Effects of Abdominal Bracing Versus Abdominal Hollowing on Abdominal Muscle Thickness and Activation During Static and Dynamic Trunk Stabilization Tasks in Healthy Adults

BACKGROUND Abdominal hollowing selectively targets deep abdominal muscles, whereas abdominal bracing emphasizes global activation of the superficial musculature. This study aimed to compare the effects of abdominal bracing and abdominal hollowing on abdominal muscle thickness and activation during trunk stabilization tasks in healthy adults. MATERIAL AND METHODS Eighteen healthy adults (9 males, 9 females) participated in a randomized crossover study. Abdominal bracing and hollowing were performed in random order with a 1-week wash-out period. Muscle thickness of the rectus abdominis (RA), external oblique (EO), internal oblique (IO), and transversus abdominis (TrA) were measured using ultrasound imaging. Surface electromyography assessed activation of the RA, EO, and IO during a static hook-lying position and a dynamic supine toe-tap task. RESULTS RA thickness was greater during bracing than hollowing (10.07±1.55 mm vs 9.22±1.46 mm, p=0.032), whereas IO and TrA thicknesses were greater during hollowing (IO: 8.65±3.18 mm vs 6.63±2.14 mm; TrA: 4.08±1.23 mm vs 2.41±0.80 mm, both p<0.05), representing an approximately 31% increase in TrA thickness. During the static task, activation of the RA, EO, and IO was higher during hollowing (p<0.05), whereas no significant differences were observed during the dynamic task. CONCLUSIONS Abdominal hollowing preferentially enhances deep muscle activation during low-load static conditions, whereas abdominal bracing promotes global muscular engagement. With increasing task demands, stabilization strategies converge, indicating task-dependent neuromuscular coordination.