2.2. SWE Measurements and Subject Positioning

For optimal measurement conditions for each muscle, SWE-recordings were completed within 20–30 s. Since the utilized ultrasound device Canon Aplio i800 records one sample per second, 20–30 images/samples were available for estimating SWE. Of these 20–30 images, three random images were chosen and a circular region of interest (ROI) was placed in a homogeneous region of the muscle for SWV measurements (Figure A1). The three SWV measures were averaged, resulting in a single value used in subsequent analyses.

In Protocol 1 (clinical feasibility), for the muscles of the upper extremity, volunteers were asked to lay on their back, in the supine position, with their arms stretched out at their side, resting on the examination table, palms facing up. When measuring the SWE of the back muscles, participants were asked to lay on the examination table in the prone position, with their hands at their side. For the lower extremity muscles, volunteers were asked to lay on their stomach or back depending on if the muscle was on the ventral or dorsal side of the body.

In Protocol 2 (optimized, rigid SWE-protocol), each muscle was examined in a position in which the muscles were not strained in terms of stretch or contraction, but in the most optimally relaxed state. Since positioning could not be optimized for the paravertebral back muscles, these were not included again (see also Results). A detailed description of the joint positions used for each muscle, as well as a comparison of our protocol to other studies can be found in Table 1.