Muscle Size and Arrangement of Muscle Fascicles

The size of a muscle influences its capabilities. When a muscle fiber (muscle cell) contracts, it can shorten to nearly half its relaxed length. The longer a muscle fiber, the greater range of movement it can generate. In contrast, an increase in the number of muscle fibers increases the strength of the contraction.

Muscle fibers are grouped into fascicles, which in turn are grouped to form a muscle. The size (length) and number of fascicles determine the strength and range of movement of a muscle. Common fascicle patterns include the following:

  • Parallel fascicles have their long axes parallel to each other. Parallel fascicles can be flat or straplike, or they can bulge at their bellies and be spindle‐shaped or fusiform.

  • Circular fascicles are arranged in concentric rings. Muscles with this pattern form sphincter muscles that control the opening and closing of orifices.

  • Pennate fascicles are short and attach obliquely to a long tendon that extends across the entire muscle. In a unipennate pattern, the muscle resembles one half of a feather (the tendon is represented by the shaft of the feather). A bipennate pattern resembles a complete feather, with fascicles attached to both sides of a central tendon. A multipennate pattern of fascicles resembles three or more feathers attached at their bases.