I’m not consciously doing anything with the upper arm/shoulder area, but I feel them tense up a bit when I bring the hand the bridge, then tense up a lot when I start picking.
With the guitar on my left leg like it is in the above clip, the upper arm doesn’t reach the guitar, but with it on the right leg (and with the attached stand lifting it up) I can rest my upper arm on it.
While I was playing around with this I noticed something that might be part of the problem though:
(I have the guitar on my right leg with the bicep resting on the upper bout in this clip.)
When I bring my wrist up to the bridge from a relaxed, hanging position, it doesn’t actually want to be on the bridge, my arm is most relaxed with the wrist being held slightly above the bridge (relative to the guitar, not the ground). To bring the wrist onto the bridge, I have to flex at the elbow to bring the forearm in towards the bridge. This also pulls the body of the guitar towards me, away from where it rests naturally, and my whole upper arm / shoulder area has to tense up to hold it there. So all the picking I do is from this position where I’m fighting to even keep the wrist on the bridge.
I switched back to electric to see if it had the same problem, and it did, but for a slightly different reason:
This time it’s the forearm resting on the upper bout, and when I bring the wrist up to the bridge from a resting position it’s even further away than it was with the acoustic. I can press the forearm into the upper bout to bring the body of the guitar towards me and get the wrist on the bridge, but the effect is the same as it was with the acoustic.
I can slightly alleviate the problem by pointing the neck of the guitar more in front of me or tilting the guitar like in the examples in your last post, but it’s not enough to stop the chain of tension through my arm and shoulder. I also tried playing standing up with various strap heights.
As for how to fix this, I have no idea. I’m not even 100% sure it’s the problem as I’ve never seen anyone else talk about it. If I get the chance I’d like to try out one of these nerd guitars since the upper bout is almost non-existent and maybe I wouldn’t have this problem:
Regarding the grip, I don’t think I’m suspending the middle finger, it just doesn’t reach the guitar when I let it hang loose. I tried curling it in more but it seems very resistant to being flexed like that.
Edit: Another thing making me unsure if this is the problem is that I was also unable to get a working motion with the guitar lying flat on the desk in front of me.