The sit-up with a twist
is a popular exercise not only in bodybuilding, but also in aerobics.
This exercise is responsible for many of the back problems frequent
The sit-up with a
twist targets two midsection muscle groups; the abdominals and the
internal and external oblique. The rectus abdominals is a long vertical
muscle that runs from the ribcage to the public bone, and the oblique
are the angled muscles located on the front sides of the abdomen.
Because of the
position of the spine when the twisting occurs, instead of being very
effective, the sit-up with a twist is potentially a dangerous exercise.
During a sit-up, the spine flexes (becomes rounded in a forward
direction) so that the deepest, most anterior portion of the vertebrae
move closer together. This action compresses the anterior portion of the
discs, and most of the discs fluid is pushed to the outside. This fluid
movement is made possible since the back, or outermost, portion of the
vertebrae open up during the bending of the spine.
The greater the
rounding of the spine, the more the vertebrae move together interiorly
(in front) and therefore open up posterior (in back). This places great
compressive forces on a very small portion of the discs closest to the
center of the body. Instead of the full-size discs keeping the vertebrae
separated, they're squeezed down into a small area, allowing the
vertebrae to almost touch one another on the inner side of the spinal
When the spine is
twisted in this rounded position, great shearing forces are created when
the vertebrae rub together, which can rupture the discs. Although most
of the compression and shearing forces occur in the lumbar portion of
the spine, most of the rounding takes place in the thoracic area.
Because the ribcage doesn't move or bend, the thoracic spine also has
limited movement. As a result, the ribcage and rounding of the upper
spine create even greater compression forces. During the twisting, the
range of motion is limited since the thoracic spine must move as a unit
because of the attached ribs.
The upper trunk is
capable of minimal rotation, which limits muscle involvement. To get
greater muscle action, the rotation must occur in the lumbar vertebrae,
but if you rise up high enough to lift the lumbar spine off the floor,
that part of the spine will also flex. Then when the rotation takes
place, disc compression occurs in both the lumbar and thoracic positions
of the spine, and the lumbar area receives additional shearing forces.
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