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