Bac kgr o und:Real tim e ultrasound im aging (RTUI) is used to m easure transversus abdom inis (TrA) thickness in low back pain (LBP ) populations. However, individuals with chronic low back pain (C LBP ) pose specific im aging challenges, such as older age and higher body m ass index, com pared to asym ptom atic populations or acute and sub-acute LBP groups. These challenges potentially increase m easurem ent error and m ay require different im aging m ethods. O bje c t ive s :This review describes the m ethodologies and reported reliability for RTUI m easurem ent of TrA specific to C LBP populations. Me t ho ds :A system atic database search of Medline, C INAHL, P EDro, the full C ochrane library, Scopus, and Inform it identified 20 studies that used RTUI to m easure TrA of C LBP participants. Two independent raters appraised the quality of the studies using the Q ualSyst and the Q AREL critical appraisal tools. Re s ult s :Methodological quality varied from low to high. Methods for patient and transducer positioning and m uscle m easurem ent were inconsistent between studies. Eight articles cited reliability results from past studies of non-C LBP populations. O nly two studies reported reliability in C LBP populations specifically and found higher Intraclass C orrelation C oefficients for thickness m easures at rest (0.63-0.97), com pared to thickness change over tim e or contraction ratios (0.28-0.80). Co nc lus io ns :Inconsistency of m ethodology, variable m ethodological quality, and lim ited and variable reliability reporting was highlighted in this review. This LBP subgroup poses challenges for RTUI, therefore future research should include standarized m ethods for im age acquisition. This will im prove the quality of study m ethods, reliability of TrA m easurem ent, and im prove the applicability and com paribility of research evidence available to clinicians. Ke ywo r ds Transversus abdom inis ReliabilityReal tim e ultrasound Back pain Int roduct ionReal tim e ultrasound im aging (RTUI) is used in research and clinical practice to m easure various m usculoskeletal structures. [1][2][3] In the last 20 years, a plethora of studies have m easured transversus abdom inis (TrA). 4-8 Through its attachm ent to the linear alba and thoracolum bar fascia, TrA provides spinal stability via m ultiple m echanism s, including feed-forward activation. 9 Hence it has been investigated in sports perform ance, and in relation to low back pain (LBP ). 10Several studies agree that TrA m orphology and function is altered in patients with low back pain. [11][12][13][14] When com pared to healthy populations, both delayed anticipatory activation and reduced m uscle thickness has been reported in individuals who had experienced at least one previous episode of LBP , but were asym ptom atic at the tim e of RTUI. 12 In part, this provides insight into the progression from acute to recurrent low back pain and then chronic pain via central sensitization. This system atic review identifies the current evidence for m easurem ent m...