Intrarater reliability of musculoskeletal ultrasound imaging of psoas major muscle in patients with subacute low back pain and healthy controls.

Psoas major (PM) is a challenging muscle from the functional and anatomical point of view. The dysfunction of this muscle can result in low back pain (LBP). This study aimed to assess the intrarater reliability of ultrasound imaging (USI) of PM muscle thickness in subacute LBP patients and healthy participants without LBP in rest and during muscle contraction conditions. PM thickness was measured in all lumbar segments (L1-L5) using a USI device in 10 healthy and 10 subacute LBP participants. The intrarater data were assessed on the same day with 1- hour interval and after 7 days. Intraclass correlation coefficients (ICC), standard error of measurement (SEM), minimal detectable change (MDC), and independent t test were used for analyses. Significant level was set at 0.05. PM thickness in all lumbar levels had excellent reliability (ICC range 80-98) for both groups and conditions. SEM (0.42- 2.29) and MDC (1.16-6.34) were low, and PM thickness was greater than rest in contraction condition. There were no significant differences between the 2 groups in PM thickness. The USI demonstrated good intrarater reliability for assessing PM thickness in patients with subacute LBP. The thickness of PM in patients with subacute LBP was similar with that in healthy participants.