Architectural design of the human intrinsic hand muscles.

The architectural features of twenty different muscles (18 intrinsics and 2 thumb extrinsics, n = 180 total muscles) were studied. Muscle length, mass, fiber pennation angle, fiber length, and sarcomere length were determined. From these values, physiologic cross-sectional area and fiber length/muscle length ratio were calculated. Intrinsic muscle lengths were relatively similar to one another, which we interpreted as representing a space constraint within the hand. However, several specialized architectural designs were observed: lumbrical muscles had an extremely high fiber length/muscle length ratio, implying a design toward high excursion. The first dorsal interosseous and adductor pollicis had physiologic cross-sectional areas comparable to those of extrinsic muscles and much greater than those of the other intrinsic muscles. The interosseous muscles had relatively high physiologic cross-sectional areas with low fiber length/muscle length ratios, suggesting their adaptation for high force production and low excursion. Taken together, these observations illustrate the underlying structural basis for the functional capacities of the intrinsic muscles.