The Architecture of Leg Muscles

Each of the four muscles shown in Figure 36.1 (a to d) consists of muscle fascicles (bundles of muscle fibers) connected at either end to tendons, but they show striking differences of architecture. Most authors would describe (a) and (c) as pen- nate, but (b) and (d) as parallel-fibered. It often seems convenient to use these adjectives, but the distinction that they make is not a sharp one: it is easy to imagine a continuous series of intermediates between (a) and (b) or between (c) and (d). It is sometimes suggested that the diagnostic feature of a pennate muscle is that its fascicles attach obliquely to the tendons. However, the cross- sectional areas of the tendons are always much less than the total of the cross-sectional areas of the muscle fascicles, so geometry requires that the attachment be oblique even in muscles such as (b) and (d) that would generally be described as parallel-fibered. Muscle (e) has fascicles that attach at one end directly to a bone rather than to a tendon.