Method
Sitting down changes the bone landmark
A note on how a seated posture can fold the lower abdomen and shift where the ruler meets the bone.
Guides on Method: Adding up the errors, Repeatability and agreement, start to finish, Bone-pressed length, start to finish, Stretched flaccid length, and why the clinics take it, Girth, done the way the studies do it, The measurement glossary
Sitting folds the lower abdomen, and the fold can change where a ruler's end can reach the bone landmark. That makes a seated reading a different measurement from a standing one, even with the same ruler on the same day. The fix is a protocol one: choose a posture, write it down, and keep it.
What changes
The bone-pressed landmark is found by pressing the end of the ruler in at the base until it meets bone. In a standing posture the lower abdomen is relatively flat and the approach to the bone is more direct. In a seated posture the trunk flexes, the abdomen folds toward the thighs, and the tissue at the base is pushed forward and compressed.
Two things follow for the reading.
- The ruler may reach the bone at a different angle, or have to press through a different depth of tissue.
- The end of the ruler may be held off the landmark by the fold, so the zero sits a little short of where it would stand if you were upright.
Both are mechanical, not medical. They are the same sort of effect as a ruler held at the wrong angle, a topic covered in ruler angle parallel to the shaft.
The direction of the effect
I will not give a magnitude, since none has been checked, and a figure would be invented. The direction is plain enough: a fold that keeps the ruler off the bone shortens the apparent reach, and a fold that lets the ruler be pushed deeper in could lengthen it. Which dominates depends on the person and the posture, and that is the reason to treat posture as a condition rather than guessing at a correction.
Posture is one of the free choices
Standing versus lying down is its own comparison, set out in standing or lying down. Seated is a third posture, usually chosen for convenience rather than for any reason of method. That is a bad reason to let it into your protocol silently.
If you measure seated one day and standing the next, you have introduced a source of between-session difference that looks like drift. Within-session versus between-session variation explains how to spot that pattern, and it will not tell you the cause unless you recorded the posture.
The related soft-tissue effect
How thick the tissue sits over the bone, and how much that changes the pressed reading, is its own topic in the pubic fat pad and the reading. This note is narrower: it covers the way posture alters access to the landmark, and it says nothing about anyone's body composition.
How the Joint Committee frames it
The Joint Committee for Guides in Metrology's Guide to the expression of uncertainty in measurement (JCGM, 2008) lists an incomplete definition of the quantity measured among the recognised sources of uncertainty. A posture left unspecified is exactly that: the quantity "length from the bone" is only fully defined once the conditions under which the bone is reached are stated.
What to do
- Pick one posture for the series and name it in the method line.
- If you must change posture, take a short paired set in each and treat them as separate series.
- Keep the instrument angle and press consistent within the posture, as described in how hard to press.
- Do not compare a seated figure with a standing one, or with a published figure measured standing, without saying so.
The published reference in Veale et al. (2015) in BJU International does not let you correct for your posture, which is one more reason to state it.
Where other sites pick up
How a score is derived from a photograph, and what the photos have to show, is covered by the scores hub and the photo-tool accuracy page, and neither has a posture rule of this kind. How human judges differ is a separate matter, and what Rate Cock does is a different kind of number from a pressed length.