Fitness, Ageing, and Bilingualism (FAB): The Benefits of Regular Exercise and Bilingualism for Language Abilities in Healthy Ageing.

Inclusion Criteria: * Non-smokers for at least 5 years prior to enrollment. * Normal or corrected-to-normal vision and hearing. * No diagnosed language impairments (e.g., dyslexia, stuttering). * No illness or disease that would prevent safe participation in high-intensity interval training (HIIT). * Self-reported not meeting global physical activity recommendations (i.e., \<150 minutes/week of moderate-intensity physical activity). * Norway only: Written health certificate from the participant's general practitioner confirming eligibility for participation. Exclusion Criteria: * Any cardiovascular, metabolic, respiratory, neurological, kidney, liver, or cancer disease that would prevent safe participation in HIIT. * United Kingdom only: Severe electrocardiogram abnormalities (e.g., ST depression, long QT, heart block, wide QRS). * United Kingdom only: Elevated blood pressure at screening (systolic \>160 mmHg and/or diastolic \>90 mmHg).