This from the Glimakra website, and a tip I learned from taking Jason Collingwood's 2/2 Twill Rugweaving workshop at 2015 MAFA.
Peter Collingwood’s Comments on Bench Height and How to Beat a Rug Tightly
“I only learnt about these some years ago when I found I could no longer raise my arms sideways sufficiently to throw a ski-shuttle. The chiropractor, who cured me practically in a single acupuncture session, emphasized that the strain on the muscles of shoulders and neck is greatly reduced if the loom seat is as high as possible, so that your arms go DOWNWARDS to the batten, [hence Jason’s advice that your thighs should be jammed up against the underside of the breast beam]. Obviously the more horizontal your arms are, the more muscle power is needed to hold them up.
“That was the main thing… the other is the oft-repeated maxim of stopping every half hour or so, and stretching your arms in the reverse way to how they are in weaving. The moment of turning the warp on is the obvious time to do this.
“Another help is not to beat by pulling the batten towards you, as you sit upright, but to lean backwards as far as you can with your arms straight out in front of you, gripping the batten. I used to tell students, “Hold the batten and fall off the seat backwards!" This adds some of your weight to the power of the beat; this plus a weighted batten gives you a well-compacted weft.
“Another help is to have the treadles slung as low as is convenient. It requires much less muscle power to push them down, if your leg is almost extended, than if it is bent at the knee.”
- Peter Collingwood