Taking only gravity into account, the quadruped vacuum should be the more difficult exercise, but due to the fact that other spinal stabilizing muscles come into play, the seated version is actually more difficult. Begin by sitting on a stable surface, but without leaning against anything. As with the other versions, exhale and pull your navel in toward your spine. Again, work up to a few sets of 60-second vacuums.
To speed up progress, perform your seated vacuums on an unstable surface like a Swiss ball. The next step on the progression ladder is what I call the functional (real life) vacuum. Don’t stop doing the above variations. Instead, do one or more of them in addition to the functional version.