Functional reserve mitigates cognitive-motor dual-task costs in older adults: insights from age, cohort, and behavioural strategies

BACKGROUND: Age-related declines in somatosensory and cognitive function impair gait and increase fall risk, particularly under dual-tasking where cognitive and motor demands compete for limited resources. Functional reserve, the difference between one's maximum capacity and the minimum required for daily functioning, may buffer cognitive-motor interference. This study examines dual-task costs (DTCs) in healthy older adults, focusing on the roles of functional reserve, age-cohort effects, fall history and behavioural strategies such as "Stops Walking When Talking" (SWWT). METHODS: A sample of 4,443 healthy older adults (mean age: 74.8 years, range: 60.05-97.43) completed the Basel Motor-Cognition Dual-Task Paradigm. Participants walked at preferred speed (single task) and performed concurrent cognitive tasks during dual-task walking, targeting semantic (animal naming) and working memory (counting backwards) during walking or as single-tasks. Functional reserve was calculated as the proportional difference between preferred and fast walking speeds. Cognitive and motor DTCs were defined as proportional performance decline from single- to dual-task conditions. Linear mixed models identified predictors of DTCs and assessed logistic-linked fall history. Cohort was residualized from age. RESULTS: Significant cognitive-motor DTCs emerged, with both semantic and working memory tasks reflecting substantial performance declines when simultaneously walking. Working memory task induced less interference on walking whereas walking interferes more on working memory task, than semantic task, indicating domain-specific competition. Higher functional reserve predicted lower DTCs (motor, p < .001; cognitive, p = .006), suggesting its protective role. Age and cohort independently influenced DTCs, with more recent cohorts exhibiting higher cognitive-motor DTCs despite comparable ages. Fear of falling strongly predicted fall history (p < .001). SWWT was rare (2.8%), yet associated with higher motor DTCs but not falls, suggesting compensatory strategy. Subject-specific cognitive DTC did not correlate negatively to motor DTC after accounting fix effects, thus no trade-off was detected (r = - .024, p = .270). CONCLUSION: Functional reserve mitigates cognitive-motor interference in aging, while SWWT may reflect adaptive compensation. These findings identify functional reserve as potential intervention target and highlight the complexity of resource allocation, given the absence of a motor-cognitive trade-off and the presence of domain-specific effects. Future longitudinal studies should examine behaviour markers and refine cognitive-motor metrics.