Changes in intra-abdominal pressure during postural and respiratory activation of the human diaphragm

In humans, when the stability of the trunk is challenged in a controlled ma nner by repetitive movement of a limb, activity of the diaphragm becomes to nic but is also modulated at the frequency of limb movement. In addition, t he tonic activity is modulated by respiration. This study investigated the mechanical output of these components of diaphragm activity. Recordings wer e made of costal diaphragm, abdominal, and erector spinae muscle electromyo graphic activity; intra-abdominal, intrathoracic, and transdiaphragmatic pr essures; and motion of the rib cage, abdomen, and arm. During limb movement the diaphragm and transversus abdominis were tonically active with added p hasic modulation at the frequencies of both respiration and limb movement. Activity of the other trunk muscles was not modulated by respiration. Intra -abdominal pressure was increased during the period of limb movement in pro portion to the reactive forces from the movement. These results show that c oactivation of the diaphragm and abdominal muscles causes a sustained incre ase in intra-abdominal pressure, whereas inspiration and expiration are con trolled by opposing activity of the diaphragm and abdominal muscles to vary the shape of the pressurized abdominal cavity.