The Stomach Vacuum!!!!!
This technique can be performed in any position but I will explain how to do it on all fours.
It's harder than it sounds actually and technique is key to this being effective so take it slow.
This workout may also elicit a little pelvic floor workout too - if so, lucky you!
Support your body on your hands and knees
Knees under hips
Hands under shoulders
Back in neutral (not rounded or arched)
As you exhale - tighten the abdominal muscles
Hold for 15 seconds
Release and repeat as many times as you can
As always, if you cannot hold for 15 seconds at first, don't worry, build up to it.