en Physiotherapy Physiotherapy Original Research Original Research <div><strong>Background: </strong>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.<br> <strong>&nbsp;&nbsp; Methods:</strong> 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.<br> &nbsp; &nbsp;&nbsp;<strong>Results: </strong>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.&nbsp;<br> &nbsp;&nbsp; <strong>Conclusion:</strong> 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.</div> Reliability, Ultrasonography, Psoas major, Thickness, Low back pain 999 1004 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-4999-1&slc_lang=en&sid=1 Zahra Yaghoubi zahra230@yahoo.com 200319475328460063684 200319475328460063684 No Department of Physiotherapy, School of Rehabilitation Sciences, Iran University of Medical Sciences and Health Services, Tehran, Iran Ismail Ebrahimi Takamjani ebrahimitakamjani.e@iums.ac.ir 200319475328460063685 200319475328460063685 Yes Department of Physiotherapy, School of Rehabilitation Sciences, Iran University of Medical Sciences and Health Services, Tehran, Iran Javad Sarrafzadeh j.sarrafzadeh@gmail.com 200319475328460063686 200319475328460063686 No 1. Department of Physiotherapy, School of Rehabilitation Sciences, Iran University of Medical Sciences and Health Services, Tehran, Iran Asghar Rezasoltani Rezasoltani a_rezasoltani@sbmu.ac.ir 200319475328460063687 200319475328460063687 No School of Rehabilitation Sciences, Shahid Beheshti university of Medical Sciences, Tehran, Iran Nader Maroufi naderpt10@gmail.com 200319475328460063688 200319475328460063688 No Department of Physiotherapy, School of Rehabilitation Sciences, Iran University of Medical Sciences and Health Services, Tehran, Iran