How Animals Run

1. What they did

Many animals can run two or three times faster than a person. The author set out to explain how the “living running machine” works. He noted that the movements of a running animal are too fast and complex for the unaided eye, and that most earlier work had relied on deductions from skeletons and muscles.

He combined comparative anatomy with the analysis of motion pictures. With an electric camera taking 200 frames per second, he filmed a trained cheetah chasing a paper bag in an enclosure 65 yards long, and horses running on race tracks. He projected each frame onto tracing paper to follow how the parts of the body moved in relation to each other and to the ground. He was also collecting film of other running animals from commercial and private sources.

2. What they found

Speed comes from stride length times stride rate

• Speed is the length of the stride multiplied by the rate of stride. A giraffe reaches a moderate speed with long, slow strides; a wart hog matches it with short, fast ones. Fast runners need long strides taken quickly.
• Bigger is not simply faster: larger animals tend to have a slower stride rate.

Ways of lengthening the stride

• Longer lower legs. In running animals the foot, shank and forearm are lengthened more than the thigh and upper arm. The bones matching the human palm and instep are the most lengthened.
• Foot posture. People, bears, opossums and raccoons walk on their soles (plantigrade). Carnivores, birds and some hoofed animals walk on their toes (digitigrade), and other hoofed animals walk on the tips of their toes (unguligrade), which lengthens the leg further.
• Swiveling shoulder blades. In carnivores the collarbone is reduced to a trace, and in hoofed animals it is gone, so the shoulder blade can pivot. This added about 4.5 inches to the stride of the walking cheetah.
• A flexible spine. Cats and dogs extend and flex their backs as their legs swing, adding the change in body length to each stride. They extend the back only while the hind feet push against the ground. The author estimates the cheetah could run about six miles per hour without any legs.
• Jumps. Running is a series of jumps. Hoofed runners have one main unsupported period in each stride; galloping carnivores have two, one with the back flexed and one with it extended. In the article’s stride diagram the cheetah’s two unsupported periods made up about half its stride, and the horse’s one made up about a quarter.
• Both the horse and the cheetah cover about 23 feet per stride. The race horse takes about 2.5 strides per second and the cheetah at least 3.5, so the cheetah reaches about 70 miles per hour to the horse’s 43.

Ways of quickening the stride

• Larger muscles contract more slowly, which is why larger animals have slower strides.
• Running animals have their leg muscles attached close to the joint they move, giving a high “gear ratio”: more speed, less power. Walking animals have lower gear ratios, and digging and swimming animals lower still. Runners keep some low-gear muscles, probably for slow movements and starting.
• Moving several joints in the same direction at once adds their speeds together, like a person walking up an escalator. The extra joint from toe or hoof walking, the swiveling shoulder blade and the flexing back all add to the speed of the foot.
• Because the flexing spine moves the front and back halves of the body at different times, the body as a whole can travel faster than the legs alone could drive it. For the cheetah this adds about two miles per hour.

Size and endurance

• Body weight rises with the cube of body length, but muscle force rises only with the square. This helps explain why the largest animals can neither gallop nor jump, why small runners such as rabbits and foxes can match race horses without special adaptations, and why large runners must be highly adapted to run at all.
• Large runners built for endurance have light limbs: muscles that move the legs sideways or rotate the forearm are reduced or lost, the ulna and fibula are reduced, and the heavy muscles sit close to the body while the lower legs stay light.
• Running carnivores have compact feet with box-like bones. Hoofed animals have lost side toes, and the remaining toe bones are fused into a single cannon bone. Their joints work as hinges in the line of travel.
• In horses and other hoofed runners, the impact of the hoof bends the fetlock joint and stretches springy ligaments, which snap back as the foot leaves the ground, straightening the joint and giving the leg an upward push. The author reports Charles L. Camp’s finding that these ligaments evolved from foot muscles when the animals left the river valleys for open plains.

3. What they concluded

• Running animals (“cursorial” animals, those that can run far, fast and easily) evolved from good walkers. Running lets them forage over wide areas, find new food and water when supplies fail, and cope with seasonal changes. Predators use speed, relay tactics, endurance or surprise; prey are often as fast as their pursuers or have better endurance or agility.
• Natural selection has made fast runners by lengthening the leg, especially its lower parts, raising the gear ratio of the muscles, adding joints that move together, and lightening the limbs.
• Through these and other adaptations, nature has reconciled the limits of muscle mechanics with the demands of speed.

6. Other technologies, skills or innovations

• An electric motion-picture camera exposing 200 frames per second, filming from the side of animals running over open ground.
• Frame-by-frame tracing of film onto paper to measure stride, footfall timing and the movements of the limbs, shoulder blade and spine.
• The article builds on Eadweard Muybridge’s 1872 sequence photographs, taken with a battery of still cameras before motion pictures existed, which settled whether a galloping horse ever has all four feet off the ground.