How do comic-book artists draw all those complex, dynamically-posed figures without a model for reference? I used to think it required a very detailed knowledge of anatomy, an abilty to name and describe every little muscle and bone in the body.

Actually, it's much simpler than that. In fact, knowledge of anatomy alone doesn't get you far, as I and many others learned the hard way. In order to draw figures from imagination, all one has to do is learn basic simple shapes to represent all the major bodyparts.
The torso should be sketched in as two simple tapered blocks, one representing the chest, the other the hips. Note the difference between the male and female torso. Female hips and chest are very similar to each other in size and shape -- remember, the female waist falls about halfway between the shoulders and the crotch. The male ribcage is bigger, the hips smaller and narrower. As you can see, even just those basic simple block shapes do a good job of defining gender, without having to use big muscles or hooters.

Pay attention also to the side view. The female waist is higher, so the small of the back is as well. As a result, the female torso is again a pair of symetrical halves, and the female booty sticks out farther (Duh).
Legs can be drawn as a simple combination of spheres and cylinders. Draw the ball joints first, using the stick figure as a guide, then line up the cylinders between the joints. It's much harder if you do it the other way round. legsbasic1
Construct the arms the same way as the legs. Note that the ball joints of the shoulders overlap the chest block. Keep in mind that the ribcage is quite narrow at the top, and it's only the shoulders themselves that create the illusion of width. Shoulders are actually pretty tough to draw, but the ball and cylinder method simplifies things a lot. basicarms1
Another thing the ball and cylinder method simplifies is forshortening. If you use spheres for the joints, they are the same shape no matter what angle you're viewing from; so just sketching three balls in a line is a quick way to lay out foreshortened limbs. armforsh