What magi said. But also, with action poses, it's probably better to get the motion right before even worrying about the shapes. Maybe even with passive poses--the "motion" of a composition is a big part of what makes it interesting to look at, even when the objects in the image aren't actually moving themselves.
I've been having to work on that lately myself. I've been in the habit of trying to get shapes right and worrying about proportions and anatomy to the detriment of my ability to make anything look alive. Ultimately I don't regret learning anatomy first, because now even my drawings where the characters don't have the right proportions or anatomy still look less like they're wrong and more like they're stylized or doing a rubber hose-type thing.
But it's an uphill battle, nonetheless.
Anyway, here's my own attempt at the gesture drawing for the reference image.