Because they are easy to obtain, clinically relevant, and widely used, the reflection index (RI) and stiffness index (SI) have become prominent fea-tures derived from arterial pulse wave (APW) analysis. To explore their poor-ly understood physiology, we focused on the association between RI and SI responses and the systolic blood pressure (SBP) and heart rate (HR) incre-ments induced by active standing (AS). Therefore, in 23 healthy volunteers, we assessed the effects of AS on the beat-to-beat time courses of SBP, di-crotic wave pressure (DWP), RI (DWP/SBP), SBP-to-DWP time (Δt), SI (height/Δt), interbeat period (IP), and systolic time (ST) series, all features obtained from APW analysis. AS provoked: 1. An initial decline in RI, followed by a return toward base-line, which was rapid at first and then slow (Table), sustained until the end. During the declining phase, DWP decreased at a slower rate than SBP; in the rapid rising phase, DWP increased at a faster rate than SBP, and during the slow increment phase, SBP reduced its rate of increase. 2. A slight decrease in SI, followed by a first rapid and then slow increase, which was maintained until the end of AS (Table). This response exhibits a similar pattern to ST and Δt, characterized by a slight initial increase followed by rapid shortening, sustained until the end. The ST-Δt correlation was 0.77±0.13. Our findings indicate that the RI response of decline and return to baseline is linked to the unequal responses of SBP and DWP, and that the SI response of a slight reduction followed by an increase is closely correlated with the slight widening followed by shortening of ST provoked by the rise in HR. Thus, the RI and SI responses are produced by the fluctuating increments of SBP and HR caused by sympathetic activation triggered by AS.