your friendly, neighborhood forehead muscle
The frontalis is a muscle that elevates the eyebrows. As the frontalis contracts, it pulls up the brows, creating dynamic (expression-based) wrinkles across the forehead. Because frontalis muscle fibers are vertical, frontalis contractions form horizontal wrinkles.
NOTE: Wrinkles form perpendicular to the direction of muscle fibers.
The way in which these horizontal wrinkles form is directly related to frontalis shape, size, and placement.
- Are the wrinkles full lines spanning across the forehead?
- Are the wrinkles displayed in one or more columns?
- Are the wrinkles straight or curved?
i went deep in the rabbit hole of frontalis
For the last few months, I’ve been obsessing over studies on anatomical variations of frontalis. Given the limited visuals provided by anatomy diagrams and the Facial Action Coding System manual, I was left with many unsolved mysteries regarding my own wrinkle formations as well as the wrinkle formations of others.
After hours and hours of observation combined with meta-style research, I have finally shed light on some burning questions I’ve had for years.
what i discovered
Contrary to most anatomy diagrams, the frontalis is widely variable in shape, size, and location. Most illustrations of frontalis tend to display one frontalis shape – the large, bifurcated shape.
NOTE: Bifurcation simply refers to the split in the muscle.
The popular frontalis shape displayed above features:
- the frontalis muscle with a division in the middle
- the frontalis muscle with large sections at each side of the split
- the split connected by a wide, sheetlike tendon referred to as galea aponeurotica (or aponeurosis epicranialis or epicranial aponeurosis)
HOWEVER! The mainstream frontalis shape is by no means the only frontalis shape that exists. Your forehead is a site of diversity.
After analyzing many cadaver photos, studying forehead facial movements, and hunting through research papers, I have developed a sense of the different looks frontalis can take on.
To give you a general idea of the different shapes your forehead muscle can assume, I have illustrated a set of frontalis shape examples below.
- a, b, and c have no bifurcation
- d and e have a small bifurcation
- c, d, and i-l are rather small
- h and k have a wider angle split
- b, c, and d are medially concentrated
- h-l are laterally concentrated
for art & entertainment
After surveying ~40 artists of varying levels (from students to 20+ year industry professionals) it is apparent that frontalis diversity is not widely known.
- expression shape libraries
- Because muscle shape, origin, and insertion directly affect muscle movement, awareness of frontalis shape range is important for those building FACS or anatomy-based expression shape libraries.
- character art
- In general, for any character-based art, knowledge of frontalis variation will allow you to draw, paint, or sculpt individual-specific anatomy – including each person’s unique wrinkles and facial expressions.
- facial mocap
- For those working on facial mocap, awareness of variable frontalis shapes will allow you to focus on forehead hotspots more effectively.
If you are using electromyography (EMG) to track brow movements, it is imperative to place the electrodes on areas that will provide the best source of information for muscle activity. Though the fully divided frontalis type is estimated to be in about 10-15% of the population (I’ve noted a similar trend in my own observational studies.), it is useful to know when it is or is not effective to place electrodes on the medial portion of the forehead to measure frontalis activity.
If a researcher were to concentrate electrodes on the medial portion of the forehead for an individual with lateralized frontalis, they would be missing out on key information. Same goes if a researcher were to concentrate electrodes on the lateral portion of the forehead for an individual with a medially-concentrated frontalis.
NOTE 1: EMG measures the underlying electrical activity generated by muscle contraction.
NOTE 2: I would love to get involved with some EMG studies and am available to consult on optimal electrode placement. E-mail me! facetheFACS@melindaozel.com
If you are using the Facial Action Coding System to label or code expressions, knowledge of frontalis diversity will be extremely helpful when coding AUs 1 and 2 vs. 1+2 vs. 1+4 vs. 1+2+4.
for face tracking
Anatomical variability in the human face is largely ignored in face tracking. In fact, in all my years in this field, I have never once heard anyone mention anything on this subject.
Common facial poses and concepts like, “inner brow raiser,” are not uniform. If an individual has a fully separated frontalis shape (as shown in the fourth row of the infographic) the movement of AU1 for inner brow raiser will look different when compared to individuals with a non-bifurcated frontalis, concentrated medially (first row, third face – pink).
Patterns of aging in the face are significantly affected by muscle shape, size, and placement. How wide your frontalis muscle spreads, where it inserts in relation to your eye muscles, how far it extends vertically, etc. will all influence how your face changes over time.
- Wrinkle formation is based on muscle activity.
- Face sagging will vary based on where your muscles are located as well as how they intertwine with each other.
Because of these differences, ideal botox and other cosmetic solutions are unique to each individual
I believe our concept of “inner brow raiser” (frontalis, pars medialis – AU1) and “outer brow raiser” (frontalis, pars lateralis – AU2) is idealistic and flawed.
Most consider “inner brow raiser” to be a difficult muscle action to perform voluntarily. When capturing expression data from research participants – or when recording expressions from actors for facial mocap – there is typically a set of standard facial poses for each participant or actor to make. These sets undoubtedly contain inner brow raiser and outer brow raiser.
Based on my findings, the look of inner brow raiser and outer brow raiser will vary considerably depending on individual frontalis shape, size, and location.
Perhaps inner brow raiser is not a difficult pose to make after all. Perhaps our interpretation of what inner brow raiser means was simply limited by the underrepresented diversity of the frontalis muscle. Where and how frontalis is situated will surely impact where the most medial raising will occur vs. where the most lateral raising will occur.
Inner brow raiser and outer brow raiser are moving targets and should be reconsidered as such.
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