Not sure how to perform the low-abs jackknife exercise correctly? Here are the two most common mistakes a trainer sees on the regular. When your fitness instructor cues jackknife exercise (also known ...

If you're looking for the best workout combo for your triceps and lower abs, try this skull crusher jackknife exercise. It will give you the ultimate burn. Who doesn’t love a good two-in-one exercise?

Beloved of the home-workout brigade, the jackknife, or V-up, is a fail-safe way to tighten your abs. In fact, it’s one of Kayla Itsines’ go-to ab moves. But beware: it’s one you’re going to want to ...