When I situp on solid floor it is harder for me to lift my body upwards versus on a soft/foamy floor which I can do a lot.
There are obviously many biological factors contributing to this, but from a physics point of view one could talk about degrees of freedom: since a foamy floor has more of them it is easier for yourself to get in the optimal position to get up. I would also like to add a simple biological factor: it hurts more to move around when you are trying to get up on a solid floor, which you will perceive as "harder"
On the other hand you can most certainly take the softness too such an extreme that it gets more difficult to get up (e.g. if you're lying on quicksand). Here the degrees of freedom have become too large for you to control, i.e. all stability is lost.
When you do a sit-up on a hard floor, your abdominal muscles are doing all the work to bring your upper body to a certain height above the floor. On a springy surface, as you start to sit up, the weight of your body becomes concentrated on the part of the floor beneath your hips, causing it to sink. This gives two advantages:
- The final height of your upper body is not as high as it would be on a hard floor, meaning your muscles have to do less work.
- A part of the floor remains in contact with your lower back for a longer time, providing some push to aid your ascent.