Muscle weakness

Shari325

I need help identifying which specific muscles are weak.
When I attempt to complete a standard crunch/sit-up (lying on the floor, knees bent), I can only lift my shoulders off the ground. However, when I attempt a crunch/sit-up while lying on a weight bench (feet on the floor), I can sit all the way up into an upright position. Confused by this, I attempted a sit-up from the floor with my legs straight and flat out in front of me, and could sit all the way up again. For some reason, bending my legs causes weakness that is overcome by straight legs. I assume that the bent knee position causes some muscles to be shortened and therefor weakened in this position, but I am clueless as to which ones.

Which muscles, or muscle groups, would cause this?

Cherimoose

Totally normal. It is partly to do with the longer lever when your hips are straight, and partly due to the farther center of gravity, which lets you generate more force without your feet popping off the floor. It's more about physics than an anatomical deficit.

You mentioned elsewhere that you have tight hip flexor muscles. Sit-ups will tighten them even more, since the hip flexors are the primary movers. I don't recommend doing sit-ups or leg lifts. Crunches are fine, since they only work the rectus abdominis ("6 pack"), not hip flexors.