This is where all the movements you did in your life are recorded as deep folds.
Doing and redoing the same actions over and over creates a groove at the intersection between the moving parts.
Some of these behaviors are natural, such as walking or eating, while others may be artificial, such as cutting, screwing, writing, typing, cycling, driving, smoking, ...
We can divide them into two categories:
•In-line action folds: where the folding is perpendicular to the member (fingers, wrists, elbows, knees, ...). They produce less damage because they correspond to the pre-existing horizontal-vertical grid.
•Angled action folds: where the folding is not perpendicular to the member (legs, shoulders, hands, ...). They are very damaging, because the skin is not made to be folded that way. Their path crosses repeatedly the horizontal-vertical grid and has to fight its pull.
Many movements that generate action folds also include some rotation.
Some simple moves, such as cutting with scissors, are very bad on your skin, because they are repeated continuously and that their action isn't in-line.