Thank you all for the full and insightful replies – “It’s not Strength that you need to build, it’s coordination, relaxation, patience” sums it up very nicely – but I would like to probe this one step further if I may 🙂

On the harmonic slurs, the measures of progress are 1.) the length of time you can keep it up for, and 2.) the speed at which you can oscillate between the two notes. What are we actually trying to build through these exercises – does there not need to be a level of corner strength built in order to rapidly make the required shape change and sustain this, without the corners tiring? Or are we solely trying to develop feeling of the shape of each note – but it should never cause the corners to tire and ‘shape’ to be lost?

A simpler version of the same question might be: Once I have the ‘shape’ right and am sufficiently relaxed – should I be able to hold a C or D in the staff indefinitely, with the only limiting factor being quantity of air? As it stands, the muscles holding my ‘shape’ in the right position will give up before my supply of air does – is this normal/something to develop, or is it an indication of too much tension still? I don’t feel I am squeezing – but the small amount of muscle activation required to hold the shape as air passes through (passively) gets harder to hold the higher i go.