-1 maneuver. Muscle tensing involves the muscles of the abdomen, buttocks, and extremities. The L-1 maneuver consists of a Valsalva maneuver with a totally closed glottis as well as breathing at intervals of 2.5–3 s [10,11]. Many studies have reported the association between G-tolerance and physical conditioning such as weight training to enhance the effect of AGSM [12–14] and the association between G-tolerance and respiration [15–17]. However, most of them did not find any effect on physical conditioning on G-tolerance or any analytical errors in earlier studies that had assumed the effect of physical conditioning on G-tolerance [3,18,19].

Because aviators are now exposed to increasing maximum acceleration levels with developing high performance aircrafts, the importance of enhancing G-tolerance is being stressed again [20,21]. However, controversy remains on whether physical condition affects G-tolerance [19]. Furthermore, only few studies have targeted well-experienced aviators on active flight duty.

The purpose of this study is to evaluate the association between G-LOC and physical condition parameters among young and well-experienced aviators. Targeting only young and well-experienced aviators who are on their active flight duties in high performance aircrafts, we investigated their basic parameters of physical conditions such as body composition, physical fitness, and pulmonary function and then compared those parameters of the aviators who underwent G-LOC during human centrifuge trainings with those who did not.

Materials and Methods

This is a retrospective study. This study was ethically approved by the Aerospace Medical Center–Republic of Korea Air Force Institutional Review Board (ASMC-ROKAF IRB).The first ethical approval was obtained on April 4, 2013, and the data collection began on the next day, April 5, 2013. All the data gathered were originally intended for routine medical checkups and education program, not for the study. No further data were gathered or requested for this study. We collected participant data from patient files and medical records retrospectively. The informed consent was exempted from IRB because the following conditions were met: (1) it is practically impossible to obtain the consent from the subjects in the study process, (2) there is no ground to assume the refusal of consent, and (3) extremely low risks may be exposed to subjects even if the agreement is exempted.

Investigated variables

We compared the subjects’ basic characteristics, body composition, physical fitness level (cardiopulmonary endurance, muscle power, and body flexibility), and results of the pulmonary function test (PFT). All of these were measured in one day, but not on the same day as the centrifuge training. The body composition test could be affected by dehydration due to the physical fitness test, while the cardiopulmonary endurance test could be affected by the increased heartbeat due to the strength test. Therefore, body composition was measured first, followed by the PFT, general endurance, muscle power, and body flexibility.

Basic characteristics and body composition.

The bioimpedance spectroscopy (BIS) technique (InBody 720; Biospace Co., Ltd., Seoul, Korea) was used to assess the body composition. The test was performed after fasting for a minimum of 4 hours, no exercise for a minimum of 12 hours, abstinence from alcohol for a minimum of 48 hours, and abstinence from diuretics for a minimum of 7 days to minimize the bias. Finally, the subjects voided 30 minutes before the test. Each subject’s height was measured first while they stood on the electrodes embedded in the scale platform of the analyzers wearing only a light gown. The subject then grasped the handles and ensured that all of their fingers contacted the finger electrodes. During the measurement, the subjects kept their limbs slightly abducted and were not allowed to talk.

After these procedures, each subject’s height, age, and gender were recorded, and the following body compositions were analyzed: body mass index (BMI), basic metabolic rate (BMR), percent body fat, waist-hip ratio, skeletal muscle mass (SMM), muscle mass of the upper extremities, muscle mass of the lower extremities, and muscle mass of the trunk.

Physical fitness.

The InBody u-Town (Biospace Co., Ltd.) was used for the physical fitness assessment. The measured variables were as follows: grip strength test, back strength test, leg strength test (knee extension strength in a 60° flexed position), sit-and-reach test, cardiopulmonary endurance (maximum oxygen consumption), whole body reaction time, sargent jump, number of sit-ups in 30 seconds, and standing on 1 leg with eyes closed.

The grip strength test was performed for 3 seconds with the subject’s mid-phalanx of 2–5 fingers facing the handle. The better of the 2 results was chosen for each hand. Maximum oxygen consumption was measured for cardiopulmonary endurance. Each subject was seated on a bicycle with his knees in a 10° flexed position when fully extended. After the resting heartbeat was checked for 1 minute, the subject pedaled at 50–60 revolutions per minute (RPM) for 6 minutes, and their heart rate was checked again. In the back strength test, the subject stood with their knees extended, feet approximately 15 cm apart, and trunk leaning approximately 30° forward. The subject pulled the handle with his back strength. The test was performed twice, and the better result was chosen. In the whole body reaction test, the subject stood on an electrical mat with his feet apart about his shoulder width and his knees slightly flexed. The subject then removed his feet from the mat immediately after the auditory signal rang. The time gap between the auditory signal and the foot removal was then measured. The test was performed 5 times, and the mean value was chosen. The standing position on one leg with eyes closed was used for the balancing test. The subject stood on one foot on an electrical mat with his hands on his waist. The duration until the other foot contacted the mat or the standing foot moved was measured. The test was performed on both legs, and the mean value was chosen. The sargent jump was used for the quickness test. The subject was not allowed to flex his knees after jumping. The duration from jumping to landing was measured twice, and the better result was chosen. Muscle strength or mass tends to increase according to weight and height. Hence, we additionally divided muscle strength and muscle mass by each subject’s weight and height, and included these values in the analysis.

Pulmonary function test.

In the PFT, we measured forced vital capacity (FVC), forced expired volume in 1 s (FEV₁), forced expired volume in half a second (FEV_0.5), and peak expiratory flow. We also calculated FEV₁ divided by FVC (FEV₁/FVC) and FEV_0.5 divided by FVC (FEV_0.5/FVC) and included those values in the analysis.

The demographics of the aviators in our study are shown as Table 1.

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Table 1. Demographics of aviators.

https://doi.org/10.1371/journal.pone.0147921.t001

Results

Of the 350 aviators, 29 (8.3%) experienced G-LOC during their first human centrifuge training trial. There was no difference in physical conditions between G-LOC and non-G-LOC groups (Table 2).The binary logistic regression analysis also did not show any significant association between the physical conditions and the occurrence of G-LOC (Table 3). Therefore, multivariate analysis was not performed. Muscle strength and mass, which were divided by the subject’s weight and height, also showed no statistically significant difference between the groups.

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Table 2. Differences between G-LOC group and non-G-LOC group.

https://doi.org/10.1371/journal.pone.0147921.t002

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Table 3. Results of binary logistic regression for variables associated with the G-LOC.

https://doi.org/10.1371/journal.pone.0147921.t003

Discussion

We examined the association between G-LOC and physical condition parameters such as physical fitness level, body composition, and pulmonary function by investigating a relatively large number of young and well-experienced aviators during their active flight duties in high performance aircrafts. However, there was no difference in physical conditions between the G-LOC and non-G-LOC groups (Tables 2 and 3). This raises the question of Type II errors given we found no differences in the groups, but the power analysis showed this study had over 90% power.

The InBody 720 is a well-known method of evaluating body composition [22–24]. We used body composition to conduct a more precise and objective investigation on the association between G-LOC and physical condition. To our knowledge, this is the first such study conducted. The body composition of the G-LOC and non-G-LOC groups was not significantly different in our study. Aerobic exercise was traditionally believed to reduce G-tolerance, whereas anaerobic exercise was believed to increase G-tolerance [19]. Some authors have reported that improved G-tolerance after physical conditioning is not due to muscle hypertrophy but rather to physiological alterations such as neuromuscular adaptation [12]. In our study, we examined muscle mass, strength, and general endurance and could not find any difference between the G-LOC and non-G-LOC groups.

The investigated variables in our study included muscle mass and physical fitness as well as PFT for indirectly measuring the intrathoracic pressure during AGSM. Some studies have assumed the importance of lung volume and peak expiratory pressure in the determination of intrathoracic pressure during the Valsalva maneuver [25–27]. Using the PFT, we measured FVC, FEV₁, FEV₁/FVC, FEV_0.5, FEV_0.5/FVC, and peak expiratory pressure, but we found no statistically significant difference between the 2 groups. On the other hand, Bain et al. [15] suggested that respiratory muscle fatigue is responsible for G-tolerance. This cannot be determined by our study findings because we did not measure this parameter.

This study investigated only the basic parameters of physical condition. The authors believe that the group action of the muscles or coordination is very important for G-tolerance. Although the data are not shown, almost all the aviators eventually completed the human centrifuge program at their second or third attempts. Although they eventually passed their tests, the authors do not think that their physical condition has been improved within such a short period. We think the improvement of G-tolerance originates from the improvement of coordination and adaptation to effective timing during AGSM. This is also the reason why we divided the subjects into the G-LOC and non-G-LOC groups according to only the results of the first trial. Future studies on the coordination and adaptation problem during AGSM surely are needed. The daily condition of the aviator and/or unexpectedly abrupt G-forces during flight are more likely to be responsible for very rare G-LOC related flight accidents, especially in highly-experienced pilots.

Our findings should be interpreted in the context of some considerations. First, many previous studies investigated whether G-tolerance could be increased by several weeks of weight training. However, a large study was nearly impossible, and the bias caused by the degree of weight training or learning effect from the experience of G-force was a disadvantage [18,19]. In this aspect, our study might have reduced bias by investigating a relatively large number of well-experienced aviators who had flown high performance aircrafts for >2 years after their initial flight training. Second, although the applied G-force was very high (9G), the duration was only 15 seconds. Therefore, it is difficult to determine the effect of muscle fatigue on G-tolerance in our study. Furthermore, because our study was performed using a human centrifuge test and not an actual flight, the assessment of G-LOC due to unexpected G-forces that could happen during an actual flight was also difficult. Third, we assessed the differences in physical condition in a cross-sectional study; therefore, it cannot be concluded that physical conditioning such as weight training is irrelevant for increasing G-tolerance in each individual. Fourth, our study did not include variables such as the daily condition of the aviators, nor did it include flight hours or G-force exposure in real flight environments, which might more accurately reflect each subject’s flight experience. Likewise, we only investigated the basic parameters for physical condition. The action of muscle group, muscle tensing, or coordination was not investigated. For example, although we included PTF as a substitute for Valsalva maneuver, effective timing and coordination with other muscle tensing seem to be more important than expiratory force alone. The present study also did not include other possible functional causes of G-LOC such as jugular vein geometry [28]. Fifth, the physical conditions were compared only among military aviators. The physical condition might affect the G-tolerance because military aviators are already physically conditioned subjects compared to the general population. However, authors believe this study is still meaningful because it is extremely rare for the general population to be exposed to such high G force during flight.

Conclusions

Physical conditions including muscle mass, strength, and general endurance were not associated with G-LOC in well-experienced aviators. Although more studies are needed, physical condition does not seem to be a significant determinant of G-LOC among the experienced aviators.