When Marey's myograph - which measured the phases and speed of muscular contractions - provided him with the first visible tracings of fatigue and the earliest understanding that fatigue was the limiting factor in the human motor's ability to produce work, it seemed imperative to him that he investigate the more complex acts of movement in order to identify and elaborate the laws governing the conservation or loss of bodily energy.

La méthode graphique… (1885) p. 513

La méthode graphique… (1885) p. 156

And so in the 1870s he turned from graphing the inner dynamics of the body to graphing the phenomena that produce locomotion. Here the primary difficulty was maintaining contact with two, or in the case of the horse, four moving legs, or two wings - movements that are independent and created by the combined motion of the body's different rods, joints, and levers. By designing special shoes that contained a hollow chamber connected by rubber tubing to the receiving tambour and stylus, Marey could trace on his revolving cylinder the number, length, and frequency of steps and variations of foot pressure on the ground.