Mechanics of animal movement

Mechanics is the science of structure and movement that engineers need when they design things like bridges, aeroplanes or machines. It is equally applicable to the structure and movement of organisms, in the branch of science known as biomechanics (Box 1). Biomechanics is concerned with humans, animals, plants and individual cells. Some of its practitioners are motivated principally by curiosity, whereas others seek solutions to practical problems such as the design of joint prostheses or replacement heart valves, or the improvement of performance in athletics. Here, I focus on the biomechanics of movement of animals and healthy humans, ignoring medical biomechanics and plant and cell mechanics. What is the movement? The first requirement in a biomechanical investigation is a description of the movement; how fast is the animal (or human) moving, what is the timing of events such as footfalls or wing beats, and what are the angles and rates of movement of joints? Scientists' ability to obtain information of this kind was enormously enhanced in the late nineteenth century by the invention of cinematography. One of the most famous early programmes of research was carried out, shortly before true cinematography was developed, by Eadweard Muybridge. Starting in 1878, he set up rows of 12 or 24 still cameras with a mechanism that fired them in rapid succession as an animal or human moved past. Thus, for the first time, it was possible to make a reliable record of movements that are too fast to be followed by the unaided eye. Among other things, Muybridge resolved a controversy by showing that there are stages in a horse's trotting stride in which all four feet are off the ground. Also, he showed that the stretched-out position in which artists of his time painted galloping horses — with both forelegs extended forward and both hindlegs extended back — does not occur during the stride. Huge technical advances have been made since then: cinematography has been superseded by video recording; automated systems are available that will generate records of the movement in three dimensions of marked points on the body, from the output of several synchronised video cameras viewing the body from different angles; X-ray video systems have been developed. One of the most spectacular X-ray movies was made by Farrish Jenkins and shows a flying starling as a moving skeleton, demonstrating, for example, that the wishbone bends and recoils in every wing …