Thomas Eakins' investigations into motion grew out of Eadweard Muybridge's animal locomotion project at the University of Pennsylvania. Eakins assisted Muybridge in finding models for his motion studies, and, erecting a separate shed on the campus, Eakins began to conduct his own experiments. This photograph is one of a series of what Eakins called "differential-action" studies, which culminated nine years later in a lecture entitled "The Differential Action of Certain Muscles Passing More than One Joint." This lantern slide image was probably used as a projected illustration during the talk. In order to demonstrate the tensile strengths of a horse's muscles, the man on the ladder balances his weight on the horse's skinned hind leg.