Patterns of shortening and thickening of the human diaphragm

Wait, J. L., and R. L. Johnson. Patterns of shorteningand thickening of the human diaphragm. J. Appl.Physiol. 83(4): 1123-1132, 1997.To study how the human diaphragm changesconfiguration during inspiration, we simultaneously measured diaphragmthickening using ultrasound and inspired volumes using apneumotachograph. Diaphragm length was assessed by chest radiography.We found that thickening and shortening were greatest during a breathtaken primarily with the abdomen. However, the degree of thickening wasgreater than expected for fiber shortening, assuming parallel musclefibers and no shear. So, to clarify this unexpected finding, weconsidered geometric models of the diaphragm. How a muscle thickens asits fibers shorten is critically dependent on geometry. Thus, if a flatrectangular sheet of muscle shortens along one dimension, surfacearea-to-length ratio along this dimension should remain constant, andthickness would be inversely proportional to length during shortening.The simplest model of the diaphragm, however, is a cylindrical sheet ofmuscle in the zone of apposition capped by a dome; the ratio of surfacearea to radial fiber length in the dome is substantially less than theratio of area to length of the cylindrical zone of apposition; hence,as the zone of apposition shortens while the dome radius remainsconstant, the ratio of total surface area to combined length (i.e.,dome + zone of apposition) must decrease and thickening of the musclecorrespondingly must increase more than expected for a simplerectangular strip. A similar relationship can be derived betweenthickening and length in a muscle sheet with a wedge-shaped insertioninto a thin flat tendon. Comparison of calculations with these types ofmodels to data from human subjects indicates that the unexpectedthickening in the zone of apposition is explained by the peculiargeometry of the diaphragm. The greater thickening of the diaphragm inthe 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 byreducing wall stress in the zone of apposition and reducing the work orenergy requirements per sarcomere.