Reliability and Validity of Lumbar-Mounted IMU for Postural Sway Assessment in Healthy Adults

Inertial measurement units (IMUs) are increasingly used as portable tools for postural sway assessment, for which force platforms are the gold standard. This study aimed to evaluate the short-term test–retest reliability, concurrent validity, and construct validity of postural sway metrics derived from a lumbar-mounted IMU, using centre of pressure (CoP) measures obtained from a force platform as the reference standard. Sixty-nine healthy adults performed two quiet standing conditions, with eyes open (EO) and eyes closed (EC), while lumbar centre of mass (CoM) acceleration and CoP displacement signals were recorded simultaneously. Ten postural sway metrics in both the time and frequency domains were extracted from the CoM and CoP signals. The IMU-based approach demonstrated moderate-to-good reliability for most time-domain parameters. Mean velocity exhibited the highest reliability (intraclass correlation coefficient, ICC = 0.779 in EO and 0.813 in EC), comparable to that obtained with the force platform. Moderate-to-strong correlations were found between IMU- and force platform-derived measures for distance-, velocity-, and area-related metrics (r = 0.52 to 0.73). Furthermore, both approaches detected significant differences in postural sway between the EO and EC conditions, supporting their sensitivity to changes in sensory input. Frequency-domain parameters showed lower reliability and weaker associations, and systematic differences between the two approaches were found for several parameters. These findings suggest that lumbar-mounted IMUs may provide a practical approach for assessing specific postural sway metrics, particularly mean velocity and distance-related parameters, when force platform assessment is not feasible, although the two approaches should not be considered interchangeable.