I just got back from a weekend with drum corps and for a while we focused on not moving the jaw when we did lip slurs. We felt it with our hands and sang it to see if it moved. The main focus was to do the lip slur with your tongue, not your face. Today I was working on some flexibility in the Arban's Book (the lip slur that goes from quarter notes, to eighth notes, eighth note triplets, sixteenth notes, sixteenth note sextuplets) and after doing that for a bit VERY slow (around 70 bpm) or so, I felt my tongue getting tired. Does this mean I was doing it correctly? It's the first time I've had it happen to me.