In the process of trying to keep the lumbar spine neutral, the pelvis must not tilt posteriorly and flex the lumbar spine. The anterior abdominal wall should stay flat. Dominance of rectus abdominis causes the anterior abdominal wall to ‘bulge’ out, flexing the trunk and increasing the pressure in the unit. A pressure increase of more than 10 mm Hg (increase to more than 50 mm Hg) indicates gross posterior tilt and uncontrolled lumbar flexion.