From what I’ve heard, the old-fashioned sit-up, the kind where someone holds your feet down and you raise your whole truck, doesn’t exercise the abs so much as some muscle or muscles in the upper leg. What muscles are these? I need to know because I injured one about 3 years ago, and it still hurts.
I think you are looking for the group of muscles grouped together as the hip flexors.
The iliacus and psoas major. Cite.
Once your shoulder blades are off the floor, the abs function mostly isometrically.
Regards,
Shodan
The iliacus and the psoas.