This study aims to examine the role of exercise intensity in the acute effects of a single bout of aerobic exercise on neurotrophic factors, biomarkers of stress and inflammation, working memory task-related changes in prefrontal cortex oxygenation in adults with sub-clinical levels of stress-related symptoms.

This study initially set out to include 43 adult (20 - 40 years) participants experiencing stress-related symptoms and 43 healthy controls, matched by age, sex, and fitness. These procedures were approved by the Swedish Ethical Review Autority (Approval number 2022-07297-01 and 2023-08181-02). Due to recruitment difficulties the design was later changed to include all individuals with sub-clinical stress symptoms and using stress as a covariate instead of comparing groups of healthy and stressed individuals (Ethical review approval number 2025-01786-02). This is a randomized-order crossover experimental trial with three experimental conditions separated by a wash-out period of 7 days: 20-min leg cycling exercise at moderate intensity, 20-min leg cycling exercise at vigorous intensity, and 20-min seated rest (passive control condition).

Prior to entering the study, study inclusion/exclusion criteria will be controlled using digital screening. Before the experimental sessions, all participants will complete two introductory study visits. At the first introductory visit, the participant's maximal oxygen uptake (VO2max) will be determined using a fitness test. This information will be used to determine the individual workloads (% of individual VO2max) that the participant will be cycling at during the experimental session. At the second introductory visit, participants will complete a familiarization session, in which key methodological aspects of the study will be introduced to participants. Participants' augmentation index, as a measure of arterial stiffness, will also be completed during this visit.

In each of the three experimental sessions, participants will perform a computerized working memory task (n-back task) pre- and post-exercise (or seated rest). During the working memory task, functional near-infrared spectroscopy (fNIRS) will be used to measure task-related changes in prefrontal cortex oxygenation. As such, each participant will complete six fNIRS measurements, two measurements (pre, post) for each of the three experimental sessions (moderate, high, control). Simultaneous with the fNIRS measurements, blood flow velocity in the left and right middle cerebral artery will be assessed during the n-back tasks using Transcranial Doppler ultrasound. At each experimental session, blood will also be sampled from the antecubital vein at six time points: pre the first n-back task, pre and post the cycling exercise (or seated rest), pre and post the second n-back task, and post the final 10-min seated rest. Whole blood samples will be assessed for blood gases, acid-base status, electrolytes, hematocrit level, platelet count, and glucose. The separated blood plasma and serum samples will be used for measurements of neurotrophic factors, lactate, biomarkers of stress and inflammation, and factors involved in blood glucose control. Blood samples will also be used for targeted DNA screening for alleles of the BDNF.

Additionally, the participant's current mood, mental and physical fatigue, heart rate variability (HRV) and blood pressure will be assessed during four 10-min seated rest periods: pre and post n-back, and pre and post cycling exercise (or seated rest). Prior to each experimental study session, participants will have assessments of sleep quality the night before, current health status, physical activity level during the week prior to the test day, sleepiness level the last 10 min, and current mood. Participant's sleep quality and heart rate variability during the night before and after each experimental session will also be assessed.

A release of an updated version of this study was attempted prior to completion of the study on October 3 2025, but due to a shutdown of the US goverment this was not possible to register until in November 24 2025.

Inclusion Criteria:

Participants experiencing sub-clinical levels of stress-related symptoms:

• Age: 20-40 years
• Absence of contraindications to physical exercise
• Do not participate in any other study
• Total score on the self-assessment scale 4-factor Shirom-Melamed Burnout Measure-19 lower than or equal to 4.00

Exclusion Criteria:

• Medical contraindication to catheterization of the antecubital vein and blood sampling
• Dementia
• Alcohol abuse
• Smoking
• Chronic medication that is considered to affect study outcomes
• Chronic disease/syndrome such as migraine, fibromyalgia, chronic fatigue syndrome, bipolar syndrome, high blood pressure, cardiovascular disease, chronic obstructive pulmonary disease (COPD) or other lung disease
• Post-COVID
• Other somatic diseases, disorders, or injuries that may affect the physical ability during physical exercise

There is no current plan to make individual participant data available to other researchers.

The outcomes assessors follow standard operation procedures with automatic analyses and are blinded to study condition and individual stress ratings when analysing the collected data.