From the day they are born, every person has a certain amount of tension in their muscles, known as muscle tone, which is also present when the body is at rest. The natural movement of the muscles, for example when a person frowns, causes them to move to a greater or lesser degree, briefly forming wrinkles. The skin is capable of compensating for this pattern of tension and release for a very long time. However, with increasing age and decreasing skin elasticity, there comes a point at which these expression-related lines no longer disappear. They become deeper and are visible as forehead lines even when the facial muscles are relaxed.
Wrinkles caused by facial expressions (expression lines) develop as a result of the activity of the underlying facial muscles. Expression lines include not only forehead lines but also frown lines between the eyebrows and crow’s feet (laughter lines) around the eyes.