Assessment of abdominal muscle function during a simulated unilateral weight-bearing task using ultrasound imaging.

STUDY DESIGN: Cross-sectional study. OBJECTIVE: To investigate the function of the transversus abdominis (TrA) and internal oblique (10) muscles bilaterally during a simulated weight-bearing task using ultrasound imaging. BACKGROUND: An important aspect of neuromuscular control at the lumbopelvic region is stabilization. Biomechanical models have predicted that activation of transversely oriented muscles, such as the TrA and 10 muscles, can stiffen the sacroiliac joints and actively stabilize the pelvis for weight bearing. METHODS AND MEASURES: Nineteen healthy subjects were positioned in supine lying with their right heel against a footplate linked to a force transducer. Each subject performed a static simulated weight-bearing task of the right lower extremity. Ultrasound imaging was used to assess resultant changes in thickness of the 10 and TrA muscles, as well as the lateral slide of the anterior abdominal fascia on each side of the abdomen alternately. Muscle thickness and slide of the fascia were assessed at standardized force levels (0% and 25% of body weight). RESULTS: Substantial increases (P < .0001) in mean (+/- SD) thickness of the 10 (18.5% +/- 9.7%) and TrA (24.7% +/- 175%) muscles during the weight-bearing task were measured. Lateral movement (slide) of the anterior abdominal fascia of the TrA muscle also occurred (mean +/- SD, 1.3 +/- 2.0 mm; P = .014) with weight bearing. Changes in muscle thickness and amount of slide were similar for the left and right side of the abdomen (P > or = .11). CONCLUSION: The findings are consistent with biomechanical models that predicted symmetrical activation of the deep transversely oriented lumbopelvic muscles in healthy subjects in response to a unilateral functional weight-bearing task. Ultrasound imaging as a measurement tool represents a noninvasive method for measuring abdominal muscle function in functional axial loading.