Trunk orientation. Trunk orientation was defined by the black line from the point halfway between the superior posterior iliac crest markers to the spinal processus thoracic 8. The red line (V) represents the line perpendicular to the plane of the pelvis (pointing upward). Lateral trunk angle α refers to the angle between the two lines, projected in the frontal plane. PSIS = posterior superior iliac spina. ASIS = anterior superior iliac spina. XP = xiphoid processus. Acr = acromion. V = vertical line.