Commercial flights are currently used worldwide by more than 2.7 million people annually for business and tourism purposes (Peterson et al., 2013). Besides its efficacy and relative comfort, commercial flights have been considered among the safest modes of transportation (Norbäck et al., 2006). Previously, stressful factors inherent to flights such as increased anxiety during take-off and landing, changes in body position induced by aircraft acceleration and deceleration, aircraft noise, and fluctuations in the aircraft's cabin pressure have been demonstrated to interfere in the well-being and mood state of the crew (Chandra and Conry, 2013). These flight experiences along with the stress of daily living situations such as those related to work, may lead travelers to endure a high allostatic load (McEwen and Seeman, 1999; Tonello et al., 2014). Repeated exposure to a high allostatic load could be associated with physiological imbalances leading to health threatening risk factors (e.g., inflammatory process, hostility, depression; McEwen and Seeman, 1999; Olson et al., 2015). Therefore, monitoring allostatic load and improving allostatic tolerance may be vital for health and well-being.

Allostatic capacity can be monitored by the activity of the autonomic nervous system (ANS) through its regulation of the cardiovascular, respiratory and neuroendocrine systems (Verlato et al., 2011). The analysis of cardiovascular control, termed heart rate variability (HRV), has been as reported as an effective tool to identify changes in ANS activity (Task Force, 1996) in response to different stressors (Teisala et al., 2014). In addition to traditional HRV measures of time and frequency domains, other non-linear measures of HR dynamics have been suggested to provide useful prognostic information in various clinical settings while their reproducibility may be better than that of traditional indices (Huikuri et al., 2009). Consideration of both linear and non-linear HRV measures could be especially important during ambulatory conditions in which the validity and reproducibility of these measures could be different depending on different factors as physical activity levels or sample characteristics (Hautala et al., 2010; Tonello et al., 2015). Previous studies with military pilots demonstrated important HRV and autonomic adjustments during flights (Dussault et al., 2009; Oliveira-Silva and Boullosa, 2015). These flight-induced autonomic changes may trigger several physiological adjustments to maintain homeostasis during a potentially detrimental activity (McEwen and Seeman, 1999). It is important to point out that the reported incidence of medical emergencies during civil aviation is low with a rate of 1 per 604 flights or ~0.002% (Peterson et al., 2013). Despite this low rate, some individuals experience difficulties during aviation with the most prevalent problems being: syncope (37.4%), respiratory distress (e.g., dyspnea, 12.1%), and nausea (9.5%) (Peterson et al., 2013). As a result of these difficulties, Limper et al. (2014) expressed the need for medical screening of individuals before flights and the importance of maintaining satisfactory health in order to cope with gravitational and pressure changes associated with commercial flights.

Recent medical guidelines for air travel have emphasized the necessity to consider different aspects of passengers' health or medical state (Thibeault et al., 2015), however these guidelines do not address the role of physical fitness for passenger well-being during commercial flights. Typically, these air travel guidelines encompass physical and psychological issues regarding specific health-related problems and diseases rather than generic well-being (Aerospace Medical Association Medical Guidelines Task, 2003; Thibeault et al., 2015). Consideration of other important health factors such as physical fitness or physiological function like ANS activity though are rarely considered important for air travel (Oliveira-Silva and Boullosa, 2015). Individuals with a greater physical fitness have been reported to demonstrate an increased parasympathetic modulation at rest (Hautala et al., 2010), a better ANS modulation during stressful situations (Teisala et al., 2014), and reduced risk of future cardiac events (Fu and Longhurst, 2009). Recently Oliveira-Silva and Boullosa (2015) demonstrated that military pilots with better physical fitness (i.e., higher aerobic capacity and lower body fatness) exhibited a greater cardiac autonomic control (i.e., HR complexity) during training flights when compared to a control day which highlighted physical fitness as an important factor to counteract the effects of stress.

While physical fitness parameters have been considered important for managing physiological responses to stress, to the best of our knowledge, there have been no studies examining the possible role of physical fitness in regulating cardiovascular stress induced by commercial flights, a common and growing activity experienced by millions of people worldwide each year. Thus, the purpose of the present study was to examine the influence of physical fitness on the autonomic modulation of HR and blood pressure (BP) responses of passengers during commercial flights. Our hypothesis was that greater physical fitness (e.g., greater aerobic capacity and muscular strength, and lower % body fat) would result in better cardiovascular control (HRV and BP) for passengers during flights. Greater cardiovascular control and HRV via enhanced physical fitness may enable passengers to cope better with the physiological stresses associated with air travel and maximize passenger well-being.

Table 1 depicts the mean values for the physical tests performed by the participants.

Twenty-four hours HR and HRV results during the control and experimental days are shown in Table 2. All variables were similar between days except for a significantly greater HR, and significantly lower LnHF and α₂ for the experimental day (ES = 0.49–1.01, Table 2).

Linear and non-linear HRV measures (1-h recordings) prior to, during and after the flight on the experimental and control days are shown in Tables 3, 4, respectively. Compared to the Control day, HR was significantly greater (>12%) prior to, during and after the flight on the Experimental day (ES = 0.58–1.13, Table 3). In contrast, RMSSD was significantly lower (>20%) in the hour prior to, during and the immediate hour after the flight compared to the control day (ES = 0.42–0.95, Table 3). During the flight, LnHF was significantly lower (>25%, ES = 0.94) compared to the control day while SDNN (< 15%) and LnLF (< 7%) were similar between days (ES = 0.34–0.52, Table 3).

Compared to the Control day, SampEn was reduced in the hour prior to the flight (>13%, ES = 0.55) while α2 was reduced in the hour after the flight (>9%, ES = 0.40) (Table 4). During the flight, SD1 and SampEn were significantly lower (>26%, ES = 0.79–0.95) while α1 was significantly greater (>14%, ES = 0.94) compared to the control day (Table 4). All other variables were similar between days and times/moments (Table 4).

Values of 24-h SBP, DBP, MBP, and RPP on the control and experimental day are shown in Table 5. All BP derived measures were not significantly different between days although a small ES (ES = 0.38) for RPP was evident. Hour-by-hour analyses of these BP values before, during and after flights for both days are shown in Table 6. Mostly, there were significant increases for all variables during and after flights (ES = 0.54–1.43, Table 6) with an anticipatory effect only for SBP and RPP (ES = 0.45–0.75, Table 6).

Correlations between anthropometric, physical fitness and absolute values and relative changes (Δ%) in 24-h HRV measures are shown in Tables 7, 8, respectively. Absolute values of HRV measures during the experimental day only were significantly correlated with measures of aerobic fitness (positive relationships) and body composition (negative relationships, Table 7). Relative changes in HRV measures were significantly correlated with measures of aerobic fitness (negative relationships) and body composition (positive relationships, Table 8). There were no correlations identified between 24-h BP measures and physical fitness indices. Finally, significant and high correlations were observed between indices of aerobic fitness and body composition (r = −0.725 to −0.861; p < 0.01).

To our knowledge, this is the first study relating physical fitness, body composition and hemodynamic responses to flight induced stress (~1 h) in physically active individuals. The main findings were that a 1-h commercial flight was associated with enhanced cardiovascular stress (i.e., enhanced HR, SBP, DBP, MBP, and RPP) with vagal withdrawal and reduction in HR complexity, as indicated by the change in several HRV indices (i.e., RMSSD, LnHF, SD1, SampEn, α2). Moreover, greater aerobic fitness was positively associated with higher HRV during the experimental day (i.e., LnLF, LnHF) and less HRV change during a commercial flight day. Similarly, significant inverse relationships were identified between measures of body composition (% body fat and BMI) and HRV (RMSSD, LnLF, LnHF, SD1) during the experimental day with greater body fat associated with greater HRV changes during a commercial flight day. This study demonstrated that short-term commercial flying significantly altered cardiovascular function with greater physical fitness (e.g., greater aerobic capacity) and lower body fat composition associated with greater cardiovascular control (HRV and BP) for passengers during flights. Enhanced physical fitness and better body composition may enable passengers to cope better with the cardiovascular stresses associated with air travel for enhanced passenger well-being.

Compared to the control day, the 24-h HRV recordings on the experimental day indicated a significant increase in HR with reductions only in LnHF and α2 with the other HRV measures remaining relatively stable. Interestingly, when shorter periods of HRV were analyzed (i.e., hour to hour), flight-related stress was accompanied by a vagal withdrawal within the hour prior to take-off and was maintained for up to 2 h after destination arrival. This reduction in HRV with flight stress was previously noted in experienced (Oliveira-Silva and Boullosa, 2015) and novice military pilots (Sauvet et al., 2009) and reinforces that anticipatory autonomic responses occur prior to an announced stress (Hynynen et al., 2009; Boullosa et al., 2012). It is well known that adaptation in the face of stress is a major priority for all organisms (Ulrich-Lai and Herman, 2009) with an autonomic anticipatory change occurring in response to an expected biological stress (Boullosa et al., 2012). As the participants of the present study were passengers that had accumulated < 50 flying hours on commercial airlines, the anticipatory response could have been associated with flight inexperience (Waterhouse et al., 2002; Hynynen et al., 2009). Moreover, it has been shown that in experienced individuals (>500 h of flight), this anticipatory response is overridden (Oliveira-Silva and Boullosa, 2015). Therefore, it is likely that the observed vagal withdrawal of the current participants was associated with an acute, adaptive response of the ANS to cope with the upcoming stress. Likewise, it has been suggested that changes in the environment and daily routine could also contribute to a higher HR that would affect HRV (Zhao et al., 2015). Consequently, both anticipation of flight and changes in daily living conditions could modulate the cardiac autonomic responses as a biological preparatory mechanism for dealing with unusual and changing situations.

The observed greater 24-h HR during the experimental day was in agreement with previous studies (Sauvet et al., 2009) with stresses of multiple origin reported to increase HR (Kenefick et al., 2008; Boullosa et al., 2012). Despite the greater HR during the flight day, other 24-h cardiovascular measures (e.g., SBP, DBP, MBP, and RPP) were similar between days with this long-term assessment likely to be insensitive to acute flight stress. Importantly, short–term, 1-h measures were able to clearly identify the acute stress prior to, during and post-flight with significant changes and moderate to large effects (ES). Taken together, our results demonstrated that a commercial flight of only 1 h exposed travelers to a high allostatic load (McEwen and Seeman, 1999; Tonello et al., 2014), which exerted considerable neural and cardiovascular stress (Brundrett, 2001; Fowler et al., 2015). Furthermore, these stress-induced responses were capable of inducing alterations in the circadian rhythms such as blood pressure (Hansen et al., 2010), that can be maintained up to 2 h after the flight. It is important to note that these undesirable hemodynamic effects, although transient, may result in functional alterations in the hypothalamic-pituitary-adrenal axis (HPA) and hippocampus (Frodl and O'Keane, 2013), and negative influences on health in the long-term. Subsequently, attenuation of these transient changes associated with flight would be beneficial in reducing the negative impact of flight on the health of flyers. Generation of greater physical fitness may be an important means to modulate these flight-induced transient changes (Huang et al., 2013).

The relationship between indices of HRV and physical fitness was not surprising (Fronchetti et al., 2006; Hautala et al., 2010; Dutra et al., 2013; D'Agosto et al., 2014; Dolezal et al., 2014). However, to date there has been no consensus regarding the impact of the different components of physical fitness on HRV modulation during stress responses (Hautala et al., 2010; Bravi et al., 2013). We identified significant relationships between high HRV (absolute and change) and high aerobic fitness (MAS), and low % body fat. The current correlations between % body fat and HRV indices, specifically SD1 and RMSSD, were in line with the results of Yi et al. (2013) who noted that % body fat was inversely associated with several HRV measures after adjusting for age, gender, and cardiovascular risk factors. More recently, Tian et al. (2015) reported an inverse relationship between % body fat and HRV, and demonstrated that the decrease in % body fat was associated with an increase in HRV. Given that SD1 and RMSSD reflect parasympathetic activity (Fronchetti et al., 2006), the negative correlation between HRV and % body fat highlights the importance of maintaining desirable levels of % body fat (Pillay et al., 2015) to optimize HRV for stress management. The incorporation of physical exercise has been well documented as an effective means to manage % body fat (Golubic et al., 2015) as well as providing an important intervention to moderate stress-related autonomic modulation (Dolezal et al., 2014). In fact, we observed significant correlations between measures of aerobic capacity (MAS and T_UMTT) with ΔLnHF (r = −0.53 and r = −0.52, respectively) that highlights physical fitness as an important modulator.

In line with the previous discussion, it is worthy to note that the current participants were healthy and physically active men with good levels of physical fitness (Garber et al., 2011). Despite this degree of physical fitness, the current participants still experienced flight-induced stress with their low % body fat and high VO₂max likely providing a degree of protection against stressful situations. It remains to be seen whether less fit individuals can cope with flight-induced stress in the same manner or whether they exhibit substantially greater stress and cardiac autonomic disturbances. Meanwhile, given that aerobic fitness and body composition measures were also correlated between each other, further studies should clarify the relative influence of these health related components on cardiovascular responses to commercial flights, with special attention to differences between evaluation protocols (e.g., Treadmill vs. Cycle vs. Field running) and selected parameters (e.g., actual VO₂max vs. Estimated VO₂max).

Finally, it should be pointed out that muscular strength was not significantly correlated with HRV indices, except for SampEn (r = −0.44; p = 0.04), a contrary finding to that of Heffernan et al. (2007) who demonstrated that a program of resistance exercises enhanced SampEn in young healthy men. An explanation for this lack of association was not apparent in the current study however, others (Barbosa et al., 2014) have demonstrated that resistance training could positively alter HRV of healthy men and hypertensive women. Therefore, we suggest that enhancement of muscular strength through resistance training should not be disregarded as a strategy to improve cardiac autonomic modulation as it has been shown to be of great value for HRV and performance (Caruso et al., 2015). Further studies are encouraged to examine the role of resistance exercise for cardiac autonomic control and coping with stress.

In summary, the results of the present study partially confirmed our hypothesis that specific aspects of physical fitness (e.g., aerobic capacity) were related to better cardiovascular control (HRV and BP) for passengers during flights. Further, these results reinforced the need for future studies to investigate the role of physical fitness in reducing the flight-induced stress and related cardiac autonomic alterations for the general population. Current recommendations for air travel such as alcohol and tobacco abstention (Mills and Harding, 1983; Newman, 2004), maintenance of adequate hydration (Meir et al., 2003) and careful activity planning on destination arrival (Reilly et al., 2009) may assist with reductions in flight-induced stress and allostatic load that remain to be investigated. Finally, our results indicate that persons who frequently fly should develop and maintain suitable levels of physical fitness to reduce the potential impact of exposure to the cardiovascular stress associated with allostatic loads during air travel.

Study Design: IO, DB. Data collection: IO, Data analyses: IO, Interpretation of the results: IO, AL, MM, HS, SR, CC, DB. Manuscript writing: IO, AL, MM, HS, SR, CC, DB. Approved the final manuscript version: IO, AL, MM, HS, SR, CC, DB.

DB, HS enjoy a productivity grant in research from CNPq.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer BM and handling Editor declared their shared affiliation, and the handling Editor states that the process nevertheless met the standards of a fair and objective review.