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

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



(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



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.

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

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

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.

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.