Minimum Upright Period Selection in ActivPAL: Effects on Sit-to-Stand Detection and Reproducibility in Healthy Young Men

Background : The minimum upright period (MUP) and minimum nonupright period (MNUP) in PAL Technologies processing rules define time-based posture-classification thresholds for activPAL recordings. Although activPAL shows high validity for posture classification, the influence of these thresholds on detecting brief sit-to-stand (STS) transitions remains unclear. Research Question : Do different MUP/MNUP settings (1, 2, 5, and 10 s) affect the accuracy and equivalence of STS counts, and do they influence walking-related outcomes? Methods : Twenty-three healthy men performed 10 STS transitions at fixed interstand intervals (1, 3, 5, and 10 s) while wearing a thigh-mounted activPAL4. Data were reprocessed using symmetric MUP/MNUP settings (1, 2, 5, and 10 s). STS count, walking time during a standardized 100-step task, and step count were compared with reference values using Friedman tests, Bland–Altman analyses, mean absolute error, and equivalence testing with two one-sided tests (equivalence bounds ± 1 STS). Results : STS detection was strongly dependent on MUP/MNUP selection. At interstand intervals ≥ 3 s, MUP/MNUP 1–2 s yielded STS counts equivalent to the reference (two one-sided test p < .001, mean absolute error ≤ 0.1), whereas longer thresholds (5–10 s) substantially undercounted STS. At 1-s intervals, all settings underestimated STS. Walking time and step count showed minimal bias and were unaffected by MUP/MNUP selection. Significance : STS quantification from activPAL data depends on processing thresholds rather than device hardware. Short MUP/MNUP settings (1–2 s) are required for accurate STS detection when transitions are brief, whereas walking-related outcomes are robust. Explicit reporting of MUP/MNUP is essential for reproducibility.