Action on lumbar curvature of the psoas muscle
Psoas muscles have other functions than to be powerful flexors of hip. They put the lumbar spine in lordosis, which increases the arch.
I read a Frederic Delavier’s book « Strength Training Anatomy » and I learned good stuff.
Abs exercises (especially for rectus abdominis) must work with the round back (round your spine). It’s the opposite of other exercises.
For vertebral coil exercises, the mechanicals stresses on vertebral joints aren’t the same as for exercises such as squat, deadlift and other exercises with weight. For example, for squat, the spine isn’t arched at the lumbar level. The vertical pressure with the spine’s rounding pushes the nucleus pulposus intervertebral disc backwards, which can compress nerve elements and create sciatic herniated disc.
For abs exercises, it’s the opposite. If during the abs execution you forget to round your back with an intense contraction of abs and obliques, the powerful flexors of hips (psoas major) will increase the lumbar arch. This will move the intervertebral discs forward as they aren’t stabilized by the vertical pressure. Then an excess of pressure at the back of the lumbar vertebral joints can create lumbago or more serious, joints damage by compression and shear.
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