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Asymmetrical gaits (that is, gallops and bounds) have the footfalls of a pair of feet unevenly spaced in time. Such gaits were studied from slow motion film for 79 genera. All information about the timing of events at the ground can be expressed by five variables. Foot contact intervals range from 16 to 70 percent of the cycle. Fore and hind contacts are nearly equal for most ungulates and carnivores; fore contacts are the shorter for most rodents and rabbits. Fore contacts are proportionately shorter at higher speeds. The size of the fore lead is less than the hind for some apes; fore and hind leads are about equal for many carnivores and ungulates; fore leads are the longer for most mammals-particularly for smaller, more agile genera when moving fast. Actions of the forefeet as a pair are related to those of the hind feet by “midtime lag.” When this variable is plotted against the percentage duration of ground contact by one or both hind feet, a basic gait graph is derived on which are distinguished gaits with no suspensions, with a gathered suspension, an extended suspension, and both suspensions. The distribution of plots on the graph also correlates roughly with body size, maneuverability, and lead sequence (that is, transverse, rotary, half bound, or bound). A terminology of asymmetrical gaits is presented. The distribution on the graph of 104 identified footfall formulas is shown, and formulas characteristic of 55 genera are depicted. Asymmetrical gaits probably evolved, in amphibians and several times in reptiles, to benefit escape. Gaits with short leads or none, and an extended suspension are considered primitive. All lead sequences evolved early.
Milton Hildebrand (Tue,) studied this question.