Skeletal muscle is not only highly organized to function at the microscopic level, the arrangement of the muscle fibers at the macroscopic level also demonstrates a striking degree of organization. Skeletal muscle architecture is defined as "the arrangement of muscle fibers relative to the axis of force generation." The functional properties of a whole muscle depend strongly on its architecture. The various types of arrangement are as numerous as the muscles themselves, but for convenience we often refer to three types of fiber architecture.
Muscles with fibers that extend parallel to the muscle force-generating axis are termed parallel or longitudinally arranged muscles (Left). While the fibers extend parallel to the force-generating axis, they never extend the entire muscle length. Muscles with fibers that are oriented at a single angle relative to the force generating axis are termed unipennate muscles (Middle). The angle between the fiber and the force-generating axis generally varies from 0° to 30°. Most muscles fall into the final and most general category, multipennate muscles--muscles composed of fibers that are oriented at several angles relative to the axis of force generation (Right). As we will discuss, an understanding of muscle architecture is critical to understanding the functional properties of different sized muscles.
The functional effect of muscle architecture can be simply stated as: muscle force is proportional to physiologic cross-sectional area (PCSA), and muscle velocity is proportional to muscle fiber length. PCSA is the sum of the areas of each fiber in the muscle. It may be apparent, based on the brief discussion of architecture presented above, that neither fiber length nor PCSA can easily be deduced based on gross muscle inspection. Detailed dissections of cadaveric muscles are required for architectural determination (see Sacks and Roy, 1982, for a description of the methodology). However, after determining architectural properties, it is possible to understand how much force the muscle generates and how fast it contracts (or how far it contracts). Let's look at two specific architectural examples and their impact on the length-tension and force-velocity relationships.