Facial Expressions in Art, Science, & Technology
A visual reference guide for the Facial Action Coding System (FACS), featuring action units (AUs) and their corresponding musculature.
NOTE: The former “cheat sheet” is available to premium members here: “FACS Study Guide.”
NOTE 1: Incisivus labii superioris and incisivus labii inferioris are considered accessory bands to the orbicularis oris muscle.
NOTE 2: Lip pucker typically co-activates vertical lip tightener (as seen in this example).
NOTE: “AD” refers to “action descriptor.” An action descriptor is basically a less fleshed out action unit (AU). ADs differ in that they function more as event descriptors. In this case, AD19 – tongue show, simply means the tongue is protruding in a notable way beyond typical protrusion for speech, eating, breathing, etc. For a deep dive into tongue actions and tongue-related muscles, book a mini course on tongue movements and tongue anatomy.
NOTE: Risorius is one of the most variable facial muscles in humans. Depending on the study, it has been reported missing in anywhere from 1-94% of research subjects. Risorius is narrow and difficult to locate; so it is possible that this discrepancy in statistics is partly inflated due to methodological study errors. For more on anatomical variation, book a studio lecture on facial muscle diversity.
NOTE: You may see some AU20 – lip stretcher in the AU21 – neck tightener example and vice versa. This concurrence is due to a close relationship between the risorius and platysma muscles.
NOTE: This 2-type distinction is a deviation from official FACS. Only “horizontal type” qualifies as lip tightener in original FACS. I have chosen to divide lip tightener into two types, because the lips tighten in distinct manners. The muscle behind both movements, orbicularis oris, possesses rich variation in fiber directionality; such directional variation yields more potential actions for orbicularis oris than have thus far been documented. The distinction of horizontal vs. vertical lip tightening proves especially useful when breaking down speech. For speech references, visit the Viseme Cheat Sheet.
NOTE: In FACS, AU25 – lips part, refers to the state of the lips being parted. This parting can be caused by any action that separates the lips – e.g. relaxation of mentalis, relaxation of orbicularis oris, contraction of other muscles, etc.
NOTE 1: AU17 – chin raiser appears during the in-between steps. I cannot perform this action without assistance from AU17 during the transition to the final pose. Mentalis appearance during lips suck is likely the case for many others as well.
NOTE 2: Jaw drop is almost always required for lips suck.