The classic front plank provides a challenge to your core muscles but doesn't target the obliques specifically. However, variations of the plank target the obliques, providing an exercise that strengthens the muscles that support the flexion and rotation of your torso. These exercises require either a mat or stability ball.
Side Plank
Similar to the front plank, the side plank is an isometric hold. That is, you assume the correct form and hold the position for 15, 30 or 60 seconds or more, depending on your fitness level. Do this exercise in front of a mirror to check your form. To do a side plank, lie on your side with your feet, hips and shoulders aligned. Place your lower arm under your body, forearm perpendicular to your torso so that your shoulder is directly above your elbow when you lift. Rest your upper arm on your side. Brace your core and raise your hips toward the ceiling until your spine and legs form one straight line. Don't lift so far that your body is arching toward your arm. Keep your neck in line with your spine, don't allow it to drop. Switch sides to work the obliques on the other side of your waist.
Variation
If you find the side plank too difficult, try a less-challenging variation. Instead of raising your torso and legs, bend your knees behind you and raise your torso and upper thighs. This minimizes the amount of weight and distance you need to lift. As you build strength, increase the time your hold this position and move to the full side plank if you become strong enough.
Side Plank Twist
Introducing a stability ball into your plank provides a dynamic, challenging exercise. This exercise works both sides of your waist at the same time. Correct form is critical to this exercise to support your lower back. Roll forward on a stability ball until your bent knees are resting on the ball and your hands are on the floor, shoulder-width apart. Keep your neck aligned with your spine and brace your core. Rotate your hips to twist your torso and move the ball to one side, rotate back across the midline to the other side. Do not twist too far since the movement is continuous -- you don't want to get stuck on one side.
Front Plank With Knee Drag
If the front plank is your preferred plank position, introduce a challenge to your obliques by adding a drag of your knee. Assume the front plank position, a full body raise resting on your forearms. Bring one knee up and forward to rest between your forearms. Point the toes of your straight leg and sink your hips toward the mat, being careful not to dip the hip of the bent leg. Hold for 15, 30 or 60 seconds, depending on your fitness level. Come back onto the toes of the straight leg, drag the bent leg back to its starting position and switch legs.
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