Background and purpose: Deep lumbar muscles have a major role in lumbar stabilization and evidence suggests that these muscles especially multifidus muscle becomes dysfunctional in the presence of low back pain. Therefore, measurement of these muscles in healthy subjects and patients may be of great value to understand the stabilization function of the muscles. The purpose of this study was to assess the reliability of lumbar multifidus muscle size measurement using ultrasonography (US) in healthy adolescent males.

Materials and methods: This methodological research was performed in 20 healthy boys (15-17 years of age). Lumbar multifidus muscle size was measured at the level of L5 using a real time US device. Two images were taken on the same day with an hour interval to assess the within- day reliability and the third image was taken one week later to determine between- days reliability. Cross sectional area (CSA), anterior posterior dimension (APD), and lateral dimension (LD) were measured in each image.

Results: The Intraclass Correlation Coefficient for within day and between days reliability in healthy adolescents varied, ranging between 0.72 to 0.76 for CSA, 0.79 to 0.89 for APD, and 0.79 to 0.84 for LD.

Conclusion: The results indicated that US could be considered as a reliable tool to measure the muscle thickness in children. Therefore, this non-invasive method can be used in the assessment of lumbar multifidus muscle and also to monitor the effect of different interventions in this population.