Cardiorespiratory Fitness and Acute High-Intensity Exercise Effects on Brain and Cognitive Function in Older Adults

Cognitive decline is one of the most consequential changes associated with aging, and executive functions-the set of higher-order cognitive processes that allow people to plan, adapt, and control their behavior-are especially susceptible to age-related change because they depend heavily on the prefrontal cortex, a brain region that shows some of the earliest and most pronounced structural and functional changes with age. Cognitive flexibility, the ability to shift rapidly between competing task rules or mental sets, is one specific executive function that supports many everyday activities, including driving, multitasking, and decision-making, and it can be measured experimentally using a task-switching paradigm that compares performance on trials requiring a rule switch to trials that repeat the same rule.

Physical exercise is increasingly recognized as a non-pharmacological approach that may help preserve cognitive and brain health in older adults, but the evidence on the acute (single-session) effects of exercise on cognition is inconsistent. Outcomes appear to depend on exercise intensity, duration, the timing of cognitive testing relative to exercise, the population studied, and the specific cognitive domain assessed. High-intensity interval training (HIIT)-brief, repeated bouts of vigorous exercise separated by recovery periods-produces strong cardiovascular, metabolic, and neuroendocrine responses in a short amount of time and has been associated with greater gains in cardiorespiratory fitness and cognition than continuous moderate-intensity exercise in some populations. However, one influential framework, the reticular-activating hypofrontality (RAH) model, proposes that intense physical exertion can temporarily divert neural resources away from the prefrontal cortex toward brain regions controlling movement and basic physiological regulation, which could transiently impair executive function immediately after high-intensity exercise. This model has rarely been tested directly in older adults using simultaneous brain imaging and cognitive testing, and it is not yet known whether an individual's cardiorespiratory fitness level protects against this kind of transient impairment.

This study is designed to address that gap by examining, within the same older adults, how a single bout of HIIT affects both performance on a task-switching test of cognitive flexibility and underlying prefrontal brain activity, and by testing whether cardiorespiratory fitness-measured on a continuous scale rather than as a simple high/low grouping-moderates these acute responses. Brain activity will be assessed using functional near-infrared spectroscopy (fNIRS), a non-invasive optical imaging method in which a sensor cap placed on the participant's forehead emits and detects near-infrared light to track relative changes in oxygenated and deoxygenated hemoglobin concentrations in the prefrontal cortex; increases in oxygenated hemoglobin coupled with decreases in deoxygenated hemoglobin are interpreted as increased regional brain activation. Compared with functional MRI, fNIRS is more portable, less expensive, and more tolerant of movement, while offering better spatial resolution than EEG, making it well suited for studying exercise-related brain responses in older adults.

Participants will attend two laboratory visits on separate days at the Exercise and Cognition Lab. The first visit establishes each participant's cardiorespiratory fitness and familiarizes them with study procedures: after health screening and baseline physical measures, participants will practice the task-switching paradigm on a laptop using PsychoPy® stimulus presentation software until their performance stabilizes, in order to minimize learning effects during the later experimental session. They will then complete a submaximal graded exercise test on a cycle ergometer, following a validated multistage protocol (American College of Sports Medicine submaximal cycle ergometer protocol) consisting of four consecutive 3-minute stages of progressively increasing workload. Heart rate and workload recorded during each stage are used to extrapolate, via the established linear relationship between heart rate and oxygen consumption, an estimate of maximal oxygen uptake (VO₂max) at the participant's age-predicted maximal heart rate, providing a safe and time-efficient index of cardiorespiratory fitness without requiring participants to exercise to true maximal exertion.

The second visit constitutes the experimental exercise session. After a health status check, participants will be fitted with the fNIRS sensor cap, positioned over prefrontal regions according to the standard EEG 10-20 placement system, and will complete the task-switching cognitive assessment while brain activity is recorded (pre-exercise timepoint). The fNIRS cap will then be removed and participants will complete the HIIT protocol on a cycle ergometer: four 4-minute bouts of high-intensity cycling, controlled to approximately 85-90% of individually determined maximal power output and heart rate, separated by 3-minute periods of passive or light active recovery. Heart rate, rating of perceived exertion, and pedaling cadence are monitored and recorded each minute throughout the exercise bout, and workload is adjusted as needed to keep participants within the prescribed intensity range; exercise may be modified or stopped at the study team's discretion based on participant signs or symptoms. Heart rate throughout both visits is tracked continuously using a wearable optical heart rate sensor worn on the upper arm or forearm. Immediately after the exercise bout, participants are refitted with the fNIRS cap and repeat the task-switching assessment while brain activity is again recorded (post-exercise timepoint), allowing within-participant comparison of cognitive and prefrontal neural responses before versus immediately after a single session of high-intensity exercise.

Cardiorespiratory fitness values derived from the submaximal test will be examined as a continuous moderating variable in statistical models of the pre-to-post exercise changes in cognitive and brain outcomes, alongside relevant covariates such as age, sex, grip strength, and lean body mass, using repeated-measures and mixed-effects statistical approaches. This design allows the study to test not only whether HIIT produces measurable shifts in executive function and prefrontal activation immediately afterward, but also whether individual differences in fitness help explain who is more or less susceptible to such shifts.

This project builds on preliminary pilot work conducted by the same research team using a related experimental design, which demonstrated that the task-switching paradigm behaves as expected (slower responses on switch versus repeat trials) and that reliable, good-quality fNIRS signal can be obtained from older adult participants using this cap montage and analysis pipeline. Findings from this study are intended to clarify the neurocognitive mechanisms underlying acute responses to high-intensity exercise in older adults and to inform more precise, individualized guidance on how and when high-intensity exercise might be safely incorporated into the daily routines of older adults, including consideration of timing relative to cognitively demanding activities.