This demonstration for work surface height is with the best seated posture, and done after the strongest chair height has been determined.
The first work surface height is shown that is the same height as the elbow. Ask the subject to reset the posture, take a breath and then relax. Watch the body relax with the forearms on the work surface. When the torso bends forward there is a loss of supported spinal posture. This surface is too low, and will encourage slumped posture.
The second work surface height is shown one inch above the measured elbow height. Again, ask the subject to over correct, or “reset” the posture, take a breath and then relax. Watch the body at the neck and shoulder when the subject relaxes with the forearms on the work surface. There should be minimal collapse of the spine, and the forearms can move across the work surface without lifting the shoulder. That way the weight of the arms are relieved from the neck and shoulder, unloading the spine.
The last demonstration is with the surface another inch taller. The forearms can rest on the work surface, but the shoulders are elevated; and the forearm cannot move without lifting the shoulder. This surface is too high, and the shoulder cannot relax, causing a problem.
Use trial surface heights above and below the best guess to find the height where the arms can move without raising the shoulder or relax without slumping.