Mechanical role of the abdominal muscles in relation to posture

Abdominal muscle activity was studied in 10 normal naive subjects during quiet breathing in different body positions. Electromyograms of the upper and lower portions of the external oblique and rectus abdominis were recorded with bipolar needle electrodes and changes in abdominal and rib cage displacements were measured using linearized magnetometers. The abdominal muscles were always silent in the supine posture. In contrast, 8 of the 10 subjects showed tonic abdominal muscle activity in the standing posture and 4 of the subjects did so also in the 45 degrees head-down position; when present, this activity was always greater in the dependent (where the hydrostatic pressure was greater) than in the non-dependent portion of the muscles. Tonic abdominal muscle activity was associated with a reduction of abdominal volume throughout the respiratory cycle and a reduction of the end-expiratory lung volume below the neutral position of the respiratory system. We conclude that (1) tonic activity is present in the abdominal muscles in most standing subjects, (2) this activity is primarily related to the magnitude of hydraulic pressure exerted by the abdominal contents on the abdominal wall, and (3) in most normal individuals standing at rest, the end-expiratory position is at least in part actively determined. Such an abdominal muscle use, although causing quiet breathing to depart from the relaxed abdominothoracic configuration, may be associated with minimum respiratory work.