A century and more ago Silicon Valley was a place of shady orchards and sunny farms. Fast horses offered a pastime to wealthy landowners such as Palo Alto magnate Leland Stanford, who once wagered a skeptical friend that horses on the gallop often had no hoof touching the ground. The disputants organized a direct test, placing its execution in the hands of Eadweard Muybridge, an ingenious San Francisco photographer. He aligned a dozen or two still cameras to view a stretch of Stanford’s racecourse, each camera fitted so that the line of shutters would be tripped one after another by threads extended across the fast horse’s path. The image sequence was compelling: Stanford won the bet, and Muybridge, whose whole career was redirected by this success, became a celebrated world pioneer of the nascent cinema. His later 1887 volumes of photographs, still frequently reproduced, provide wonderful motion sequences of every condition of humanity and many creatures of the zoo.

That early inquiry has grown, mostly since World War II, into a science, animal locomotion (the very title of Muybridge’s book). The human animal is of course a major subject, with wide applications, from physical training to sports, dance, orthopedics … (Your authors, too, bedecked with glowing red diodes, have capered on foot before the computer- feeding cameras of a local “gait lab,” to generate sequences of stick- figure moving images.)

The state of the art nowadays is suggested by physiologist Alberto E. Minetti at a well-known lab in Milan, Italy. His motion analyzer, he says, consists of “four infrared cameras capable of detecting … the three-dimensional positions of 18 reflective markers positioned on the subject’s joints of interest.” The images are computed by the custom software every inch or two as the motorized treadmill glides underfoot.
Two ubiquitous patterns of human locomotion are the walk and the run, both apparently innate. The time and place of the repeated grounding of each foot define the basics of any gait. Timing makes the difference: the two-beat alternation does not differ between walk and run, but a walker always has one foot on the ground, sometimes two. When either foot is raised, its replacement has already made contact. But runners fly free more than 15 percent of the time; in running, contact by the replacing foot is regularly delayed each time it is due. The runner usually does not notice the resulting free-flying intervals; their total duration is less than a tenth of a second at usual speeds.

Two rough models sketch the very extensive analyses. Walking resembles the motions of a pendulum. Consider the walker at the moment when one leg is slanted behind, the other slanted ahead, making an inverted V. Soon the lagging leg rises to pass the other. When they pass more or less straight beneath the torso, both feet are in ground contact, and the walker’s center of gravity is raised a little, given that the legs, nearly straight and close to vertical, hold the body’s center of gravity higher off the floor than the slanted inverted V did. The inverted V forms again, only now with the legs exchanged. The total mechanical energy alternates in form, changing from the kinetic energy of swinging legs and rising body to gravitational potential energy near maximum body height, and back again. A pendulum handles energy similarly, exchanging kinetic energy near the midpoint of its arc for gravitational energy around the point of least height, then up again. The energy so stored is largely recovered and serves to reduce the energy cost of walking by about half of what would be needed were the swinging legs not also stores of gravitational energy
The walker rolls up and over the high midpoint of a step on near- straight legs, then smoothly down again under gravity. But the runner moves in longer, freer steps, with muscle-driven legs rapidly bending and unbending. The weight of a runner’s leg is not enough to bring the extended limb down soon enough. The runner has in effect jumped a little, until at last the limb comes down. Both the kinetic energy– horizontal and vertical–and the potential energy of the center of gravity reach maximum at the same time during the periods of “flight.” This substantial energy peak is stored briefly and then released during contact time to feed the alternation.

Yet muscles are not to be recharged by mechanical energy, and gravity is simply too slow to keep pace. Energy is stored in the tight-strung tendons and ligaments that form the linkages that bind muscles and bone; they stretch and relax, smoothing the energy peaks and saving much overall effort. The runner bounces elastically like a ball, enabled by the resilience of the tendons. Those cords are subtly woven elastic cables of the protein collagen, a structural polymer much less lossy than any Super Ball.

A third gait of human locomotion has been studied systematically as recently as 1998. We all knew the trick, although mostly we have dismissed our old skill. It is skipping, the joyful gait of the young. (Adults often skip at tight corners and going downstairs. The Italian lab found that almost all its adult male subjects got used to treadmill skipping after a few minutes of practice.) Skipping follows a basic pattern, usually L-L, R-R, different from the unvarying L-R, L-R of walking and running. Unlike the walker but like the runner, the skipper spends some time in flight. Unlike the runner but like the walker, the skipper normally has intervals of support by both feet at once.

In some respects, the skip is rather extreme; it costs more energy than other gaits, works typically at a high step rate and requires stronger leg forces than the run demands. Skipping suggests the gallop of horses. There is a rising case that force may be as important as energy in locomotion; peak muscle tension may be as much a limit as is the work muscles do. On the moon the astronauts preferred forms of skipping to the other gaits they tried. In low gravity the energy cost of locomotion is low, so anyone can afford to skip, and high force demand is also eased. Maybe a youthful skipping crew–smaller, faster, cheaper?–would be preadapted for field trips on Mars?

It is plain that the last word is far from final, even about human gaits. But both splendid results and admirable popular accounts now ornament this diverse science, which is at home with motion by living things ranging from bacteria to brachiosaurs, on and under land and water, and through the air. Not all these topics require the high technology of modern gait studies; many still invite amateur observers and home and field experimenters.
To read more, one cannot do better than R. McNeill Alexander’sExploring Biomechanics: Animals in Motion (Scientific American Library, 1992, distributed by W. H. Freeman and Company), and his CD-ROM Animals on the Move (Expert Software, 1998) adds visual resources for the wired.

Philip Morrison and Phylis Morrison