Shikibutons are thinner and thus offer less sink / give for a firmer feel.
Shiki comes from the word hiku meaning to lay, and when attached to another word, the futon is now pronounced with a b and becomes buton, forming the word shikibuton.
In Japan, these simple mattresses are part of a style of minimalist living that goes back centuries. According to Handmade Style: Japan, a shikibuton is usually laid out on the floor atop a straw mat called a tatami mat and then, when not being slept on or used as a sitting cushion, folded up and stored in cupboards.
The Shiki is multi-functional, multi-purpose, and there will be no wasted space in your home. What could be more minimalistic?
People think that very soft comfortable beds are better for bad backs, but according to many types of research over the years, it has shown that a firm (but not hard) bed is better for your back. It keeps your spine aligned, and you do not wake up with back pain on a shikibuton.