Allenhm,
I find at 'times' that during my PR session my shoulder area will Tense at times.
The mind is a wonderful tool and a crutch at times. You see, whatever it is you become consciously aware of ... you target it.
So whenever PR'ing and you notice a tensing of the body, guess what, you become consciously aware to the point you concentrate on the tense state as opposed to conentrating on the PR session in a fully relaxed way. Let the feeling go and try to get back into the task of PR'ing.
Their is a strong relation between the body and mind when it comes to learning. To prove it ... next time before PR'ing, try jogging for 15-20 minutes (or any exercise of your choice,however jogging gets arms,legs and torso all involed), shower up, drink plenty of water and then start your PR session ... I think you'll be amazed!
There is another way to relax but until I can learn more about it ... I'll post it then.