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When the abdominal muscles contract, a very
simple thing happens: They pull the rib cage and the pelvis toward each
other in a short, "crunching" motion. No matter what kind of abs muscle
exercise you do, if it is really a primary abs muscle movement this is
what happens. In the past, before the physiology of abdominal training
was well understood, bodybuilders used to do a lot of "conventional"
abdominal muscle exercises such as sit-ups and leg raises.
Unfortunately, those are not primary abs muscle exercises but instead
work the iliopsoas muscles - the hip flexors. The hip flexors arise from
the lower back, go across the top of the pelvis, and attach to the upper
thigh. When you raise your leg, you use the hip flexors. When you hook
your feet under a support and lift your torso up in a conventional
sit-up, you are also using the iliopsoas muscles.
Try this exercise: Stand up, hold on to
something for support, and lift one leg up in front of you while putting
one hand on your abdominals muscle. You will feel a pull at the top of
the thigh but it will also be obvious that the abdominals are not
involved in lifting the leg. The abdominals attach to the pelvis, not
the leg, so they have nothing to do with raising the leg up in the air.
The same thing is true of a sit-up or
slant-board sit-up. This exercise is really the reverse of a leg raise.
Instead of keeping the torso steady and lifting the leg, you are keeping
the legs steady and lifting the torso - and the same muscles are being
used, the hip flexors. When you do any of these exercises, the primary
role of the abdominals is as stabilizers. They keep the torso locked and
steady. But this is directly opposite of what we want to achieve in our
abs muscle training because the role of the abs, as pointed out earlier,
is simply to draw the rib cage and pelvis together - to crunch them
together in a very short movement which involves the back curling
forward. The back doesn't bend much doing a sit-up, while it curls a lot
doing a crunch. That is the secret to full range, quality isolation
training of the abdominals muscle.
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