The relation between abdominal muscle asymmetry and trunk postural stability: An ultrasound imaging study

BACKGROUND: The core stability theory has emphasized symmetric co-contraction of both anterolateral abdominal muscles (AAM), but there is a lack of research on whether the symmetric co-contraction of AAM is related to the postural stability of the trunk. OBJECTIVE: This study aimed to investigate the correlation between the symmetric co-contraction of bilateral AAM and trunk angular displacement during sudden trunk perturbation. METHODS: Eighteen subjects were asked to maintain half-seated posture against backward trunk perturbation. Muscle thicknesses of external oblique (EO), internal oblique (IO), and transversus abdominis (TrA) were measured using ultrasound imaging. 3D motion analysis system was utilized to calculate the amount of trunk angular displacement during perturbation. RESULTS: There were significant differences of side-to-side muscle thickness in TrA (P= 0.02) and EO (P= 0.02), but the difference disappeared during sudden external loading. No significant correlation was identified between the asymmetry of abdominal muscle thickness and the amount of trunk angular displacement. CONCLUSIONS: These findings indicate that the side-to-side asymmetry of AAM measured in a static position is rarely related to the core stability because the asymmetry can disappear during trunk stabilization.