Straight legs, locked-in lower abs.
If your lower back lifts off the floor, don't drop your legs as low.
The lying leg raise turns your straight legs into a long lever the abs must control, with the load peaking as the legs hover just above the floor, right where the core has to fight hardest to keep the lower back pressed down. Raising is only half the exercise, the slow lower against gravity is where most of the work lives. It earns its place as the standard floor drill for lower-ab strength, scalable from bent-knee beginner reps to brutal slow-motion sets.
Your abs are losing the fight against the lever of your legs near the floor. Press your lower back into the floor before the set, and shorten the range, stopping the descent higher, until you are strong enough to go lower without the arch.
No, they are the correct scaling. Bending the knees shortens the lever and reduces the load on the abs, letting you build the pattern with a protected back. Straighten the legs gradually as you get stronger.
It is a legitimate option the setup allows, tilting the pelvis slightly and giving the lower back a touch more support. Use it while building strength, then progress to hands at your sides for a stricter test.
