Obliques are the muscles that run along the sides of your torso, from the lower eight ribs to the front of the hip bone. They contract when you rotate your torso, so a great way to develop them is with an exercise called the Russian twist.
Lie flat on the floor in a spot where you can tuck your feet under a low piece of furniture to keep them from moving or have a workout buddy hold them in place for you. Bend your knees and raise your upper body to make a V-shape with your thighs. Extend your arms in front of you from the shoulders, lifting them until they’re parallel with your thighs, hands together.
Tighten your abdomen — think of trying to bring your navel to your spine. Now twist your entire upper body, arms included, to the right side. Hold, then return to start. Repeat to the left. That’s one full repetition. Do two sets of 12 to 15 reps in all.
As you get more advanced, you can hold a weight in both hands for added resistance. One type of weight that’s easy to hold is the medicine ball. This is a weighted ball, not the kind of inflated exercise ball that you might sit on for balance exercises.
Choose a weight that slows down your movement. Each rep should take longer to complete with good form, and this results in increasing the time that your muscles contract. But the weight shouldn’t be so heavy that it prevents you from using proper form. Hold the medicine ball in both hands and, as you inhale, rotate your torso along to the right. Exhale and return the ball to the center. Repeat to the left. At first, you might aim to complete just one set of 12 to 15 reps. As you progress, increase to two full sets.
For more moves you can do with a medicine ball, the American Council on Exercise has a step-by-step workout that targets the entire body.