This is from an old book called "The Complete Guide to Drawing, Illustration, Cartooning and Painting." For a book with such an ambitious title it doesn't really deliver as much as you might hope. These two pages are the best thing about it.
This is by Willard Mullins, a great newspaper cartoonist who specialized in cartoons about baseball. This is an interesting piece about how he approaches his work by starting with the line-of-action.
Some drawings he did of a baseball player. Interesting, of course, because the poses look ones you'd associate with a ballerina, not a baseball player.
Again, note how E-V-E-R-Y-T-H-I-N-G on the figure is made to follow the Line Of Action. I used to have a life drawing teacher who said that even the bellybutton should reflect the pose. It should bend, twist, squash or stretch to fit what the figure is doing. As it does in real life, of course!