Leg-lefts and crunches are often mistakenly thought to be exercises that work the lower abdominal muscles. Both can definitely help you in your quest for a flatter stomach, but each has its own function, and they might work differently than you think. Understanding the different muscle groups that leg-lifts and crunches target is important for getting the best abdominal workout.
Abs in a Crunch?
A crunch is performed with both feet on the floor and the knees raised. The upper back is curled forward while the lower back remains on the ground.
Images of six-pack abs may suggest a matrix of many muscles lurking beneath the surface of your torso. But that six-pack is really one long muscle called the rectus abdominus, which flanks each side of the front of your torso. Although you can’t really isolate one part of the muscle from another, some exercises work the top end more than the bottom end, and vice verse.
Crunches mainly work the top part of the rectus abdominus. “You can acquire a fair amount of definition doing crunches, but there’s a limit to how far they’ll take you,” says Los Angeles-based personal trainer and yoga instructor David Knox, author of Body School: A New Guide to Movement in Daily Life. “The hips and legs don’t move during crunches, so you don’t really get the lower abs going.”
Leg-lifts and Hip Flexors
Leg-lefts can stimulate fatigue in the pelvic region that can make it feel like you’re working your lower abs. But that’s because you’re working the iliopsoas and rectus femoris. These two hip-flexing muscles are found beneath the rectus abdominus and they’re key to stabilizing the lower back. That benefits posture and alignment in a way that can enhance the work you do directly on toning your abs.
More to the point, leg-lifts strengthen the lower back, preventing injury and reducing lower back pain. Because double leg lefts involve the hip flexors, which originate in the lumber spine, they raise the potential for back injury. Do them with careful attention to form.
Should You Include Them in Your Workout?
Crunches were once the gold standard of ab work, but their reputation among fitness experts has declined somewhat over the years. In fact, in a 2001 report by the American Council on Exercise, the Biomechanics Lab at San Diego State University that examined 13 of the most common abdominal exercises ranked the traditional crunch at eleventh. That doesn’t mean they don’t work. Rather, there are a number of exercises that were found to work better. At the top of the list: the captains chair and the bicycle manuever.
But crunches and leg-lifts both have the advantage of being great for at-home exercise. All you need is a floor and motivation. Don't leave them out of your workouts entirely, just remember that they are not the only moves that you should do to work your abs.
- University of New Mexico: Superb Abs Manual
- Body School: A New Guide for Movement in Daily Life, by David Knox
- RxEX.net: Lower Ab Myths
- Journal of Physical Therapy Science: The Effect of Complex Rehabilitation Training For 12 Weeks On Trunk Muscle Function and Spine Deformation of Patients with SCI
- ACE Fitness: New Study Puts the Crunch on Ineffective Ab Exercises