Eadweard Muybridge was a pioneering photographer in the late 1800’s. The story is that he set out to answer a question: “Is there a time in a horse’s gallop when all four hooves are off the ground?” To do this he developed an ingenious method of rapidly taking sequential images. As a result, he both answered the question (“Yes.”), and embarked on the creation of the first formal sets of high-speed sequential photographs of both animals and people in motion.
His photographic sequences of humans, horses and other animals walking, running, turning, carrying and moving in other ways are still an invaluable resource for artists today. They are particularly useful for comic book artists, illustrators and animators.
There are number of books of his photographs in print that serve as terrific reference for drawing people and animals in motion. (There are much better sources for anatomy, but Myubridge is great for drawing motion.) The Male and Female Figure in Motion from Dover Books is inexpensive. I find The Human Figure in Motion is a good reference for the price. There is also a multiple-volume complete series.
It’s an interesting sidenote that the process used for the famous “Bullet-time” special effects sequences in The Matrix was essentially a descendant of Myubridge’s multiple-camera technique for capturing motion.