Two facial muscles play the primary role in the formation of nasolabial folds: the zygomaticus muscle
and the levator labii muscle (Fig. 1). As we age, these muscles weaken, just like all other muscles in our body (a process known as physiological sarcopenia).
Over time, muscle volume decreases, but the ligaments that anchor these muscles remain in their original positions. Because of this, as the muscle loses volume, its ligaments become increasingly taut.
Both the zygomaticus and levator labii muscles attach at their lower ends (via tendons) to the orbicularis oris muscle (the muscle surrounding the mouth). Due to this constantly increasing tension
on the tendons, the skin in the nasolabial area begins to pull upward and outward. This is precisely what triggers the formation of nasolabial folds.