Loom Bench Ergonomics
I was wondering if there is a rule of thumb regarding how a loom and bench should be set up to prevent back problems. I have been weaving for a few years as a hobby only and it has not been a problem yet, but my family has a history of weak backs and I can imagine that back problems could occur if the loom and bench are not of the proper height.
There have been several articles in Handwoven about the importance of taking care of your posture and health at the loom (look under Ergonomics in the 2005–2011 Index). When I got your question, I also did an internet search for Ergonomics in Weaving and found quite a few sources, all of which were good. As a teacher at my school, I've observed the way students sit. Many of them re-adjust bench height as they move from loom to loom, not always to a height that I think looks right for them.
The online sources share my general view. Weaving can be a very good activity for strengthening back, shoulder, and thigh muscles. The most important thing, which is probably obvious, is to choose the height at which you are most likely to sit with your back straight as you weave. As I've observed with my students, this happens more if the bench is relatively high. As I move from loom to loom in my school (and move the benches students have left at various heights to see how they place me), I can see that for my back to be straight, my elbows should be just above the breast beam if my arms are bent at a 90 degree angle, upper arms straight, forearms resting on the breast beam. In this position also, my hips are above my sitting knee height (thigh slanted downward to the knee). If I am sitting so low that my legs form a 90-degree angle (with my foot on the floor), treadling requires much more effort to move my leg (I have to lift my thighs) from treadle to treadle. Also, with the bench too low, I have to raise my upper arms to throw the shuttle. This is why a chair doesn't work well at a floor loom.
So, in short: If the bench is too low, you have to work harder to throw the shuttle and use the treadles. If the bench is too high, you will have to bend over too much. Keep adjusting the bench height until you feel best weaving with a straight back. And remember to take frequent breaks and do some stretching!