Actually it is not that strange. The brain is very economical. It may keep a lot information together to make it easier to memorize. If you always tense your shoulder and your jaw when palying something difficult or demanding, your brain will save those things as the same "memory". So when you think the passage is difficult, you bring that memory and tense shoulder and jaw altogether.RaphaelPazos wrote: So strange that clenching your jaw would cause shoulder pain....
If you focus relaxing one thing you may brake the pattern and you loose all associated tensions. So if you simply relax your jaw you may relax the shoulder together and vice-versa. If you simply relax and breath deeply and think with another mood, without anxiety, tension or without thinking the passage is difficult, you may get rid of those tensions without even thinking about the jaw or shoulder.
You may also do massages to the muscles, to increase your perception and relax them.
Another think is to deliberately move or tense the problematic area while playing, and then relax it. For example, you hold a chord tightly while you move your shoulder in circular motion. Then relax the shoulder while keeping the chord tight. Feel the relaxing and the contrast. The objective is simply to notice what are the sholder muscles and what are the fingers muscles, so you can learn to detach them when you want to.