As you know with teaching, one does not always get time to practise some of the most interesting things to draw - movement. This is one of my most loved forms of drawing; full of spontaneous lines and odd squiggles, which make up many of my fast and furious sketches. Subjects are never static and neither should ones style of drawing. However, relaying that movement to paper, in the form of a drawing causes all sorts of conundrums.
Keeping a sense of proportion as the subject continues to move, requires well developed perspective skills, as one need to remember where the subject was in order to place it on paper. The rest is filling in minimal details, as the subject moves closer or further from the original view. So often, limbs can get distorted in length as one comes to terms with the difference in space.
And then, when I think I have had a good crack at movement, I return to something a little more static - but still using the same 3-4 minutes sketch arrangements. My students are good subjects for this and it allows me to confirm proportion again. I believe if one gets comfortable drawing only one form, style or using a single technique - that style which you tend to practice most, becomes lifeless. Strokes lose spontaneous energy and so too does the drawing.