Jeroen Van Cutsem is a postdoctoral Researcher at Royal Military Academy, Belgium. We talk about resilience and Circulating biomarkers but also about mental fatique.
We still don’t fully understand why some individuals perform under extreme stress—while others break down. Traditionally, resilience has been framed as something psychological: mindset, grit, mental toughness. But this research takes a different lens—one that looks inside the body.
The focus in this line of research is on Circulating biomarkers such as cortisol, IGF-1, neuropeptide Y (NPY), and DHEA(S).
Resilience was defined in a performance-oriented way as the ability to maintain cognitive and physical performance under challenging conditions. A decline in performance was therefore interpreted as lower resilience. Based on this definition, the review examined whether circulating biomarkers are associated with the ability to sustain performance under stress.
These biomarkers provide a physiological window into resilience.
Key insights are:
- Resilience is not about avoiding stress.
- It’s about how your body responds and recovers from it.
High performers often show a pronounced but controlled stress response, followed by a rapid return to baseline.
This pattern reflects what the researchers call physiological flexibility.
Not too flat. Not too extreme. But adaptive.
This raises important questions:
- How strong should a stress response be to be “optimal”?
- How fast should recovery occur?
- And how do we define these thresholds across individuals and contexts?
If we want to understand performance under pressure—whether in military, police, or high-stakes environments—we need to move beyond behavior alone. We need to understand the biology of adaptation. Not just: Can you handle stress?
But: How does your body handle stress—and how quickly does it reset?
If you’re working in training, leadership, or performance environments: this is a space worth watching. Because the future of resilience may be measurable, trainable—and physiological.
The conversation shifts to mental fatigue, which is defined as a psychobiological state with both a subjective (how it feels) and physiological (biological processes) component. It is typically caused by prolonged cognitive effort and fluctuates over time, meaning it is a state rather than a fixed trait. Mental fatigue can negatively affect both cognitive and physical performance.
Importantly, someone can be physically fit but mentally fatigued—for example after a long workday. Even without physical exertion, mental fatigue can still impair physical performance. This happens not because the body changes physiologically, but because tasks feel more effortful.
The key mechanism behind this is rating of perceived exertion (RPE): when mentally fatigued, effort feels higher, which limits performance. This effect is linked to brain processes, possibly involving increased neural drive (motor-related cortical potential), meaning the brain has to work harder to produce the same physical output.
Measuring mental fatigue remains challenging. There is currently no reliable wearable that can directly assess it, making simple self-report (“how mentally fatigued do you feel?”) still the most practical method.
In terms of countermeasures, some strategies show promise. Creatine supplementation can help sustain cognitive performance over time, while mouth rinsing with a bitter solution (e.g., caffeine-based) can provide an acute boost by stimulating the brain. Other approaches like breathing techniques, yoga, and recovery strategies may also help reduce mental fatigue.
Quote’s
“What we eventually would like to be able to get out of it is that the special forces operator… creates more insight in the functioning of his own body so that he or she can improve their performance and their resilience.”
“If you could develop… something that looks like [a continuous glucose monitor] and that could monitor other things than glucose, but for example cortisol, then that could be used… to increase the insight in your own body.”
“What I’m very enthusiastic about is actually to have such a sensor that enables us to continuously monitor more of the body.”
What is resilience?
“There are a lot of different definitions of resilience. It’s such a broad term. It’s used in so many ways that it eventually becomes a problem, because nobody knows what exactly is meant by the term resilience and how it exactly can be measured.”
“We went looking for a pragmatic solution in terms of how resilience can be measured, and we defined it in terms of performance.”
“For the special forces unit it is often: we want to perform at a high level and we want to keep performing at this high level.”
“What always comes back in each definition of resilience is your reaction to a stressor.”
“So it’s the reaction to a stressor and the ability to uphold your performance.”
“Is resilience now a trait—something that is fixed in time—or is it a state, something that can be trained and developed over time?”
“That makes the difference between screening for resilience or monitoring and training it.”
Topics
Introduction of the researcher (background, Royal Military Academy, psychophysiology, mental fatigue, performance)
Context of the research project (special forces, performance optimization, PESO project 2020–2025)
Reason for the study (growing importance and ambiguity of “resilience” in military settings)
Purpose of the systematic review (create a baseline of existing research on biomarkers and resilience)
Operational definition of resilience (ability to maintain cognitive/physical performance under stress)
Research question (are there circulating biomarkers linked to military-specific performance resilience?)
Main conclusion (yes: cortisol, IGF-1, DHEA(S), NPY are associated)
Discussion on definitions of resilience (broad concept, need for pragmatic definition)
Trait vs. state debate (resilience as stable vs. trainable; likely both)
Objective vs. subjective measurement (why biomarkers vs questionnaires; bias vs objectivity)
Explanation of stress and stress response (homeostasis disruption → neuroendocrine activation)
Role of key biomarkers (cortisol, DHEA, etc. and their physiological functions)
What constitutes a “resilient” biomarker response (peak + fast recovery vs flat or prolonged response)
Current limitations (no clear thresholds yet for “optimal” responses)
Classification of biomarkers (neuroendocrine, anabolic/growth, inflammatory)
Measurement methods (blood sampling, practical feasibility)
Future vision (continuous monitoring, wearables, personalized insight)
Follow-up research design (tracking candidates during selection/qualification courses)
Use of real stressors (physical, psychological, combined—runs, boxing, navigation tasks)
Integration with subjective measures (questionnaires, traits, life experience)
Goal of research (not prediction, but improving insight, training, and attrition rates)
Transition to second topic: mental fatigue (from PhD work)
Definition of mental fatigue (psychobiological state from prolonged cognitive effort)
Difference between mental and physical fatigue
Impact of mental fatigue on physical performance (via perceived exertion)
Underlying mechanism (RPE and brain processes, motor cortical potential)
Measurement challenges (brain activity during movement)
Practical measurement (no reliable wearable yet; self-report still most useful)
Strategies to reduce mental fatigue (creatine, mouth rinse, breathing, etc.)
Future vision for mental fatigue research (better sensors, continuous monitoring) Broader research ambitions (e.g., female health, hormonal monitoring