Abstract / Summary
Resting heart rate variability (HRV) describes cardiac interval modulation under a particular physiological state; it does not establish how effectively cardiovascular control adapts when that state changes. This narrative review integrates evidence on physical activity and exercise training across HRV, blood pressure variability (BPV), and baroreflex sensitivity (BRS). We use cardiovascular autonomic flexibility to describe stimulus-appropriate adjustment and subsequent recovery of cardiovascular regulation. Exercise interventions often increase resting vagally mediated HRV, whereas evidence for BPV and BRS depends on population, measurement method, and recording timescale. Evidence that these resting adaptations translate into improved challenge responses is less consistent, including randomised evidence of improved fitness without corresponding changes in cardiovascular reactivity or recovery. Interpretation requires distinguishing cardiac vagal modulation from sympathetic vasomotor regulation, spontaneous cardiac BRS from complete baroreflex function, and beat-to-beat BPV from ambulatory or longer-term variability. Neither a larger response nor lower variability is inherently favourable. We propose an assessment framework comprising resting state, response to a standardised challenge, coupling among simultaneously recorded signals, and recovery kinetics. Isometric exercise, postural or central-volume changes, and controlled breathing provide complementary experimental demands but require protocol-specific interpretation. Cardiovascular autonomic flexibility is presented as a research framework rather than a validated biomarker. Its value depends on demonstrating reproducibility, mechanistic specificity, responsiveness to training, and incremental prediction beyond resting measures and conventional cardiovascular risk factors.