Oh, oh, I just had a flash about the habit of hitting one on the shoulders. When men went at work or at war to do physical tasks and/or accidents happened, perhaps men checked each other with a punch there. If one could stand the pain, he wasn't injured, or he was strong enough to sustain the injury. If he kneeled in pain, he was injured and he was forced to go away to rest - he would then have been regarded as weak, too old to work, etc. Kids may have seen their father doing it, and at some point the purpose of the punch was lost, but boys would keep on doing what their dad did, and the whole thing became a "manliness check". Eventually the whole point may have been lost, and it would just be some kind of cultural evolutionary relic. It could also be related to checking broken collar bones in knights' battles.