Summer is right around the corner, and if you're looking for a challenge—and a six-pack—Jeff Cavaliere, C.S.C.S., of Athlean-X has you covered with an ab workout that’s built for all fitness levels.

So, your diet has been on point, and you've been cranking out tons of situps at the end of your lifts, but still, the set of defined washboard abs you were promised has yet to show up. What gives? You ...