Study aim: To determine the reliability and validity of a time domain heart rate variability (HRV) index during free-living physical activity (FLPA).

Material and methods: Eight white-collar workers participated in this study. RR intervals (time between consecutive R-peaks of the PQRS complex) were recorded using the Polar V800 heart rate (HR) monitor upon awakening and at work on 16 different days. A total of 127 cycles of sitting periods followed by walking breaks were included for consecutive pairwise analysis for reliability. The HR values from the orthostatic test (OT) were compared with the corresponding values at work.

Results: The HR values showed high levels of repeatability [the coefficient of variation (CV) during sitting and walking at work was 4.71 and 3.99%, respectively, with a typical error (TE) of 3.73 (3.34-4.25) and 3.65 (3.31-4.09)], but they did not correlate with the corresponding OT HR upon awakening (r = 0.28 for supine vs. sitting and r = 0.05 for standing vs. walking, p > 0.05). The root-mean-square difference of successive normal RR (RMSSD) was revealed not to be repeatable [CV values during sitting and walking were 19.99 and 29.05%, respectively, with a TE of 7.9 (7.15-8.85) and 9.43 (8.53-10.57)].

Conclusions: Analyzing RMSSD from daily routine activities was not reliable, and therefore validity cannot be assumed. RMSSD should therefore be calculated from RR intervals recorded in standardized conditions, such as during the OT upon awakening.