BACKGROUND AND AIM: We previously showed that colonic gas infusion increases the girth and modifies the muscular activity of the anterior abdominal wall. We hypothesized that abdominal accommodation to volume loads is an active process instrumented by the coordinated activity of the anterior wall and the diaphragm. METHODS: To increase intraabdominal volume in healthy subjects, a gas was infused into the colon (1.44 L in 1 h) while measuring girth (by tape measure) and electromyography (EMG) activity of the anterior wall (via four pairs of surface electrodes) and the diaphragm (via six ring electrodes over an esophageal tube in the hiatus). After preliminary feasibility studies (N = 12), postural activity (N = 6) and responses to colonic gas loads, both with the trunk erect (N = 8) and in supine position (N = 8), were studied. A morphometric analysis was performed by computed tomography, image analysis (N = 8). RESULTS: In the erect position, anterior wall tone was higher and diaphragmatic tone was lower than in the supine position. With the trunk erect, gas infusion induced diaphragmatic relaxation (by 21 ± 3%; P < 0.05) and anterior wall contraction (16 ± 4% EMG increment; P < 0.05). By contrast, in the supine position, it induced diaphragmatic contraction (15 ± 6%, P < 0.05), while the anterior wall, in the absence of postural tone, showed no change (3 ± 2%, NS). Gas infusion was associated with girth increase (7.3 ± 1.0 mm with the trunk erect and 8.6 ± 1.4 mm in the supine position) and diaphragmatic ascent (17.6 ± 5.2 mm; P < 0.05). CONCLUSION: The degree of abdominal distension produced by intraabdominal volume increments results from posture-related abdomino-phrenic muscular responses. © 2008 by Am. Coll. of Gastroenterology.