Action Unit Reference#

Written by Eshin Jolly

The Facial Action Coding System (FACS) is a system to taxonimize human facial movements based upon their underlying musculature. It was first developed by Carl-Herman Hjortsö in 1978 and then adopted and expanded upon by Paul Ekman and Wallace V. Friesen in 2002. The table below lists most of the possible Action Units (AUs) that are coded for along with with their muscular basis, expression involvement, and model support (not all models are trained to detect all AUs). Most models support a subset of of about 25 AUs corresponding specifically to facial muscles. Additional AUs are part of the FACS system but not commonly used to train models as they are associated with eye or head movement.

You can sort the table by clicking on any column (shift+click to sort by multiple columns). You can also use the search bar (case insensitive) to filter the table by any of its content such as:

  • which AUs are commonly associated with which expressions (e.g. search “happiness”)

  • which muscles are associated with which AUs (e.g. search “corrugator”)

  • which Py-Feat models support which AUs (e.g. search “svm”)


Emotion expressions (e.g. sadness) are often characterized by patterns of AUs (AU1 + AU4 + AU15), but individuals vary in the way the exact ways that they emote. For this reason, some AUs may not be observed on some faces for the same expression and some expressions may consists of additional AUs.