Whenever I try to do a lower ab workout my legs get tired before my abs feel a burn. What do I do?

Everytime I try to do a lower abdominal workout now (the bottom 2) it always involves my legs doing like 6 inches off the ground and in and outs and such, but now my abs are stronger and my legs get too tired to do the exercise before my abs even feel a burn. What do I do to enhance the burn my lower abdominals feel?

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