Sketches from photographs.
Walking involves all sorts of subtle shifts of balance: the heel tends to land towards the midline; the top of the foot splays outwards; even if there isn’t an obvious arm movement, the weight of the opposite side of the body provides a counter-swing.
I don’t know of any other way to practice sketching these poses other than from photographs. It’s hard enough to sketch someone unless they remain quite still; the slightest change in position throws off the remembered line. But it must be possible as there are plenty of artists who create scenes including action poses completely from imagination. Perhaps they made many sketches from life (or photos), and that, combined with knowledge of anatomy and human proportions, is enough to create a novel scene.