Facial expressions can be systematically coded using the Facial Action Coding System (FACS) that describes the specific action unit (AU) or combination of AUs elicited during different kinds of expressions. This study investigated the thermal patterns concomitant to specific action units performance. As thermal imaging can track dynamic patterns in facial temperature at any distance (>0.4m), with high temporal (<20ms) and thermal (<20mK@300K) resolutions, this non-invasive technique was tested as a method to assess fluctuations of facial heat patterns induced by facial muscles contractions. Four FACS trained coders produced nine different AUs or combination of AUs at various speeds and intensities. Using a spatial pattern approach based on PCA decomposition of the thermal signal, we showed that thermal fluctuations are specific to the activated AUs and are sensitive to the kinetics and intensities of AU production. These results open new avenues for studying patterns of facial muscle activity related to emotion or other cognitively-induced activities, in a non-invasive manner, avoiding potential lighting issues.