It takes practice to quickly capture body positions. Figure drawing is used for sketching at a live event, pinning down a fleeting idea while daydreaming, or working out various compositions for a project. I like to use a woodless pencil, square crayon, charcoal, or colored chalk because they can make strokes that quickly switch from wide to narrow just by twisting as I draw. These tools also can automatically create shading by tipping the pressure to one edge while drawing. Fine tipped tools tend towards drawing finer details that are not needed in quick figure sketches.
Here are two handy links for practice at home: http://artists.pixelovely.com/practice-tools/figure-drawing/ (timed intervals of 30 sec, 60 sec, 2 min, 5 min, 10 min. Clothed or Nude)
http://lovecastle.org/draw/ (timed as low as 10 secs. You choose how long each reference stays on screen, some nudity even in draped section)