Multimodal Validation of an sEMG-Based Visual Biofeedback System for Deep Abdominal Muscle Activation in Healthy Adults: A Randomized Controlled Proof-of-Concept Trial

Background: Portable biofeedback technologies are increasingly used in rehabilitation; however, the validity of surface electromyography (sEMG) as a surrogate indicator of deep abdominal muscle function remains unclear. This study aimed to validate a portable sEMG-based visual biofeedback system by examining its relationship with ultrasound-derived measures of deep abdominal muscle activation. Methods: Twenty-nine healthy adults were randomly assigned to a Visual Biofeedback group (n = 14) or a Verbal Feedback group (n = 15). Both groups performed a standardized 2-week core stabilization program. Muscle activation of the deep abdominal muscle complex (transversus abdominis–internal oblique; TrA–IO) and external oblique (EO) was measured using sEMG (%MVIC), while ultrasound imaging was used to assess transversus abdominis thickness and contractile activity (ADIM–Rest index). Between-group differences and correlations between EMG and ultrasound variables were analyzed. Results: The Visual Biofeedback group demonstrated significantly greater improvements in TrA–IO activation and in the preferential activation ratio (TrA–IO/EO) compared to the Verbal group (p = 0.004). Ultrasound analysis revealed significantly greater increases in TrA thickness and contractile activity in the Visual group (p < 0.001). A significant positive correlation was observed between changes in TrA–IO activation and TrA thickness (ρ = 0.51, p < 0.001). Conclusions: Portable sEMG-based visual biofeedback demonstrated physiological relevance by reflecting ultrasound-derived changes in deep abdominal muscle function. These findings support the use of sEMG as a practical surrogate tool for monitoring deep core muscle activation and highlight the potential of portable biofeedback systems in scalable and accessible rehabilitation.