Even when they are heavily exerting themselves. Don't fight yourself. Muscles work in pairs often times, and if you are tense, one muscle that you don't need to be using may be fighting a muscle that you do need to be using. To relax the proper muscles, you first have to become aware of what muscles you are using, and which ones you need to be using . Relaxation takes control over your muscles.
Relaxation is a form of control .
Adding relaxation is adding control. Most of us play the harp with our mouth most of the time. Unfortunately, there are exceptions, but let's try not to think about that. Well, pretty much you have to breathe out or in to compensate, if only a little. You can quickly take a bite of air, or let some go, by momentarily taking your lip off the cover plate and breathing through your mouth but not through the harp.