The members of the Hellenic Navy Special Operations Command (SOC), the elite unit of the HN Special Operations Forces (SOF) community, should be able to demonstrate optimal performance in extreme conditions. These conditions require extraordinary physical stamina and excellent control of the cognitive executive functioning. As Paulus and colleagues have pointed out “Extreme environments are characterized as those situations that place a high demand on the physiological, affective, cognitive, and/or social processing resources of the individual.” (1).
Hellenic Navy SEa Air Land (SEALs), known in Greece as N.H.S., are expertly trained to deliver highly specialized, intensely challenging warfare capabilities that are beyond the means of standard military forces. In order to do so, they must successfully attend the Basic Underwater Demolition School (BUD/S) of the Hellenic Navy Special Operations Command. A 32-week extremely intensive and high-risk training schedule with very low ratio of success. This military training program aims to develop the mental and physical stamina of the SEAL candidates and assesses them in physical conditioning, water competency, teamwork and mental tenacity. More specifically, special training objects such as combat diving, unconventional warfare techniques, amphibious operations, use of man-portable medium range weapon systems and standard explosives are included. A very strenuous and demanding part of the training program, is the so called ''Hell Week''.
The idea behind the structure and the intensity of the BUD/S, is to create training conditions similar to a combat. It is known that combat is one of the most stressful situations that human can face. It includes unexpected attacks and uncontrolled threads, that demand attention and rapid decision making. Under these circumstances, sympathetic nervous system and the “fight or flight” response are activated (2-5). Acute stress response, leads to certain physiological and psychological reactions. While it has an adaptive function, as it prepares the individual to cope with uncertainties and threats, negative effects may arise when this reaction is maintained in time (6).
Heart Rate Variability
Among the various psychophysiological reactions, the ones being associated with the cardiovascular system, such as heart rate and heart rate variability (HRV), are the first to be affected during stress. The cardiovascular system is under control of both the sympathetic and parasympathetic branches of the autonomic nervous system (7). It is regulated by complex central mechanisms, including lower-level reflex systems as well as higher neurobehavioral mechanisms. It has been suggested that resting measures of heart rate variability, is a reflection of the interplay between the aforementioned higher and lower mechanisms (8,9). There is a large body of research that has shown a direct link between cognitive processing and the cardiovascular system through autonomic vagal control (10). Based on evidence that prefrontal cortical substrates of top-down self-regulation influence cardiac activity primarily through the parasympathetic nervous system (11), two theoretical perspectives have emerged; Porges’s Polyvagal Perspective (12) and Thayer’s Neurovisceral Integration Model (8,13). In addition, Blons and his colleagues have pointed out (14), that heart rate variability is an index of how the brain achieves flexible control and adaptive regulations, allowing us to explore coordinated heart-brain interactions through the concept of neurovisceral integration. That is, by measuring time and frequency domains of HRV, we can assess central nervous processes, such as attentional control and emotional regulation (15).
An optimal level of HRV is associated with health and self- regulatory capacity, and adaptability or resilience (16). A growing body of evidence indicates that vagally mediated HRV have linked to self-regulatory capacity (17-19), emotional regulation (19,20), social interactions (21), one’s sense of coherence (22), the personality character traits of self-directedness, and coping styles (23). In this framework there are also evidence showing a relationship between vagally-mediated resting HRV and performance on cognitive performance tasks requiring the use of executive functions (24).
In this sense Porges advocated that the evolution of the ANS, was central to the development of emotional experience and the social engagement system. Thus, human beings, are not limited to fight, flight, or freeze responses but simultaneously have the capacity of self-regulating behaviors when encounter challenges or stressors. His theory suggests that function of the ANS determines the boundaries for the range of one’s emotional expression, quality of communication, and the ability to self-regulate emotions and behaviors. Consequently, the measurements of ANS activity could serve as a marker for one’s ability to self-regulate (12).
Specifically, HRV are linked to performance of executive functions like attention and emotional processing by the prefrontal cortex (24). Afferent information processing by the intrinsic cardiac nervous system can modulate frontocortical activity and impact higher-level functions (25). A recent study found differences among the two sexes to stroop color word test (SCWT). In particular, cardiovascular autonomic response was more towards sympathetic activation in males than females, even with a stress of short duration, such as the SCWT (26).
Psychophysiological Measurements in Military
Until recently, there was limited literature on the psychophysiological responses of the military population in stressful situations, let alone of the special operation forces. On 2007, Morgan and his colleagues, conducted one of the first studies to assess the relation between cardiac vagal tone and performance in male military personnel exposed to high stress (27). They concluded that reduced vagal tone was associated with enhanced performance. This result was attributed to the ability of better emotion regulation and cognitive functioning when vagal tone is suppressed.
Helping to better understanding and enabling an improvement of current training methods of soldiers, Clemente-Suárez and Robles-Pérez, analyzed psychophysiological changes in a simulated urban combat (28). They focused on changes in cortical arousal, blood lactate, muscle strength, autonomic modulation and rate of perceived exertion. Their results led them to the conclusion, that urban combat produces high sympathetic nervous system stimulation. Nevertheless, despite the fact that there were alterations in basal psychophysiological states, rates of perceived exertion of the soldiers were below the physiological response. Others have compared elite and non-elite soldiers (3), acute high stress combat situations in professional soldiers (4) and autonomic and cortical response of soldiers in different combat scenarios (29).
Delgado-Moreno and his colleagues studied the effect of combat stress in the psychophysiological response and attention and memory of warfighters in a simulated combat situation (2). Their results led to the conclusion that higher psychophysiological activation correlated with poor memory performance, confirming the hypothesis that the activation of the sympathetic nervous system affects executive functions. It is worth mentioning that soldiers remembered more easily subjects that could pose physical harm on them in comparison to neutral stimuli indicating that cognitive processing is associated with emotional disturbance.
Taking into account the above considerations, it would be much advantageous if it is possible to quantitatively and objectively measure the individual resilience for the efficient stress management. The aim of the present study was to investigate the suitability of heart-rate variability (HRV) as a measure of stress resilience, by evaluating it both for mental (color wort & arithmetic stroop test) and emotional stroop test. To do this, we evaluated HRV parameters belonging both to time and frequency domain of HRV, by calculating the correlation of reactivity and recovery from the acute mental and emotional stroop stress, respectively, in Hellenic Navy SEALs (HNS) and healthy controls (HC). We hypothesized that HNS would have higher adaptive responses of reactivity and recovery from both mentally and emotionally stressful events.
It should be noted that the ability to actively resist and recover from the stressors in the environment is called resilience (30). Resilience is the ability to maintain or quickly return to a stable physical and psychological equilibrium despite experiencing stressful events. Flexibility of the autonomic nervous system is particularly important for adaptive stress responses and may contribute to individual differences in resilience (31).