Nearly every expression is at times opposed by the muscle of the lips, the orbicularis oris. Its action, tight compression of the lips, is likely both an unconscious effort of restraint and a sheer physical outlet. In suppressed sadness, this action doesn't prevent the other lower-face muscles of grief from contracting, but it partly overcomes their effects—a suppressed expression is often just as expressive as the unrestrained look.
In the angry mouth (top), the contraction of mentalis and triangularis is the strongest action, creating the upside-down smile shape. Orbicularis oris is only partly contracted; the lips are not fully compressed.
In the suppressed smile (middle), the orbicularis oris is again only moderately contracted, but this is strong enough to prevent the zygomatic major, the smiling muscle, from lifting the mouth corner. Mentalis aids in compressing the lips (A); triangularis aids in holding the mouth corner down (B).
In the suppressed sob (bottom), the mouth is similar to that of anger with two key differences: it's stretched sideways by risorius/ platysma, and the compression of the orbicularis oris is more intense—the lips are straightened and thinned.
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