Ab exercises with no equipment

Why don't abs need added load?

Because you are already carrying it. On most muscle groups your own bodyweight is hard to line up against the working muscle, which is why the lists for biceps or hamstrings are short. Abs have the opposite situation: their job is to keep the torso and the legs in check, and both of those weigh plenty and can be moved further from the hips.

That distance is the only variable you need. Leg raises with the knees tucked tight, then bent at a right angle, then with the legs straight are the same movement in three versions, differing only in how far the center of mass of your legs sits from your hips. With a barbell the equivalent would be adding a plate.

The ceiling only shows up right at the top. Once a full hollow hold, straight-leg raises and a plank with a hand and a foot lifted stop being a challenge, the lever is used up and the next step needs something in your hands.

What is the difference between a plank and a crunch?

A crunch shortens the abs: the torso comes closer to the hips and the muscle travels through its range of motion. The plank, the hollow hold and the dead bug do something else. The abs don't change length there, they just refuse to let the spine bend under the weight of the legs and hips. Both are real work, but you measure them differently.

You count crunches in reps, so progress shows itself. Held positions tempt you to measure them with a stopwatch, and that is where the mistake creeps in: the seconds stop meaning anything the moment the hips sag or ride up too high. Practically speaking, the set ends when the line from your head to your heels stops being straight, not when the planned time runs out.

Which is why it is worth having both in a week. The plank and the hollow hold teach you to hold your ribs and pelvis in place; crunches and leg raises add the range of motion that a held position by definition does not have.