Mechanics of the human diaphragm during voluntary contraction: statics.

We related diaphragm electromyographic activity (Edi) to transdiaphragmatic pressure (Pdi) in man during graded inspiratory efforts. Estimates of rib cage and abdominal volume displacements were based on their anteroposterior (AP) diameter changes. The diaphragm was assumed to contract isometrically when subjects performed inspiratory efforts against a closed airway at specified abdominothoracic configurations, increasing Edi and Pdi while holding lung volume and rib case and abdominal AP diameters constant. The relationship between Pdi and Edi depends primarily on abdominothoracic configuration rather than lung volume. For equal increments in lung volume, the Pdi developed at constant Edi is four to eight times more sensitive to changes in abdominal than in rib cage AP diameter. We demonstrate an isofunctional state of the diaphragm at different lung volumes, when increases in lung volume and rib cage AP diameter are compensated for by slight decreases in abdominal AP diameter, resulting in a constant relationship between Edi and Pdi. We conclude that diaphragm shortening is reflected more directly in abdominal displacement than in lung volume change.