The modern office chairs are designed to cave in your body itself.
Notice how most chairs have rounded side edges. This design causes your thighs to roll together and the deepest part of the seat is in the center of the seat - as opposed to under each butt cheek.
The back is also curved inward. This design rolls your shoulders together. Rarely does the lumbar 'roll' fit the body correctly.The arm rests are always too long and frequently too high. You can't get the chair close enough to the desk forcing the upper body to roll forward trying to reach the keyboard or mouse.
We now face orthopedic and medical problems as a result of sitting for most of every day.