How to Interpret the Illustrations of Muscles

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Most chapters in the manual that contain descriptions of AUs begin with illustrations of the muscles underlying the actions covered. Take a look now at the muscle illustrations in Figure 2-1 on page 15, which show the muscles in the upper face. The face labeled "Muscular Anatomy" shows where the muscle fibers are anatomically located. The face labeled "Muscular Action" schematically indicates the location and direction of action for each muscle. The numbers that appear in the figure refer to the numbers for each Action Unit described in the chapter. The location of the circled number in the Muscular Action illustration approximates where the muscle emerges from the bony structure (also known as its origin). The other end of the line indicates approximately where the muscle attaches to the soft tissue of the face (also known as its insertion or attachment). When a muscle is contracted, the pull is always toward the circle where the muscle emerges, drawing the soft tissue towards that point, usually bunching or wrinkling the skin perpendicularly to the line of muscle pull. For example, the muscle that underlies Action Unit 1 emerges from the bony structure high in the forehead region and attaches at its other end to the soft tissue below the medial portion of the eyebrow. Look at Figure 2-1 to be sure you understand how the origin and attachment are diagrammed in the Muscular Action illustration. When AU 1 contracts, the skin in the medial area of the eyebrow is pulled upwards towards the top of the forehead, raising the inner corner of the eyebrow. Horizontal wrinkles form in the medial portion of the forehead. These are the kinds of signs that you use to determine what muscle has acted.

The two muscular illustrations that begin each chapter show each muscle on only one side of the face. In reality, the muscles normally exist symmetrically on both sides of the face. Throughout most of the chapters on AUs, you learn the appearance changes that occur when there is bilateral action of each Action Unit or AU combination. In Chapter 11, you learn when and how to score an action that occurs unilaterally or occurs more strongly on one side of the face than the other.

When each AU is described, refer to the two muscular illustrations at the beginning of the chapter. They will help you to understand how the action occurs, why facial appearance changes in the way it does, and how to make that movement occur on your own face.