Muybridge did his human (and animal) motion studies in the 19th century (late 1800's) before movie cameras were invented. His set-up was a series of cameras set in a row and as his subjects moved past the cameras, the shutters were tripped one after the other. Each 'frame' is a single picture taken by one camera. Amazingly, in todays age of super-fast photography ranging in the hundreds to even thousands of frames per second, his work is still a valid tool for studends of motion and animation. Hope this helps and answers your question. Walter