Analysis of Asymmetrical Gaits

1. What they did

Gaits come in two classes. In symmetrical gaits (walks, the pace and the trot) the two feet of a pair land evenly spaced in time. In asymmetrical gaits (gallops and bounds) they do not: the two feet of a pair land close together, then there is a pause. The author had already analysed symmetrical gaits, and this companion paper is the first general analysis of asymmetrical ones.

He studied slow-motion film of 331 passes in front of the camera by mammals of 79 genera in 11 orders, from bandicoots and mice to horses, giraffes and cheetahs. Most of the film was his own; some came from film libraries and television and film companies. For each record he averaged three or more successive strides to smooth out the quirks of single strides, then built gait diagrams showing when each foot was on the ground.

He set out to:

• Find the smallest set of measurements that fully describes any asymmetrical gait.
• Propose a terminology for these gaits.
• Describe the asymmetrical gaits of many mammal genera.
• Interpret gait choice in relation to animal group, body size, speed, way of life and habitat.

2. What they found

Five measurements describe every gallop and bound

All the timing of events at the ground can be captured by five variables: how long the hind feet touch the ground (hind contact), how long the forefeet touch (fore contact), the hind lead and the fore lead (the delay between the first, or trailing, foot of a pair and the second, or leading, foot), and the “midtime lag” (how far the middle of forefoot support follows the middle of hind foot support in the stride).

Contact times

• Contact times shorten as animals go faster. In fast runners from the horse, dog and cheetah to the jumping mouse and rabbit, the hind foot is down for about 20% of the stride. In steady movement neither fore nor hind contact is likely to be over 70% of the stride.
• Fore and hind contacts are about equal for horses and their relatives, most even-toed hoofed animals and most carnivores. Fore contacts are about 80% of hind contacts for most rodents, several carnivores (including the cheetah at speed) and some hoofed animals, and 70% or less for hares at speed, jumping mice, kangaroo rats and springhares.
• Smaller, lighter animals tend to have relatively shorter fore contacts and more variation from stride to stride, which seems to go with agility, maneuverability and rough ground relative to body size.

Leads

• Leads vary widely within a genus. The fore lead is shorter than the hind only for most apes and a few others. It is about equal to, or up to 1¼ times, the hind lead for most carnivores, hoofed animals and horses, and more than 3 times the hind lead for the bounding and half-bounding marsupials, carnivores, rabbits, rodents and hyrax.
• The fore lead tends to grow relative to the hind lead as body size decreases, as agility and speed increase, and as contact times shorten.
• All galloping mammals use both right and left leads. When turning, the inside forefoot leads.

Suspensions and the gait graph

Plotting hind support against midtime lag gives a “basic gait graph” with four zones: gaits with no airborne phase (slow gallops of many mammals, fast ones of the giraffe), a gathered suspension with legs under the body (fast gallops of horses and camelids), an extended suspension with legs stretched out (deer, some rodents), and both suspensions (fast gallops of rabbits and many carnivores and hoofed animals).

Lead sequences

• In a transverse gallop the forefeet lead on the same side as the hind feet; in a rotary gallop on the opposite side. In a bound both pairs land together; in a half bound the hind feet land together but the forefeet do not; in a pronk (stott) all four land at about the same moment.
• The transverse gallop is typical of primates, most carnivores, horses and rhinoceros, and camelids. The rotary gallop is preferred by the tapir and giraffids, and by the fastest, most maneuverable runners (cheetah, blackbuck, gazelle). The half bound and bound are typical of most marsupials studied, tree shrews, rodents, small carnivores, sea lions and fur seals, and hyrax. Several mammals switched to the half bound or bound as speed increased.
• Overall, heavier, less agile animals and slower movement go with transverse gallops with no suspension or a gathered one; lighter, more maneuverable animals and faster movement go with an extended suspension or both, and with rotary gallops, half bounds or bounds.

Footfall formulas

He mapped 104 footfall formulas (the order in which combinations of feet support the body) onto the gait graph and showed those typical of 55 genera. There are many more formulas than had been described before, and the horse alone may use about 21.

Terminology

He proposes describing a gait in words by combining speed, lead sequence, the length of the fore and hind leads, and the kind and length of suspension. For example, a cheetah’s very fast gallop is “a very fast rotary gallop having a short hind and very long fore lead, and short gathered and extended suspensions.”

3. What they concluded

• Five variables are both necessary and sufficient to describe asymmetrical gaits. Published descriptions should give variables from which the full gait diagram can be rebuilt.
• Despite wide variation within and overlap between genera, gait choice correlates roughly with lead sequence, body weight, speed, agility and maneuverability.
• Asymmetrical gaits probably evolved in small animals, at least once in amphibians (a frog’s hop is a kind of bound), several times in reptiles and probably in some mammals, first for rapid escape. Bounds, half bounds and gallops in both lead sequences are probably all very old. Gaits with short leads or none and an extended suspension are considered primitive.
• Most small mammals are versatile in their gaits, probably more than has been reported; large or highly specialised animals such as the rhinoceros, kangaroo and giraffe give up versatility.
• Symmetrical gaits are a special case of the general relationships described here, and the two classes meet at a broad interface.
• The author hoped to prepare films of these gaits for study and teaching.

6. Other technologies, skills or innovations

• Slow-motion film analysis frame by frame, at 18 to 200 frames per second.
• Computer plotting of 100 exploratory bivariate graphs.
• Gait diagrams, the basic gait graph with overlays, and a standard footfall-formula notation (always shown with a right hind lead and starting from the left hind footfall).