Anatomical variation is a surprisingly ignored consideration for face tracking/facial mocap in tech and entertainment. Simplified anatomy diagrams are often accepted as universally applicable to all faces and few further questions are asked.
The reality is: FACIAL MUSCLES ARE HIGHLY VARIABLE.
Masterclass excerpt and links from recent webinar with CAVE Academy, via Visual Effects Society.
The frontalis muscle (forehead muscle that elevates your eyebrows) does not always follow the prototypes presented in anatomy diagrams.
Frontalis shapes are widely variable, and this variability is a major contributor to age and expression-based wrinkle formations and brow shapes.
Currently learning chimpanzee FACS, chimpanzee anatomy, and making custom chimpanzee landmark diagrams. (Chimpanzee landmarks determined by Animal FACS group. Original research work by Lisa A Parr, Bridget M Waller, and Jennifer Fugate. See: Emotional communication in primates: implications for neurobiology )
Inner brow raiser is one of the most difficult facial actions to find clean references for. Many sources fail to find actors who can separate their inner brow raiser from other facial actions such as outer brow raiser (from frontalis, pars lateralis) and brow lowerer (from corrugator).