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A new study has found a connection between a person’s heart rate patterns at rest and their likelihood of passing an extremely demanding selection course for a special operations forces naval unit. The findings, published in the journal Applied Psychophysiology and Biofeedback, suggest that this physiological measurement could offer an objective way to help identify candidates who are better equipped to handle high-stress environments.
The researchers, led by Yosef Kula of the University of Haifa, conducted this study to explore better ways of selecting candidates for elite military roles. Special operations forces soldiers must perform complex tasks under immense physical and psychological pressure.
The selection processes are designed to be incredibly difficult to identify individuals who possess the necessary resilience. These processes are not only a significant burden on the candidates but also represent a substantial investment of resources. Finding objective, reliable predictors of success could make these selection processes more accurate, benefiting both the organization and the applicants.
The scientific team focused on a biological process related to the body’s stress response. Our bodies have an autonomic nervous system that operates automatically to regulate functions like breathing and heart rate. This system has two main branches. The sympathetic nervous system is responsible for the “fight or flight” response, preparing the body for action by increasing heart rate and alertness. The parasympathetic nervous system promotes a “rest and digest” state, helping the body calm down and recover.
A key component of the parasympathetic nervous system is the vagus nerve. The activity of this nerve helps moderate the body’s stress reactions. One way to measure the influence of the vagus nerve is by looking at heart rate variability. This is not the same as heart rate, which is the number of beats per minute. Instead, heart rate variability measures the small, natural fluctuations in the time intervals between consecutive heartbeats. A higher level of heart rate variability when a person is at rest is generally considered a sign of a healthy, flexible, and adaptive nervous system, one that is well-prepared to respond to and recover from challenges.
To investigate its predictive power, the researchers designed a historical prospective study, meaning they analyzed data that had been collected in the past. The data came from 365 healthy male high school students, aged 16 to 17, who were candidates for an Israeli naval commando unit. The team analyzed a standard 10-second electrocardiogram that was taken for each candidate as part of a routine medical examination approximately two months before they attended the five-day selection course. From this recording, they calculated a specific measure of heart rate variability known as the root mean square of successive differences between normal heartbeats.
In addition to the heart rate data, the researchers had access to other information on each candidate. This included their score on a 2-kilometer run, which served as a measure of physical fitness, a score from an intelligence test, and a score from a structured psychological interview designed to assess their suitability for military combat service. The main goal was to see if heart rate variability, measured well before the stressful event, could predict the outcome of the selection course, which was categorized simply as pass or fail.
The initial analysis directly compared the candidates who passed the course with those who failed. The results showed that the successful group had a higher average heart rate variability than the group that did not pass. The successful candidates also tended to have better run scores, higher intelligence scores, and higher scores on the adjustment interview. This first finding was in line with the researchers’ hypothesis that a more adaptive nervous system would be associated with success.
However, the picture became more complex in the next stage of analysis. The researchers used a statistical technique called logistic regression to determine which of these factors was the most powerful predictor when considered all together. In this analysis, the link between heart rate variability and passing the course disappeared. The only measure that remained a significant predictor was the 2-kilometer run score. This result suggested that the initial connection might have been driven by physical fitness, as fitter individuals often exhibit higher heart rate variability.
To explore the relationship further and untangle the influence of these overlapping factors, the research team employed another analytical strategy. They created a smaller, matched sample of participants. They did this by finding pairs of candidates, with one individual who passed and one who failed in each pair. The key was that the two people in each pair had nearly identical scores on the 2-kilometer run and the psychological adjustment interview. By matching candidates on these strong predictors, the researchers could better isolate the unique contribution of heart rate variability. This process resulted in 29 matched pairs.
When analyzing this carefully matched group, the original finding re-emerged with clarity. Within these pairs, the candidates who successfully passed the selection course consistently had higher heart rate variability than their matched counterparts who failed. This finding suggests that when two individuals have comparable levels of physical fitness and similar psychological profiles, the person with higher resting heart rate variability may have an edge. It points to heart rate variability as an independent indicator of a person’s capacity to manage the intense demands of the course.
The authors believe these results support the idea that heart rate variability could be a useful, evidence-based tool in the screening process for tactical professionals. Since it reflects both physiological and cognitive capacities for self-regulation, it offers an objective glimpse into a candidate’s underlying resilience. Improving the accuracy of selection could lead to significant resource savings and reduce the number of candidates who are ultimately unsuccessful after enduring the physically and mentally exhausting course.
The study also highlights the importance of when the measurement is taken. By obtaining the reading two months before the selection test, the researchers captured a true resting state, which may be more predictive of future performance under stress.
The study did have several limitations that the authors acknowledged. Because it was a retrospective study, the researchers had no control over the exact conditions in which the electrocardiograms were recorded. The measurements were also taken from a very short 10-second recording, which is not the standard length for a detailed heart rate variability analysis. Finally, the measurement was taken at only a single point in time.
For future research, the scientists suggest that studies should measure heart rate variability using longer electrocardiogram records to ensure greater reliability. They also propose that taking measurements at several different time points, both before and during training, could provide a more complete understanding of its role in performance and adaptation. Despite its limitations, this study provides new evidence that the body’s own regulatory patterns, reflected in the subtle rhythm of the heart, may help identify those best suited for the world’s most demanding professions.
The study, “Does Vagal Nerve Activity Predict Performance in a Naval Commando Selection Test?,” was authored by Yosef Kula, Zev Iversen, Adi Cohen, Ariel D. Levine, and Yori Gidron.
