Effect of Sitting Posture on Transversus Abdominis Thickness and Trunk Muscle Activity During a Valsalva-Based Simulated Straining Task in Healthy Young Adults

Abstract

Background/Objectives: Defecatory straining requires coordinated abdominal-wall and lumbopelvic muscle activity to generate propulsive pressure. This study investigated whether sitting posture alters ultrasound-derived transversus abdominis (TrA) contractile behavior and trunk surface electromyographic (EMG) activity during simulated defecatory straining. Methods: Twenty healthy young adults (10 females and 10 males) performed quiet sitting and a maximal Valsalva maneuver in four standardized postures: neutral posture (NP), anterior trunk lean with anterior pelvic tilt (AT), posterior trunk lean/posterior pelvic tilt (PT), and collapsed posture (CO). TrA thickness was measured with B-mode ultrasonography, and the TrA contraction ratio was calculated as Valsalva thickness divided by resting thickness. Surface EMG root mean square amplitudes of the superficial multifidus (sMF), erector spinae (ES), external oblique (EO), and internal oblique (IO) were analyzed at rest, during Valsalva, and as an activation ratio (Valsalva/rest). Repeated-measures, one-way ANOVA with Bonferroni-adjusted comparisons was used. Results: Ultrasonographic measurements demonstrated excellent intra-rater reliability (ICC = 0.984–0.999). Resting TrA thickness did not differ among postures. During Valsalva, AT showed greater absolute and relative TrA thickness change than NP. EMG analyses showed higher absolute sMF and ES activity in AT both at rest and during Valsalva, whereas EO and IO activity during Valsalva did not differ among postures. The ES activation ratio was highest in NP among postures. Conclusions: Anterior trunk lean with anterior pelvic tilt was characterized by a distinct pattern of TrA behavior and posterior trunk muscle activity during a Valsalva-based simulated straining task. However, whether this posture-dependent muscle response increases intra-abdominal or rectal propulsive pressure or improves defecatory function remains unknown. The accompanying EMG pattern suggests that this advantage is associated with greater resting postural activation of the posterior trunk wall rather than generalized abdominal effort.