Crunches and leg lifts strengthen the frontal sheath of abdominal muscle known as the rectus abdominus, which is where the proverbial "six-pack" resides. But that's only part of the story. Equally important for core stability are the internal and external obliques, which function in rotating the trunk, flexing the spine and bending to either side.
The obliques are located on either side of this front muscle. The two internal obliques, left and right, stretch diagonally from the pelvis and across the abdomen, while the two external obliques start at the lower rib cage and run diagonally down across the abdomen. The obliques play an important role in stabilizing your core and your center of gravity.
Working the obliques will not only make you look better, they’ll make you less prone to injury. Just as the obliques help you twist and turn in daily life, twisting and turning is the way to tone them.
Keep Your Back Safe
Any exercise that involves twisting the trunk must be approached with extreme caution. According to the textbook Sports Chiropractic, twisting while lifting strongly increases the risk of damaging a lumbar disc. That’s important to know because effective abdominal twisting exercises depend on adding resistance either with free weights or by using machines, according to exercise researcher Len Kravitz, PhD. He notes that many gyms now have machines that create resistance to trunk rotation, but cautions that users should have a qualified trainer check their form to avoid injury.
Fitness author Frederick Devalier notes in his book, Core Training Anatomy¸ that when it comes to obliques, definition is usually preferred over bulk. He recommends training with light resistance in long sets instead of with heavy weights to avoid creating a blocky waist.
A Word About Reps
How many reps to do of a given abdominal exercise? That depends on several things, one of them being how many you're able to do. According to the American Council on Exercise, the last few repetitions should be testing your limit. ACE recommends doing 10 to 25 repetitions per set, and one to three sets of an exercise.
You may opt to use a a medicine ball or kettlebell instead of a dumbbell.
Holding a dumbbell with both hands, sit on stability ball. Roll your back downward and lie back on the ball with your hips nearly straight and feet hip-distance apart on the floor. Extend your arms straight up over your chest.
Rotate your torso to one side while keeping your arms straight and perpendicular to your torso throughout the movement. Return the dumbbell back over your shoulders by rotating your torso to the original position.
Continue lowering the dumbbell to opposite side.
Weighted Incline Twisting Crunch
To add resistance, do the exercise with a weight plate.
Lie on your back on a decline sit-up bench. Hold the weight plate to your chest with your arms folded over it.
Raise your torso from the bench, flexing to one side. Lower your torso until your shoulders touch the board.
Repeat on the opposite side, alternating twists.
Planks Twist on Stability Ball
Doing abdominal exercises on a stability ball has been shown to activate a greater range of muscles.
Kneel with your chest or waist resting on the exercise ball. Drop your trunk over the ball with your hands on the floor and your arms thrust downward to support your upper torso.
Remaining horizontal, walk your hands away from the ball until your thighs are resting on top of the ball. Bend your knees so your lower legs extend upward.
Rotate your hips so your thighs roll to the side of the ball. Rotate to the opposite side and repeat.