"Don't ever do another situp again, honey," Kardashian explains on her website and app. "There are way more effective moves for toning your abs and strengthening your core while torching major calories. All you need is a BOSU ball."
If you have no idea what a BOSU trainer is, it's that blue piece of equipment that looks like a fitness ball cut in half. You may have seen gym goers using it to intensify lunges or make their squats a bit more challenging. The unstable surface strengthens muscles in a way that flat surfaces can't compete with. That's why it has become a favourite amongst stars like Kardashian.
This article originally appeared on InStyle U.S.