PATTERNS OF SHORTENING AND THICKENING OF THE HUMAN DIAPHRAGM

To study how the human diaphragm changes configuration during inspirat ion, we simultaneously measured diaphragm thickening using ultrasound and inspired volumes using a pneumotachograph. Diaphragm length was as sessed by chest radiography. We found that thickening and shortening w ere greatest during a breath taken primarily with the abdomen. However , the degree of thickening was greater than expected for fiber shorten ing, assuming parallel muscle fibers and no shear. So, to clarify this unexpected finding, we considered geometric models of the diaphragm. How a muscle thickens as its fibers shorten is critically dependent on geometry. Thus, if a flat rectangular sheet of muscle shortens along one dimension, surface area-to-length ratio along this dimension shoul d remain constant, and thickness would be inversely proportional to le ngth during shortening. The simplest model of the diaphragm, however, is a cylindrical sheet of muscle in the zone of apposition capped by a dome; the ratio of surface area to radial fiber length in the dome is substantially less than the ratio of area to length of the cylindrica l zone of apposition; hence, as the zone of apposition shortens while the dome radius remains constant, the ratio of total surface area to c ombined length (i.e., dome + zone of apposition) must decrease and thi ckening of the muscle correspondingly must increase more than expected for a simple rectangular strip. A similar relationship can be derived between thickening and length in a muscle sheet with a wedge-shaped i nsertion into a thin flat tendon. Comparison of calculations with thes e types of models to data from human subjects indicates that the unexp ected thickening in the zone of apposition is explained by the peculia r geometry of the diaphragm. The greater thickening of the diaphragm i n the zone of apposition suggests that more of the muscle mass and mor e sarcomeres are retained in the zone of apposition as the dome descen ds. Physiologically, this greater thickening may have importance by re ducing wall stress in the zone of apposition and reducing the work or energy requirements per sarcomere.