Patterns of shortening and thickening of the human diaphragm

To study how the human diaphragm changes configuration during inspiration, we simultaneously measured diaphragm thickening using ultrasound and inspired volumes using a pneumotachograph. Diaphragm length was assessed by chest radiography. We found that thickening and shortening were greatest during a breath taken primarily with the abdomen. However, the degree of thickening was greater than expected for fiber shortening, 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 should remain constant, and thickness would be inversely proportional to length 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 cylindrical zone of apposition; hence, as the zone of apposition shortens while the dome radius remains constant, the ratio of total surface area to combined length (i.e., dome + zone of apposition) must decrease and thickening 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 insertion into a thin flat tendon. Comparison of calculations with these types of models to data from human subjects indicates that the unexpected thickening in the zone of apposition is explained by the peculiar geometry of the diaphragm. The greater thickening of the diaphragm in the zone of apposition suggests that more of the muscle mass and more sarcomeres are retained in the zone of apposition as the dome descends. Physiologically, this greater thickening may have importance by reducing wall stress in the zone of apposition and reducing the work or energy requirements per sarcomere.