Nonie's clear explanation
This is an overly simplified theory (that is not considering collagen, elastin loss). What causes wrinkles is the exercising of the same muscles over and over without ever engaging any of the antagonistic muscles so that they too can strengthen and improve in tone and pull the skin in the opposite direction. When muscles are toned, they stay in semi-contraction. So someone who frowns all the time continually strengthens forehead muscles until they get to where they maintain that semi-contracted state, which holds the skin in a creased state. Same thing with smiling. Always using the same muscles, tones them at the expense of all other related muscles and so an imbalance is created. A good face exercise program aims to engage muscles that are hardly used so that they too can strengthen and attain tone bringing all the muscles to a state close to how they used to be when you were young, before aging, atrophy and gravity took their toll. The result being you have a smoother and more lifted face that appears to have improved elasticity since no expression stays frozen in your face: you make it, and when you stop, the muscles that previously would be overpowered by the overused one are able to snap back and return your face to total relaxation.
If you consider Tom's scalp exercise, it aims to do two antagonistic movements. One of them is very hard for most people because it is a movement never done. It's no wonder forehead furrows form since the raising of eyebrows is done so easily. But as you strengthen the occipitalis muscle which pulls the scalp back, you end up creating a tug of war between the frontalis and occipitalis as both muscles attain ultimate tone...and in time the forehead lines open up as the forehead muscles find themselves pulling against a muscle that is almost now as strong as they.
Tom's ideas about lines in the face
I agree totally with Nonie about how lines in the face form. She wrote, "So someone who frowns all the time continually strengthens forehead muscles until they get to where they maintain that semi-contracted state, which holds the skin in a creased state." Some have called that semi-contracted state in which unwanted lines form bad facial posture. If you watch small kids at play you see mobile facial expressions. When they feel an emotion it is immediately registered in their faces. Their facial muscles get an intense workout, especially when these kids are playing uninhibitedly with other kids. It's good entertainment to watch them.
But when these kids grow up, mobile facial expressions often change to more stylized expressions. These stylized, static expressions as a person gets older can contribute to lines in the face. So am I against line formation in the face. I don't like to see young faces with etched lines in the forehead, at the outside corners of the eyes, or around the mouth. But I'm also not a big fan of the perfectly smooth face unclouded by thought.
What I like about sound facial exercise programs is that they tone up all the muscles in the face while still allowing for expression. Facial exercise does not deaden facial expression like Botox does; it keeps the face alive and ready for action.