Rotation is automatic and side bending in the spine because of both the same of the articular facets in the vertebrae and the spiral pathways of the muscles. To keep the action “pure” lateral flexion, there needs to be a counterrotation in the rib cage. in this case, the upper ribs rotate posteriorly and the lower ribs rotate anteriorly. To achieve this, the internal obliques on the upper side and the external obliques on the lower side are recruited.

Also, if there is tightness in the outside of the standing leg hip joint (in the tensor fascia lata, gluteus medius, or gluteus minimus), then that hip will try to flex rather than stay purely adducted. The standing leg should maintain hip extension (via the adductor magus and hamstrings) to prevent this.

When there is tightness in the latissimus dorsi, lifting the arm overhead can push the rib cage forward (compressing the floating ribs and inhibiting breathing in genral), or pull the scapula downward even as the arm is lifting—potentially creating impingement of the biceps tendon or supraspinatus at the acromion process. Folding the upper arm behind the back eliminates this concern and allows the focus to remain on the torso movement.