Walking, Running, and Jumping

1. What they did

Milton Hildebrand gives a short overview of how land vertebrates walk, run and jump. He describes four ways scientists have studied locomotion: physiology, anatomy, mechanics, and motion analysis, naming key earlier works in each.

He sketches the body features of cursorial (running) animals, then describes his own study in progress. He was analysing the gaits of animals from motion pictures taken at 64 to 200 frames per second, some filmed for the study and others bought, rented, donated or exchanged from sources in several countries. With a large film editor and frame counter he drew “progression diagrams” showing the order in which the feet touch down and how long each foot stays on the ground over one or more strides. At the time of writing, one or more gaits of about 50 genera (mostly mammals) had been analysed, and he expected to double that before publishing results.

2. What they found

Body features of runners and hoppers

• Runners are usually larger than rabbits and may be as large as the giraffe; hoppers tend to be smaller than kangaroos and may be as small as mice.
• Large runners have a fairly rigid spine. Carnivores and rabbits gain speed partly by flexing and extending the back as they gallop. Two-legged hoppers have a short spine in front of the hips for stability and a long tail as a counterweight.
• Limbs are long. Runners lengthen the leg further by standing on their toes (digitigrade: some reptiles, birds, carnivores) or on the tips of their toes (unguligrade: hoofed mammals), instead of flat-footed (plantigrade). Limb muscles sit close to the body, so a longer leg costs little extra effort to swing.
• Foot bones are compacted (cheetah), fused (birds, artiodactyls), or side toes are lost (ostrich, some marsupials and rodents, hoofed mammals), adding strength without weight.
• The shoulder blade lies upright against a deep, narrow chest. With the collar bone lost or reduced, it can swing with the forelimb, lengthening and speeding the stride.
• Hinge-like joints keep the legs moving in the direction of travel and prevent dislocation. Muscles attach close to the joints they move, which gives faster limb movement but less force.

Describing gaits

• One stride is one complete cycle of motion, counted from when a hindfoot is placed. The author lists nine timing variables for a four-legged stride, but shows that only a few matter for each family of gaits.
• Evenly spaced footfalls (walk, trot, pace): the left and right feet of each pair touch down at even intervals, so the animal has no lead foot. These gaits can be graphed using just two variables: how much of the stride each foot is on the ground (which roughly tracks speed), and how the forefoot touchdowns are timed relative to the hindfeet.
• The crawling walk, moving only one limb at a time, is used by turtles and infant mammals.
• A four-beat walk where each forefoot follows the hindfoot on the same side gives smooth, continuous support; it is used by elephants and most hoofed mammals.
• The pacing walk lets long-legged, agile animals such as the cheetah and gerenuk avoid their legs getting in each other’s way.
• The trotting walk of short-legged animals such as the weasel and hippopotamus gives the most stability.
• When sped up, these become the pace, the running walk and the trot, without any break in stride.
• A fifth walk, where each forefoot follows the hindfoot on the opposite side, is used by few animals.
• Unevenly spaced footfalls (the gallops): the animal runs with a lead. In the transverse gallop the fore and hind pairs use the same lead; in the rotatory gallop they use opposite leads. Related gaits are the half-bound (hindfeet placed together, forefeet one after the other) and the bound (each pair placed together). The type of gallop or bound matches body size, speed and manoeuvrability well, and it is usually possible to predict the gaits a particular animal uses.

Speed and stride

• Time in the air tends to increase with speed and manoeuvrability, but large animals are on the ground more of the time at top speed than near top speed.
• Top stride rates range from about 1.4 strides per second (elephant) to 10 or 12 (certain lizards and mice).
• As they speed up, hoofed mammals and carnivores lengthen their stride a lot but quicken it only a little. Small rodents also lengthen their stride but slow their stride rate.
• With more speed, the forefoot stays on the ground for a shorter time compared with the hindfoot.

3. What they concluded

• Only de la Croix had tried a full analysis of vertebrate gaits, and his conclusions often must be questioned, so some papers on locomotion are too theoretical: limbs that do not swing like pendulums are treated as pendulums, body size is linked to speed without knowing real stride rates, and some factors in speed, support, balance or manoeuvrability have been missed.
• Despite the apparent complexity, meaningful patterns in gaits are easy to find once the right variables are plotted.
• High-speed film also allows study of the paths of the limbs, the role of the spine, head, neck and tail, and the order and size of coordinated movements.
• Results of the film study were not yet ready; it was too early to include even tentative results.

6. Other technologies, skills or innovations

• High-speed motion pictures (64 to 200 frames per second), studied frame by frame on a large film editor with a frame counter.
• Progression diagrams: time charts of each foot’s touchdown and ground contact through the stride.
• Earlier, Muybridge (1899) photographed more than two dozen kinds of mammals and several kinds of birds with a battery of 24 still cameras triggered in sequence.