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Go into almost any gym and you’ll see people doing exercises like sit-ups and leg raises. They believe these are the best exercise for abs. But they’re not — it’s crunch exercises.
Here’s an experiment. Stand up and hold onto something for balance. Put your other hand on your abs. Now, raise your knee up in front of you. Notice the ab muscles are not activated. They don’t attach to the legs.
The muscles that raise the legs are the iliopsoas or hip flexor muscles. They run from the lower back across the top of the pelvis and attach to the front of the thigh. The sit-up is just the mirror image of the leg raise.
The legs stay in place and it’s the torso that lifts up, also making use of the hip flexor muscles. The role of the abs in these exercises is simply to act as stabilizers. They do not contract over a range of motion, which is what is needed in a primary ab exercise.
Let’s look at the abs and what they do. The abdominals attach between the pelvis and the rib cage, and their function, in addition to being stabilizers, is to pull these two body parts together in a “crunching” movement – where the torso rolls or curls forward as if into a ball.
The major abdominal muscles are as follows:
Although you can feel more or less stress in different areas of the abs doing different movements, there are no different exercises for upper and lower abs. The principle of non-contiguous innervation tells us that motor neurons do not stimulate specific muscle fibers in limited locations or groups but innervate muscles over widespread areas. So the upper and lower abs pretty much work as a single until. The basic variation in the effect of an ab exercise comes from whether you contract your abs though a full range of motion, or only a partial range and how much resistance you subject these muscles to.
Here are the basic crunch exercises:
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