Ultrasound Evaluation of the Abductor Hallucis Muscle in Youth: A Reliability Study

ABSTRACT Introduction Abductor hallucis (AbdH) is an intrinsic foot muscle and a stabiliser of the medial longitudinal arch. Its cross‐sectional area (CSA) is routinely measured by ultrasound (US) and is highly relevant to foot muscle development. However, the reliability of portable US for measuring AbdH CSA in healthy youth is unknown. Therefore, the aim of this study was to examine the inter‐device association, inter‐rater reliability and inter‐session reliability of portable US for measuring AbdH CSA in youth. Methods Twenty‐seven youths (11–17 years; 14 male, 13 female) participated, with inter‐device association and inter‐rater reliability assessed in a subgroup of n = 12 and intra‐rater inter‐session reliability in n = 15. AbdH CSA images were captured on two occasions one week apart using a 5–15 MHz handheld US unit (L15HD, Clarius), while a single image was captured using a cart‐based US device (3–16 MHz; HS40, Samsung) for inter‐device comparison. Reliability was assessed using intraclass correlation coefficients (two‐way mixed‐effects, absolute agreement). Inter‐device comparison was evaluated using Pearson's correlation, linear regression and paired‐sample t ‐test for systematic differences, and measurement error was quantified using the standard error of measurement (SEM). Results Portable and cart‐based US measurements of AbdH CSA showed a strong positive correlation ( r = 0.938, p < 0.001), although mean values were 0.19 cm 2 higher with the cart‐based device ( p = 0.002). Inter‐rater reliability was excellent (ICC(3, 3) = 0.934, 95% CI: 0.786–0.980) with no significant difference between assessors ( p = 0.823), minimal bias (−0.01 cm 2 ), and a SEM of 0.039 cm 2 . Inter‐session reliability was also excellent (ICC(3, 3) = 0.969, 95% CI: 0.911–0.989) with no significant difference between sessions ( p = 0.788), minimal bias (0.007 cm 2 ) and a SEM of 0.013 cm 2 . Conclusion A portable US device demonstrated excellent inter‐rater and inter‐session reliability for measuring AbdH CSA in youth and was strongly associated with cart‐based US measurements.