Background Symmetrical activation of the transversus abdominis (TrA) and internal oblique (IO) muscles is able to enhance spinal stability and functional efficiency. Previous studies have demonstrated side-to-side symmetry in the thickness of these deep abdominal muscles at rest and during contraction in the supine position. It remains unclear whether symmetry is presented in axial-loading postures such as double- and single-leg standing. Objective To examine side-to-side symmetry of TrA and IO activation in standing and single-leg standing in healthy adults. Methods Fifty-five healthy participants underwent ultrasound imaging to measure the thickness of the TrA and IO muscles at rest and during an abdominal drawing in maneuver (ADIM) in both standing and single-leg standing. Activation ratio (AR) was calculated as the ratio of ADIM thickness to resting thickness. A two-way repeated-measures ANOVA was used to compare ARs between sides and between muscles. Results In both conditions, the IO demonstrated significantly higher AR on the dominant side than the non-dominant side (p <0.001), whereas no side-to-side difference was observed in the AR of TrA. The TrA activation was significantly greater than the IO on non-dominant side (p < 0.001), while no significant difference between TrA and IO activation was observed on the dominant side. Conclusion Healthy adults exhibit symmetrical TrA activation during functional upright postures, while the IO shows a modest predominance on the dominant-side. The AR of TrA exceeds the IO on the non-dominant side. These reference values may assist clinicians in differentiating physiological activation patterns from pathological alterations of the deep abdominal muscles.
Sitilertpisan et al. (Wed,) studied this question.