A friend of mine who plays euphonium, Kristin, was telling me that she’s been having trouble lately with her jaw getting tired while playing and asked me if I had any advice. This is not an uncommon issue for players of certain embouchure types who need to play with their jaw protruded and their teeth alined. Simply practicing carefully to develop more endurance helps, but it’s helpful sometimes to isolate those muscles in particular. To help his students with similar issues Donald Reinhardt designed an away-from-the-instrument exercise he called the Jaw Retention Drill.
While the outer lower lip membrane is slightly inward and over the lower teeth, protrude the jaw as far as possible; however do not tolerate strain while so doing. Sustain this extended jaw position for at least ten seconds. When completed drop the jaw, open the mouth, exhale rather explosively, then relax. After a week or so of this routine, the amount of time for the jaw protrusion should be extended. Many students have developed this to such a point that thirty seconds is no great chore; however, it often requires several months to accomplish this.
Encyclopedia of the Pivot System, Appendix page 3.
There are a couple of other similar drills that Reinhardt recommended for building strength away from your instrument, including free buzzing and the pencil trick exercise. All of these drills are probably best done with a little time between doing them and actually playing to give your muscles a chance to rest and recover. Remember to take things easy at first, you’re trying to build up, not tear down.