We read with great interest the article by Rivas et al.,  which found that pain scores, but not opioid consumption, were inversely related to postoperative mobilization. We applaud the use of accelerometers to provide measurable mobilization data over the routine use of nonquantifiable parameters.

Nevertheless, the authors’ definitions of mobilization are worth noting. Rivas et al. defined mobilization as “either sitting or standing,” using “upright 90 degrees” posture as sitting and “walking” posture as standing. Assuming the accelerometer was placed on the patient’s chest following the manufacturer’s instructions, the upright 90 degrees position could include sitting in bed, at the edge of the bed, in a chair, or even standing still. Each of these corresponds to different static positions and potential levels of mobility performance. Such distinctions are often made to describe patients’ functional status and set mobility goals in the acute inpatient setting.  Mobility has been defined as “getting patients out of bed, including sitting in a chair, toileting at bedside or bathroom, standing, and ambulating  but the effect of these distinct positions and activities on postoperative outcomes remains unknown.  It would be informative for the authors to provide more detailed findings for different postures separately to understand how each may affect clinical outcomes.

While the use of accelerometers in clinical research has increased exponentially in recent years many non–research-grade accelerometers lack validation, particularly among the inpatient population. There is substantial variability in measurements of physical activity between device models, wear location, patient population, and study setting.  It is unclear if the accuracy and precision in identifying different postures have been established for ViSi Mobile (Sotera Wireless, Inc., USA), the device used in this study. This could explain the median of 32 (interquartile range, 23 to 40) hours of usable data during the 48-hour period and removal of a considerable fraction of monitoring gaps (up to 50% of a subject’s monitored time).

We deeply value the efforts by Rivas et al. to incorporate accelerometer measurements to characterize postoperative mobility in a much-needed quantitative manner. However, we hope the authors’ conclusion suggesting a minimal daily mobilization of 2 h/day is interpreted with caution because the definition of mobilization was broad, encompassing a range of static upright positions and active mobility, and the device measurements were incomplete and may still await validation.