Reliability and correlates of cross-sectional area of abductor hallucis and the medial belly of the flexor hallucis brevis measured by ultrasound

BACKGROUND: Weakness of the intrinsic foot muscles is thought to produce deformity, disability and pain. Assessing intrinsic foot muscles in isolation is a challenge; however ultrasound might provide a solution. The aims of this study were to assess the reproducibility of assessing the size of abductor halluces (AbH) and the medial belly of flexor hallucis brevis (FHBM) muscles, and identify their relationship with toe strength, foot morphology and balance. METHODS: Twenty one participants aged 26–64 years were measured on two occasions for muscle cross-sectional area using a Siemens Acuson X300 Ultrasound System with 5-13 MHz linear array transducer. Great toe flexor strength was measured by pedobarography, the paper grip test and hand-held dynamometry. Foot morphology was assessed by foot length, truncated foot length, Foot Posture Index (FPI) and dorsal arch height. Balance was measured by the maximal step test. Intra-class correlation coefficients (ICC(3,1)) were used to evaluate intra-rater reliability. Pearson’s correlation coefficients were performed to assess associations between muscle size and strength, morphology and balance measures. To account for the influence of physical body size, partial correlations were also performed controlling for truncated foot length. RESULTS: Intra-rater reliability was excellent for AbH (ICC(3,1) = 0.97) and FHBM (ICC(3,1) = 0.96). Significant associations were found between cross-sectional area of AbH and great toe flexion force measured standing by pedobarography (r = .623, p = .003),), arch height measured sitting (r = .597, p = .004) and standing (r = .590, p = .005), foot length (r = .582, p = 006), truncated foot length (r = .580, p = .006), balance (r = .443, p = .044), weight (r = .662, p = .001), height (r = .559, p = .008), and BMI (r = .502, p = .020). Significant associations were found between cross-sectional area of FHBM and FPI (r = .544, p = .011), truncated foot length (r = .483, p = .027) and foot length (r = .451, p = .040). Significant partial associations were found between AbH and great toe flexion force in standing by pedobarography (r = .562, p = .012) and FHBM and the FPI (r = .631, p = .003). CONCLUSIONS: Measuring the cross-sectional area of AbH and FHBM with ultrasound is reproducible. Measures of strength, morphology and balance appear to relate more to the size of AbH than FHBM. After controlling for physical body size, cross-sectional area of AbH remained a significant correlate of great toe flexor strength and might be a useful biomarker to measure early therapeutic response to exercise.