Stomach vacuums

Muscles worked

You need

How to do it

1. Get on your hands and knees with your back flat and your hands under your shoulders.
2. Exhale all the air out of your lungs and, without moving your spine, draw your belly button up and in toward your spine as far as it will go - imagine your stomach shrinking away from your waistband.
3. Hold the vacuum for a count, then release and breathe normally before the next rep.
4. Nothing visible happens on the outside; the deepest layer of your abs is doing all of it.