In the thoracic rotation measurement protocol, PSIS is mentioned as a landmark to be supported during prone twisting.

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Stabilizing the pelvis so the motion comes from the thoracic spine is the key idea. In this protocol, using the PSIS as a landmark to support the pelvis during a prone twist provides a reliable way to keep the pelvis from rotating. When the PSIS area is supported, the hips and lumbar region stay still, allowing the thoracic spine to move and all measurements reflect true thoracic rotation rather than compensatory pelvic or hip motion.

Laying prone with PSIS supported while twisting achieves this isolation effectively, which is why it’s the best choice. The other postures described would not reliably limit pelvis movement or isolate the thoracic segment: standing positions offer less control over pelvic rotation, and sitting with arms crossed doesn’t restrict the hips or lumbar motion in the same way, leading to less accurate thoracic rotation measurements.