Q. How does a moisturizer increase skin firmness?
Increased firmness is commonly used to describe the beneficial effects of moisturizers. This is confusing to the dermatologist who considers firm skin indicative of scleroderma. The substance that increases skin firmness in scleroderma is collagen while the substance that increases skin firmness with moisturizer use is water. Moisturizers, themselves, do not moisturize the skin, but they create an environment for enhanced skin water content.
Moisturizers contain substances, such as dimethicone, petrolatum, mineral oil, sesame oil, sunflower oil, etc, that reduce transepidermal water loss and increases the skin water content. When the skin is full of water, it is harder to distend with negative suction from a suction cup. This is interpreted as increased skin firmness. Anything that increases skin water content by definition increases skin firmness.
This is how moisturizers increase skin firmness. The more occlusive the moisturizer, the better the improvement in skin firmness.