One of the routine discussions that goes round the weaving world is that of how to hold the shuttle.
Now, everyone has to work within their abilities and disabilities, but here's the scoop on whether or not thumbs up or thumbs down is good for you:
"What is this “ideal shoulder position,” I speak of? Lucky for you, the ideal position of external rotation can be demonstrated by standing (or sitting) up straight, imagining that there’s a pencil between your shoulder blades (scapulae), and you’re pinching your scaps together to keep the pencil from falling. Lift your arms directly in front of you, make a fist and lock your elbows. Rotate your thumbs so they’re pointing towards the ceiling; you are now externally rotated at the shoulder (pictured below, ignore the arrow for now). This is an ideal position. Alternately, flip your thumbs towards each other then down towards the ground; you’re now internally rotated at the shoulder. This is bad."
(edited to add the link to the above quote)
I have been weaving, production weaving, in other words many hours nearly every day, for 40+ years.
I have two whiplash injuries, so my neck is compromised as well as my back and shoulders. And yet, and yet, I can still weave for 3 (or more) 45 minute sessions every day, holding my shuttle as shown in the photo above.
Over the years I have consulted with various professionals - chiropractor, massage therapist, physical therapist, dance instructor who holds a degree in movement. All, every one of them, says the thumbs down position is 'bad' for the body. Maybe not today. Maybe not tomorrow. Maybe not ever!
But for those people who are already compromised in their neck, shoulders, back, I strongly urge them to try holding the shuttle in the manner pictured above. It helps with wider warps insofar and you can more easily propel the shuttle across a wide warp. It helps with shoulder issues (especially if you have a tendency to rotator cuff problems) and I feel a smoother rhythm can be achieved much more quickly.
I have certainly had positive feedback in workshops from students who make the effort to change. (If you are one of those who experienced an improvement by changing, please comment below?)
In the end, however, if you are happy with what you are doing, no need to change anything. But I do suggest that if you hold the shuttle thumb down that you take frequent breaks. And if you only ever weave for 15-20 minutes at a time, the thumbs down position may never cause any grief.
Each to their own!