de Matos, Dihogo2025-10-082025-10-082025-10-032025-10-042025-10-07http://hdl.handle.net/1993/39419Background: The cardiovascular and autonomic systems regulate blood pressure (BP) during postural changes, preventing orthostatic hypotension (OH), linked to dizziness, falls, and mortality. OH is influenced by age, sex, and frailty, yet short-term compensatory mechanisms are underexplored. Aging impairs BP regulation; females show higher OH prevalence due to lower cardiac baroreflex gain (CBG) and greater parasympathetic activity. Frailty further compromises these systems. This thesis includes a systematic review and experimental studies examining whether age, sex, and frailty affect responses to orthostatic stress and if progressive strength training (PST) improves these responses. Methods: Participants underwent two active standing tests: sitting (5 min) or lying (10 min), followed by up to 7 minutes standing. Systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), cardiac output (CO), stroke volume (SV), systemic vascular resistance (SVR), and HR were measured at baseline, immediately on standing, and throughout four specific phases after standing: phase 1 (0-30s), phase 2 (30-60s), phase 3 (60-180s), and phase 4 (300-420s). CO-SVR matching was evaluated to assess BP regulation timing. Heart rate variability (HRV) at baseline was analyzed in time and frequency domains. CPM was measured by the ratio between HR at the 30th beat and the 15th beat (HR 30:15 ratio) on standing. CBG was determined as the ratio of HR and SBP changes (ΔHR/ΔSBP) at specific phase time points (30s, 60s, 180s, and 420s). Results: This research demonstrated that frail older adults experience greater BP drops and slower recovery after standing, reflecting impaired cardiovascular regulation (Chapter 4). Older adults showed delayed BP regulation, lower short-term compensatory responses, HRV indexes, and CBG compared to young adults (Chapters 5-6). Despite similar BP regulation, older females relied more on SVR and males on CO, with no significant sex differences in autonomic responses (Chapters 7-8). Pre-frail and frail individuals exhibited delayed BP recovery and reduced HRV indexes (Chapters 9-10). Finally, a 12-week PST improved BP regulation timing and countered frailty, though it did not significantly affect autonomic modulation (Chapter 11). Conclusion: Age, sex, and frailty influence short-term cardiovascular and autonomic responses to orthostatic stress and support PST to enhance BP regulation and counteract frailty in older adults.engCardiovascular sytemAutonomic nervous sytemOrthostatic hypotensionFrailtySexOlder adults