By convention, Hildebrand chose one hindlimb as reference to calculate footfalls variables. However, in experimental conditions, it may often happen that the forelimb stride only can be completely recorded. In such cases, the Hildebrand's model can be also applied as follows. Let us take from [5] (Fig. 3-2) the examples of the pigmy hippopotamus, characterized by a single foot in lateral sequence, and the monkey, characterized by a trotting walk in diagonal sequence. A simple horizontal shift in the associated footfalls diagram changes the reference limb from the left hindlimb to the left fore limb (Fig. 1). Consequently, the percentage that hind footfall (LH) follows fore (LF) on the same side becomes 80 in the case of the pigmy hippopotamus and 40 in the case of the monkey. The gait is plotted in the same position within the graph if we reverse the ordinate axis. One hundred percent has to be placed on the ordinate at the lower left corner of the graph when the reference limb is the hind one, whereas it has to be placed at the upper left corner when a forelimb is used. There is no change in the location of the lateral and diagonal sequences on the graph. Of course, the change of reference does not affect the sequence of limbs action. It may be noticed that a forward lateral succession and a backward diagonal crossing occur in a lateral sequence, whereas a backward lateral succession and a forward diagonal crossing occur in a diagonal sequence. The terms used by [6] and also by [7] are based on this observation and may bear sense for dynamic consideration.

We suggest a second addition to the Hildebrand model concerning asymmetrical gaits. He specified these gaits as follows [8]: “asymmetrical gaits have the footfalls of the two feet of a pair (fore or hind) unevenly spaced in time”, and in contrast for symmetrical gaits “since symmetrical gaits have the footfalls of a pair evenly spaced in time, the particular instant a given foot strikes always comes midway between two footfalls of its opposite”. For instance, the strike of a right hindlimb happens at 50% of the left hind cycle. The same time lag occurs between the right and the left forelimbs. These two variables, time lag between the footfalls of a pair of fore limbs and time lag between the footfalls of a pair of hindlimbs, can be plotted on a graph (Fig. 2). The points located at 50% on each axis (±5%) correspond to symmetrical gaits, and all those that are aside the range have to be considered as asymmetrical. Such a complement to the Hildebrand method could discriminate on the same graph the basic gait patterns of different species. The analysis of asymmetrical gaits necessitates peculiar variables [9]. To characterize the two kinds of flying phases when no limbs are on the ground that can appear in gallops, was used percent of stride interval that fore midtime (middle of the contact time by one or both forelegs) follows hind midtime, related to percent of stride interval that the body is supported by one or both hind feet. Another important feature is the lead sequence that is the time lag between the footfalls of the limbs within a pair. When a moderate fore lead (between the trailing and the leading fore limbs) appears the gait become a half bound. In the bound, forelimbs hit the ground together (no lead), followed by a flight phase before the hindlimbs hit also the ground together. In a gallop, the sequence of footfalls is transverse when the fore feet had the same lead as the hind (RF first and LF; RH first and LH). It is rotary when they had an opposite lead (RF first and LF; LH first and RH). As important character of diverse asymmetrical gaits, lead sequence can be plotted on a graph (Fig. 3). The hind lead (positive when the trailing hind leg is the same as the trailing fore leg, or negative when the trailing hind leg is opposite to the trailing fore leg) is indicated on the X-axis in percent of the stride interval; the fore lead is plotted on the Y-axis also in percent of the stride interval. Rotary gallops are placed on the left part of the graph, transverse gallops on the right, half-bounds directly on the ordinate and bounds at the axis intersection.

Gaits were usually analysed to characterize behavioural differences between taxa, and the Hildebrand grid system, based on quantitative parameters, placed this descriptive phase on sound basis. However, neuromuscular control of locomotion has also been intensively studied. There is now a demand to understand the complex organization, which underlies the diverse resulting gaits. A more integrative approach, compiling biomechanical, neurophysiologic, behavioural and ontogenetic data, is necessary [10]. Comparative locomotion studies are now questioning how animal's motion takes its place in the life history, relating form, function and evolutionary strategies. This implies the use of analytical tools, which can be scaled in correspondence: for instance, kinematic parameters have to agree with neurophysiological time scale.