Dear Mikepa

It is interesting that the only time you get this pain is when you are mountain climbing with your rucksack, from your description it seems it is onesided...If EMG results suggest its a long thoracic nerve injury then maybe it is?????...however true nerve injuries (especially motor ones as chronic as yours) should present with some weakness, in this case perhaps a Wikipedia reference-linkwinged scapula...do you have that?

I am in agreement with alophysio and to add alophysios point, it sounds like a compression problem, perhaps along the course of this nerve, neck or first rib? This I can imagine would be the case if you have to carry a rucksack as heavy as you mention in some instances two...climbing mountains...

fill us in with more details

cheers