Tai Chi Intervention for Balance Function in Older Adults: Predictive and Modifying Roles of Circadian Age

Inclusion Criteria: * Aged 55 years or older * Community-dwelling * Able to walk independently (assistive devices such as canes are permitted) * Berg Balance Scale (BBS) score ≤52 at screening * Able to understand and follow verbal instructions * Willing to wear a wrist accelerometer continuously for 7 days at baseline * Willing to accept random group assignment * Willing to undergo blood collection during the study * Provides written informed consent Exclusion Criteria: * Regular Tai Chi or yoga practice (more than once per week) within the past 6 months * Severe cognitive impairment (Montreal Cognitive Assessment \[MoCA\] score \<20 at screening, or previously diagnosed dementia) * Unstable cardiovascular disease (myocardial infarction, stroke, or hospitalization within the past 3 months) * Severe musculoskeletal disease that limits exercise participation * Neurological diseases affecting balance (e.g., Parkinson's disease, severe peripheral neuropathy) * Currently participating in another exercise intervention study * Planned surgery or extended absence during the study period * Contraindications to functional near-infrared spectroscopy (fNIRS) examination (e.g., scalp injury, metallic implants in the head region) * Severe psychiatric illness (e.g., schizophrenia, bipolar disorder) * Considered unable to safely participate in moderate-intensity physical activity as judged by the study physician or attending healthcare provider