Multidirectional Trunk Movements Reveal Hidden Symmetry Loss in Stroke: An Electromyography-Based Comparative Study.

: Stroke and hemiplegia disrupts symmetrical activation of skeletal and abdominal muscles, impairing trunk control and functional movement. Although asymmetry is also present in healthy adults, its magnitude and patterns differ with neurological impairment. Understanding trunk muscle symmetry across functional tasks in healthy individuals and patients with stroke is essential for targeted rehabilitation strategies. : A comparative cross-sectional study was conducted including healthy adults and patients with stroke. Muscle activation symmetry of the rectus abdominis, external oblique, internal oblique, and multifidus was analyzed across four trunk movements: flexion, extension, and lateral flexion to the dominant or non-dominant side. A two-way repeated measures ANOVA examined main and interaction effects of condition, muscle, and group. : Trunk muscle symmetry was significantly influenced by the movement conditions, and patterns of change differed between groups. While no consistent differences were observed across muscles, specific interactions revealed condition-dependent variations, particularly between abdominal and deep spinal muscles. Lateral flexion elicited the greatest asymmetry, with distinct response patterns in healthy individuals compared with patients with stroke. : This study highlights the importance of addressing movement-specific demands in trunk rehabilitation. Rather than focusing on isolated muscles, interventions should consider the dynamic and condition-dependent nature of symmetry to optimize functional recovery in patients with stroke.