IntroductionMagnetic resonance imaging (MRI) is frequently used to quantify and determine any association between muscle morphology and neuromusculoskeletal pathology or dysfunction through the evaluation of muscle cross-sectional area (CSA) (1,2,3,4,5,6,7) and muscle fatty infiltrate (MFI) (5,8,9,10,11,12). These parameters of muscle morphology are also often investigated to identify expected age-related muscle changes (13,14).Many, though not all, studies analysing spinal muscle CSA and MFI report intra-and/or inter-rater reliability as part of their investigations (4,5,7,15,16), many of which demonstrate strong intra-rater reliability of lumbar erector spinae and multifidus CSA, with an intraclass correlation (ICC) ranging from 0.85 (5, 15) to 0.98 (4) and an inter-rater reliability ranging from 0.77 (7) to 0.85 (5). However, the specific reporting of intra-and/or inter-rater reliability findings is often lacking regarding the description of the methodology of assessor agreement and assessor experience. Further, most studies have published a global muscle CSA ICC for intra-rater reliability by reporting a single value, rather than distinguishing between the right and left sides. Battié and colleagues (17) have, however, detailed lumbar paraspinal muscle volume ICC for both the left and the right sides to range from 0.90 to 0.99 in patients with lumbar radiculopathy. Published inter-and intra-rater reliability outcomes of other trunk muscles have been noticeably limited, although one study Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts was identified to report a global intra-rater reliability of the psoas muscle CSA, which was found to be high with an ICC of 0.92 (15).Intra-and inter-rater reliability of MFI is less frequently reported in the literature compared to muscle CSA. In addition, due to the range of MFI quantification methods, MFI intra-and inter-rater reliability findings may not be easily compared across studies. Using opposedphase MRI, intra-rater reliability of lumbar multifidus and erector spinae MFI was found to be high in patients with persistent low back pain, with an ICC range of 0.86 to 0.88 (5). This study also found an equally high inter-rater reliability (ICC 0.85 -0.87) for MFI in these muscles.Although the inter-rater reliability of generalised lumbar extensor muscle CSA has been identified, the reporting of inter-and intra-reliability metrics for the left and right sides of individual muscle CSA and MFI is infrequent. Furthermore, reports for other trunk muscles, such as the abdominals are, to our knowledge, lacking. Therefore, the purpose of this study was to identify an inter-rater reliability metric for the quantification of the left and right individual lumbar spine and abdominal muscle volume and MFI between two novice assessors. Materials and Methods ParticipantsAxial MRI scans of 10 healthy male participants from a University population who were recruited as part of a larger study were included. Data collectionTransverse T1-weighted magnetic resona...