We investigated the reliability of physical activity monitoring based on trunk accelerometry in older adults and assessed the number of measured days required to reliably assess physical activity. Seventy-nine older adults (mean age 79.1 ± 7.9) wore an accelerometer at the lower back during two nonconsecutive weeks. The duration of locomotion, lying, sitting, standing and shuffling, movement intensity, the number of locomotion bouts and transitions to standing, and the median and maximum duration of locomotion were determined per day. Using data of week 2 as reference, intraclass correlations and smallest detectable differences were calculated over an increasing number of consecutive days from week 1. Reliability was good to excellent when whole weeks were assessed. Our results indicate that a minimum of two days of observation are required to obtain an ICC ≥ 0.7 for most activities, except for lying and median duration of locomotion bouts, which required up to five days.
van Schooten, Rispens, van Dieën, and Pijnappels are with MOVE Research Institute Amsterdam, Faculty of Human Movement Sciences, VU University Amsterdam, The Netherlands. Elders is with EMGO+ Institute, Department of General Practice and Elderly Medicine, VU University Medical Center, Amsterdam, The Netherlands. Lips is with MOVE Research Institute Amsterdam, Department of Internal Medicine and Endocrinology, VU University Medical Center, Amsterdam, The Netherlands. Address author correspondence to Mirjam Pijnappels at firstname.lastname@example.org.