Barbell floor wiper. Lie on your back on the floor, holding a barbell over your chest with straight arms. Keeping your legs straight, raise your feet directly above you. Start lowering them towards the right side of your body but don’t let them touch the ground. Reverse back to the L-shaped position, lower to the left, then reverse. That’s one rep.
Top tip: Pause and hold for a second when your feet are hovering an inch from the floor. This will not only work your obliques but, because your body is working against the rotation, it’ll hit your whole core too.