This laboratory-based interventional study aims to determine which combination of physical activity intensity and cognitive activity difficulty yields the greatest acute cognitive benefits in community-dwelling older adults.

The main question it aims to answer is:

- Which combination of physical and cognitive activity results in the greatest short-term improvement in cognitive functioning?

Researchers will compare four different combinations of physical intensity and cognitive task difficulty to determine whether one combination stands out. In addition, the study examines whether these effects differ depending on baseline cardiorespiratory fitness.

Participants will:

• undergo a preparatory phase in which their cardiorespiratory fitness, individualized physical activity intensity levels, and cognitive activity difficulty levels are established
• visit the laboratory four times, with one week between visits, to complete each combination of physical and cognitive activity (light vs. moderate physical activity × easy vs. challenging cognitive activity)
• complete cognitive tests immediately before and after each condition

Recognizing the escalating dementia rates, its profound social and economic consequences and the lack of curative treatments, the World Health Organization (WHO) emphasizes the importance of reducing modifiable risk factors for dementia, as outlined in Action Area 3 of the WHO Global action plan on the public health response to dementia 2017-2025. Previous research suggests that combining physical activity and cognitive activity may produce greater cognitive benefits than either activity alone, but it remains unclear which specific combinations are most effective in older adults. In particular, little is known about how physical activity intensity and cognitive activity difficulty interact, and whether cardiorespiratory fitness influences these effects. The present study addresses this knowledge gap in community-dwelling older adults.

The study is conducted in two consecutive parts. In the preparatory phase, participants attend one laboratory visit. During this phase, baseline screening and descriptive assessments will be performed, cardiorespiratory fitness will beestimated using the 6-Minute Walk Test, individualized physical activity intensity zones will be determined using heart-rate reserve, and cognitive activities will be classified as easy or challenging based on subjective ratings. Participants will also be familiarized with treadmill walking and simultaneous physical and cognitive task performance.

In the fundamental phase, eligible participants will complete four laboratory sessions in randomized order, each involving one combination of physical and cognitive activity: light physical activity with easy cognitive activity, light physical activity with challenging cognitive activity, moderate physical activity with easy cognitive activity, and moderate physical activity with challenging cognitive activity. During each session, participants walk on a treadmill at a prescribed heart-rate-based intensity while simultaneously performing cognitive tasks. Cognitive functioning is assessed immediately before and after each session using three tasks from the CANTAB neuropsychological test battery: a) Multitasking test (MTT), (b) Delayed Matching to Sample (DMS), and (c) Match to Sample Visual Search (MTS), allowing the study to examine the short-term cognitive effects of each physical activity × cognitive activity combination. The overall aim is to identify the optimal combination of simultaneous physical and cognitive activity for acute cognitive benefits in older adults.

Inclusion Criteria:

• Aged 65 years or older
• Community-dwelling
• Able to safely walk on a treadmill and complete the 6-Minute Walk Test
• Sufficient Dutch proficiency to understand study instructions and complete study procedures

Exclusion Criteria:

• MoCA score < 23
• Severe cognitive or physical impairment that would hinder participation or understanding of study procedures
• Use of medication affecting heart rate
• Any medical condition contraindicating engagement in physical or cognitive activity
• Severe visual or hearing impairment that would prevent valid participation in the study procedures

De-identified individual participant data collected during the study will be made available to other researchers. The exact dataset(s), supporting documents, timing, and access procedures will be specified after completion of the main analyses and publication of the primary study results.

De-identified individual participant data will become available after publication of the primary study results and are expected to remain available indefinitely in an institutional or open research repository.