BACKGROUND: In recent years, the efficacy of various physical exercise programs in enhancing functional fitness among frail older adults has gained recognition. However, limited research has concurrently explored the long-term effects of exercise on brain health, stress biomarkers, and mental well-being. This study aimed to investigate the impact of two distinct chair-based exercise programs on salivary stress hormones and psychological well-being in frail older women over a 28-week period. METHODS: A total of 140 individuals participated in the enrollment phase, with 84 eligible participants randomly assigned to three groups. Following the intervention, data from 60 participants were analyzed across the multicomponent exercise (MCE, n = 23), elastic band muscle-strength exercise (ESE, n=19), and non-exercise control (CG n=18) groups. Salivary biomarkers of alpha-amylase (α-AMY) Cortisol (COR), alpha-amylase/cortisol ratio, psychological indicators and physical frailty (PF) and functional fitness were assessed pre- and post-intervention. RESULTS: Salivary COR levels exhibited a significant time × group interaction, with a moderate increase in MCE, a small decrease in ESE, and a substantial increase in CGne. Salivary α-AMY levels varied significantly over time and by group, with a small decrease in both exercise groups and a moderate increase in CGne. The α-AMY /COR ratio also displayed a significant interaction effect. Additionally, significant improvements were observed in PF compound scores, general self-efficacy, attitudes toward aging, and reductions in perceived stress and depressive symptoms (p <
0.05). CONCLUSIONS: Notably, the MCE program demonstrated greater benefits than ESE. The observed associations between changes in α-AMY levels, mental well-being, and functional fitness indicators contribute novel evidence on the psychophysiological adaptations to long-term exercise. Importantly, reductions in PF scores correlated with improvements in self-efficacy, attitudes toward aging, and handgrip strength, reinforcing the link between functional fitness, stress regulation, and psychological well-being. These findings emphasize the need for tailored exercise interventions to enhance both physiological resilience and mental health in frail older populations.