*2.2. Experimental Setup and Data Capture*

In the study, "4D average" posture analyses were performed on all the participants using the DIERS Formetric III 4D measuring device (software versions DICAM v3.7.1.7 (DIERS International GmbH, Schlangenbad, Germany) for the data collection and DICAM v3.5.0Beta11 (DIERS International GmbH, Schlangenbad, Germany) for the data export), a VRS system based on the principle of triangulation [13]. A slide projector, used as the optical equivalent to an inverse camera, projects horizontal and parallel light lines onto the unclothed back of the participant, who is standing upright on a treadmill (height: ~18 cm) at a predefined distance from the measuring device (~2 m), with the eyes looking towards a standardized point ~2 m away and 20 cm below the individual's body height (measured from the ground). Twelve series recordings of the transformed line pattern (due to back surface curvatures) were captured for a period of 6 s with an associated camera system. The three-dimensional scatter plot derived (consisting of up to 150,000 individual data points, depending on the body size) was used to create a virtual plaster cast of the surface of the participant's back. The three-dimensional position of the underlying spine and the pelvis was estimated based on this information in combination with a clinically validated correlational model [11–13].

Even though it is technically not required for static VRS posture analyses, all the participants were marked with seven reflective markers prior to the data capture (on the spinal process of C7, the spinous processes between the medial parts of the spinae scapulae (~T3) and the thoracolumbar transitions (~T12), the left and right posterior superior iliac spine (PSIS) and on both acromia). This was necessary because the superior study protocol meant that the data for the dynamic gait analyses were also captured on the same measurement appointment. In order to best control for potential palpation or measurement bias, however, the same investigator (physical therapist) always performed the complete procedure themselves, including the entrance examinations (checking for inclusion and exclusion criteria), palpation, marker attachments and the VRS measurements, following a strict and standardized protocol. A static control scan was also performed to check for the correct placement of the markers. Where there were clinically inconclusive measurement results or any uncertainty on the part of the investigator, the placement of the markers was checked, palpated again, and corrected, if necessary, until the final marker position was defined. The measurements were repeated if the first graphical data output revealed clinically incomprehensible, inconsistent measuring artefacts or apparent software misinterpretations. For reasons of quality assurance, the investigator and an additional technician, who were both highly familiar with the software and the measuring device, further inspected all the pictures and the graphical data output visually after completion of the data collection phase for further abnormal spinal representations or other measuring artefacts and corrected them if necessary. In total, 46 specific and 14 global spine parameters were exported using the export interface of the DICAM v3.5.0Beta11 software. The Statistical Analysis System (SAS version 9.4) was used to combine all the exported

files into one editable sheet of raw data. Figure 1 provides a schematic flow chart of the experimental process.

**Figure 1.** Schematic flow chart of the experimental process.
