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Warfighter Performance Department Naval Submarine Medical Research Laboratory


The views expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Department of the Navy, Department of Defense, nor the U.S. Government.

This work was supported by work unit number F1005.

The study protocol was approved by the Naval Submarine Medical Research Laboratory Institutional Review Board in compliance with all applicable Federal regulations governing the protection of human subjects

I am a military service member (or employee of the U.S. Government). This work was prepared as part of my official duties. Title 17 U.S.C. §105 provides that ‘Copyright protection under this title is not available for any work of the United States Government.’ Title 17 U.S.C. §101 defines a U.S. Government work as a work prepared by a military service member or employee of the U.S. Government as part of that person’s official duties.


Stress affects everyone in every walk of life. Submariners are no exception and there are numerous external stressors such as family or romantic relationships, health issues, and financial factors contributing to how well an individual is able to handle stressors. Furthermore, there are a number of emotional states that a submariner might experience while underway that may also affect his performance. For example, fatigue, monotony, and stress are common, and each of these can have an effect on how the submariner performs in his everyday duties. In fact, some research studies have shown that during the stress of combat-like training, the impairments in cognitive performance exceed those impairments seen following 0.10% Blood Alcohol Content. Put a different way, individuals who are highly stressed show more severe cognitive impairments than individuals who are legally drunk! The study that showed this result was done with Special Operations Forces, suggesting that these stress-induced impairments affect even highly-trained and skilled military personnel, and that performance under stress should be a concern for all branches of the military.

There is a large amount of variability in how people respond to stressors, with some people performing well as dictated by their training, whereas other people freeze or perform other inappropriate actions during stress. Certain branches of U.S. Special Operations Forces are able to spend considerable amounts of time, effort and expense to determine who is best suited to respond to extreme stressors. However, not every branch of military service can devote those kinds of resources to selecting the best performing individuals, and it would be highly desirable to have a brief and tow cost test that would help predict who responds optimally to stressors. For submariners, it would be advantageous to predict who is likely to be severely impaired by stressors. The Naval Submarine Medical Research Laboratory (NSMRL), located on Submarine Base New London, is developing protocols that will allow prediction of which individuals are best able to detect threats while under stress.

To address this in a research setting, it is first necessary to quantify an individual’s stress level, and there are a number of ways to do this. A simple method is to ask the individual questions such as, “On a scale of 1 to 10, with 10 being the most stress possible, how stressed are you right now?” This method is simple and easy to collect; however, it relies on the person telling the truth or being able to know what their true stress level is at that very moment. Often, these limitations make this self-reporting method undesirable. A more common way to quantify stress is to monitor the person’s physiological response to a stressor. This can be done by monitoring cardiac, respiratory or sweat responses. These are advantageous in that they each are very sensitive to an individual’s stress level, they are relatively easy to collect, and they are not easily biased by the person’s intentions or beliefs.

Another advantage of these physiology measures is that they can be collected continuously during a long training session, essentially allowing for thousands of data points to be collected. A slight disadvantage of these is that they require specialized equipment, and the individual must be attached to several small electrode contacts and wires in a manner similar to the setup for a clinical electrocardiogram.

A third way of monitoring stress is to examine the hormones the body produces following a stressful event. Cortisol is a common stress hormone that is produced by the adrenal glands, and it is critical to help prepare the body for a fight or flight response. Cortisol does this by increasing blood sugar and promoting metabolism to allow the organism to respond to an emergency. Cortisol, and many other hormone levels can be determined from blood, urine, or saliva. Examining hormones has the benefit of providing a global picture of how stressed the individual is- and collecting the necessary fluids can be relatively easy. When using saliva, the individual simply spits into a test tube, which is later analyzed for the relevant hormone. However, the disadvantage of this method is that hormonal changes can take many minutes to occur in the human body, so hormonal analysis does not allow for very good insight into the exact time or what specific events caused the stress.

NSMRL has combined some of these methods to heighten the ability to monitor submariners’ stress reactions. Specifically, both cardiac and galvanic skin response measures are continuously recorded, while concurrently collecting periodic saliva samples to examine cortisol and other stress hormones. This provides excellent resolution of the timing of events via the electrophysiology measures, as well as a view into the global stress level of the individual through the hormonal analysis .

One common method of inducing anxiety in humans in a research setting is through the use of a conditioned fear paradigm. In this paradigm, a neutral stimulus, such as an auditory tone, is repeatedly followed by an aversive event, such as a harmless electric shock. After experiencing multiple pairings of this tone shock combination, individuals display anxiety responses to the tone alone, even when the shock is no longer present. In effect, the person has been classically conditioned to fear something which was previously innocuous and which elicited no fear. Note that this is not/ear in the traditional sense of the word. Rather, it is an increase in anxiety or stress as measured by physiological measures, and these increases typically are similar to fearful responses, but at a very mild level. One advantage of these conditioned fear paradigms is that individuals typically learn to be fearful quickly (typically less than five minutes), making this an excellent approach for use in a research setting.

To assess performance, it is critical that there are decipherable performance measures available to analyze. For example, responding well in a combat situation involves vigilance, detecting threats, recalling proper response procedures from training protocols, initiating proper responses, and executing those responses until completion. Furthermore, combat is often complicated by a torrent of sensory information such as vocal commands, alarms, sudden noises, and random events.

Whereas replicating this complexity may be desirable for a training scenario, it is experimentally cumbersome to disentangle. If a participant fails in a training exercise, it is unclear at which particular phase they failed (e.g. Did they fail to detect the threat? Did they fail to recall the proper procedures? Did they get distracted while performing the proper procedure?).

For example, in one study examining performance on a military Combat Diver Qualification Course, personnel are placed in the water 3 miles from a target location on the beach, and must navigate underwater to this target without resurfacing, typically 45-55 minutes later. A navigation performance score was calculated by measuring how far from their intended target the students arrived on the beach. This test is advantageous in that it mimics real-life scenarios with exceptional realism and places significant stress on the divers. However, a diver’s performance score is affected by a wide variety of processes, making it unclear from the navigation score where a diver fails or succeeds. For example, did the diver fail to mark the goal correctly when starting? Or, did the diver have problems with his re-breathing device while underwater? Did the diver get disoriented and then have to find his way back, or did he proceed directly, but slowly? Each of these scenarios suggests different performance problems, and identification of these problem areas is a key component of proper training and performance.

To look at threat detection in a stressful environment, a simple task which also has been demonstrated by other researchers to be sensitive to changes in performance is required. The test selected was one where a participant sits in front of a computer screen and views a series of rapidly presented letters one at a time, in the middle of the screen. The goal of the participant is to press a response button as quickly as possible to a specific stimulus. However, when a different, but similar stimulus occurs, the participant must not respond. In this experiment, the participant must press a button every time he sees the letter “X” but not respond when shown the similar shaped letter “K”. The task demands can be manipulated to ensure that all participants are performing well, but not perfectly. The not perfect initial performance of participants is desirable because it allows easily observed improvements or impairments in performance. Certainly, this task is not threat detection in the typical use of the phrase, where a sailor might be scanning a sonar display for transient noises signaling the initial detection enemy submarine. Nonetheless, it is a simple version of threat detection, and it is an excellent task to use when first developing a testing protocol. Once researchers are comfortable with predicting performance on this simple, easy-to-understand task, then testing can move on to attempt predicting performance on tasks that are more realistic and more complicated.

Testing Method/Results

At Submarine Base New London, 27 sailors were tested on how well they detected the targets during stressful times when they knew and were anxious that they were about to hear a very loud (115dB) and strident general submarine alarm, compared to safe times when they knew they would not hear the alarm. The participants were asked to occasionally pause and provide a saliva sample that was analyzed for cortisol and other stress hormones. The results indicate that threat detection was impaired when the sailors were in the fearful state compared to the safe state, particularly in the time immediately before the loud alarm. Interestingly, it was observed that the greater the increase in saliva cortisol level, the better the threat detection. However, with the cardiac response, an opposite effect was observed; the lower the cardiac response, the better the threat detection. Hence, those who show a large increase in cortisol and a small increase in heart rate are the best performers during stress; those who show a small cortisol response and a large increase in heart rate perform poorly under stress.

The results with a submariner population are in agreement with research with Special Operations Forces indicating that stress can significantly impair performance. These results also suggest that it might be possible to predict those who are best able to perform under stress by examining their salivary cortisol and cardiac responses during stressful training scenarios. Interestingly, other researchers have also reported that salivary cortisol levels during stress are positively predictive of superior performance during military Combat Diver Qualification Course testing.

Whereas cortisol is the only reported measure derived from saliva that predicts performance, there are a variety of measures derived from the blood that have been shown to predict performance. Specifically, plasma neuropeptide-Y concentrations (released from the hypothalamus in the brain) are positively correlated with good performance under mock interrogations during a highly intensive U.S. Army survival school. And, concentrations of dehydroepiandrosterone and dehydroepiandrosterone sulfate (hormones released from the adrenal glands) are significantly and positively predictive of superior performance during military Combat Diver Qualification Course testing.

Lastly, there is a psychological measure called tendency to dissociate, which predicts performance. Dissociation is a state whereby the person distances his/her mind from the current moment and environment. People who dissociate may report being involved in a situation but watching the situation as a11 observer or outside of my body or with blunted intensity. Research has shown across a number of studies that the greater the tendency to dissociate during stress, the worse the performance.


This research lays the groundwork for predicting performance levels of submariners and other military personnel when under stress. Given that a powerful collection of predictors from saliva, blood, and psychological tests are now available, one of the next steps is to investigate training scenarios that are more realistic.

Early in the submarine pipeline, students participate in a number of stressful training scenarios, including a very realistic fire fighting trainer (students must extinguish actual fires producing significant vision impairing smoke and heat in a confined simulated engine room space), a Damage Control trainer (students must find and plug a series of increasingly difficult high pressure leaks from seawater piping systems in another confined simulated engine room space), and an Escape trainer (students must don the Submarine Escape and Immersion Equipment suit and conduct an escape from an actual submarine escape trunk located at the bottom of a 30 foot deep pool). For each scenario, stress response information derived from blood and saliva samples may be beneficial in creating an overall composite score of an individual’s ability to perform under stress. For example, if a sailor shows physiological indicators that he has poor stress reactivity during all three training scenarios, and his psychological profile (previously derived from the SUBSCREEN test that he completed at Basic Enlisted Submarine School (BESS)) suggests that he is questionable for submarine duty, this might be invaluable information for decision makers when deciding whether a sailor should continue in the submarine pipeline. Certainly, it is preferable to remove a sailor early in the pipeline during BESS, rather than finding out a year or more later while on a m1ss10n that the sailor is having major problems coping with stress on the boat, necessitating an emergency evacuation. Alternatively, a sailor identified in this manner might receive more focused training to help him improve his stress reactivity, or he can be considered for a rate that would not require peak stress responses. Conversely, a sailor who seems to perform very well under all stressors might be recommended for a leadership role in damage control situations.

Whereas this paper has discussed performance under stress, it is also important to realize that other physical and mental states such as monotony and fatigue can result in decreased arousal levels, which will also impair performance. Unfortunately, at present there is very little research examining these factors. NSMRL is beginning projects examining how these other psychological states affect performance and how performance can be optimized during fatigue or monotony. This research is in the early stages, but the work to date has laid important and solid groundwork in understanding how to predict performance during various physical and mental states. The goal of NSMRL research in this area is to help the Submarine Force distinguish and nurture its best and brightest submariners.

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