Shown here at two different paces, each with the fact-check of the knees actually making contact with the elbows in each rep.
Not, knees-to-triceps, or knees-to-thin-air.
And no butterfly, caterpillar, or anything else;
Sit-ups are to build strength and improve position for use in other applications. If we are constantly trying to outsmart their function, they’ll do neither.