Differential Effects of Abdominal Bracing and Hollowing on Scapular Muscle Activation Patterns During the Y-Raise Exercise

Background: Trunk stability is closely linked to scapular muscle recruitment during upper-limb activities, yet the differential effects of abdominal bracing and hollowing on scapular muscle activation patterns remain unclear. Objective: To compare the effects of abdominal bracing, hollowing, and normal breathing on scapular muscle activation and activation ratios during the Y-raise exercise in individuals with rounded shoulder posture (RSP). Methods: Thirty-six individuals with RSP participated in this within-subject repeated-measures study. Surface EMG was used to record upper trapezius (UT), lower trapezius (LT), and serratus anterior (SA) activity during the Y-raise exercise under three conditions: normal breathing, abdominal hollowing, and abdominal bracing. Muscle activation (%MVIC) and UT/LT and UT/SA ratios were compared using repeated-measures ANOVA with Bonferroni-adjusted pairwise comparisons. Results: The bracing instruction condition was associated with greater LT activation than hollowing and normal breathing on both sides (adjusted p < 0.05), whereas UT activation did not differ across conditions. Both bracing and hollowing were associated with greater SA activation than normal breathing on both sides (adjusted p < 0.05). Bracing was associated with lower UT/LT ratios than both hollowing and normal breathing on both sides. The UT/SA ratio was lower with bracing than with normal breathing on both sides and lower than with hollowing only on the left side. Conclusions: Abdominal bracing instruction was associated with acute changes in relative scapular EMG activation during the Y-raise, characterized primarily by greater LT activation rather than reduced UT activity. These preliminary findings reflect responses to different instructional conditions, and their clinical relevance in symptomatic populations remains to be determined.