The behaviour of the abdominal muscles during inspiratory mechanical loading.

Abdominal muscle activity was studied in eight normal subjects, while seated and supine, during inspiratory resistive and elastic mechanical loading. Electromyograms of the external oblique and rectus abdominis muscle were recorded using unipolar needle electrodes and changes in antero-posterior dimensions of the abdomen (Vab) were measured using linearized magnetometers. Elastic loading evoked abdominal muscle activity during expiration, especially in the rectus abdominis, in seven seated subjects while resistive loading did so in five. In four runs activity was also detectable throughout inspiration. Abdominal muscle activity was associated with a reduction in Vab throughout the respiratory cycle. When the subjects were supine loads did not evoke abdominal muscle activity. We conclude that abdominal muscle recruitment during inspiratory mechanical loading may facilitate inspiration by increasing diaphragmatic length.