body-mechanical Q on legs as counterweight to levering up one's upper body

Factual Questions
1

An observation that is contrary to my understanding of mechanics:

Consider me lying on my back. Trying to lever up my upper body will lift my legs up


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(the dot being my butt, as the fulcrum of movement)

Now if I bend my knees I can lift my upper body without my feet leaving the floor


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But… shouldn’t it be the other way round? With bent knees the leverage that the weight of my legs can exert as a counterweight is less, so it should be in the bent-leg position rather than the extended-legs position that my feet leave the grounds.

2

I can lift my upper body up while leaving my legs out straight on the ground. Maybe your premise is flawed.

3

That’s probably to do with different builds (more weight in the belly, for me)

4

The muscles are all outstretched when the legs are outstretched. The muscles are compressed when the legs are bent, making it easier to sit up.

My guess.

5

You don’t bend your knees and then sit up, you sit up while bending your knees. It’s the acceleration of your legs that matters here, not their position.