Dave Wirth
Guitar is an interesting pursuit. If a person has any tension in the body, as a result of habit or whatnot, it eventually comes under scrutiny because it will hold them back from playing something they want to play. It's pretty normal to begin playing guitar with tension where it doesn't need to be. After all, we practically twist our wrists to fit around the neck of the guitar. It scares the willy-nilly out of me to see students play in a way that is bad for their bodies (as a side note, beware of blanket advice such as "Your thumb needs to be directly behind the neck of the guitar, at all times." It might not be the best for you).
There are times where tension is the result of bad technique. What if you have a solid technical ability and the tension is still there? The culprit might be even more invisible. I've noticed that there is a common thing that all guitarists will do if they are playing a passage that stretches their ability: They will twist and move their mouth and jaw muscles. If this sounds familiar, then try the following.
1. Get a mirror and practice in front of it for at least two weeks.
2. Observe your mouth (don't try to change it).
If your mouth moves any small little bit, you would do yourself a big favor by merely observing it. Try not to clamp down on your teeth. Try not to grind them! Eventually, and through observation of this problem, you can rid your body of another bit of tension that is unneeded.
If you are interested in a bit of further reading, check out this article on mandible jaw syndrome.
