**3.3 Discussion**

Though definitely forming a graded spectrum, the various laterally coordinated gaits of horses can be distinguished kinematically and in terms of their footprint patterns (see **Figure 3**). The standard walk has a stride duration over a second in length, as well as a front/rear hind stance phase ratio around or over 1.0, and a stride length around 1.6 meters with a velocity of 1–2 m/s (see supplementary **Table S1**). The legs operate independently of each other (ipsilateral swing phase less than 50% and advanced lateral placement and liftoff around 20–25% of the stride length) and is very even (ipsilateral/diagonal step time ratio of 0.75 or higher). The intergascular angle, being 30 degrees or so, is small. The walk typically lays down of ipsilateral pairs of prints capping or sightly overstepping each other in two parallel tracks.

The running walk is faster than the walk at around 2.9 m/s with a stride duration of moderate size at 0.77 seconds and a lower front/hind stance phase ratio of around 0.91, and a longer stride length around 2.3 m (see supplementary **Table S2**). It has similar lateral advanced placement and ipsilateral/diagonal step time ratio as the walk, but is more laterally coordinated with the lateral advanced liftoff less than 20% and the ipsilateral swing phase overlap of 0.50 to 0.70. As it involves lots of rear leg extension it has a very high intergascular angle of 50–70 degrees. The running walk also has a diagonal/ipsilateral step distance ratio of 0.50 or more and so lays down a trackway of four independent hoof impressions nearly the same distance apart.

It should be noted that both the running walk and gallop lay down prints that are isolated from each other and so do not form pairs. However, the cycle length of the running walk trackway is shorter compared to the gallop trackway (2.2 vs. 3.1 meters), and while the step lengths of the running walk alternate between two short- and two medium-length steps which are fairly equivalent, in the gallop there is a short step (where contralateral feet spring off the ground), followed by a long step, followed by a medium-length step, in turn followed by another long step. As a result there is a noticeable asymmetry between the step lengths in the gallop, and the ratio of the stride length to horse height is around 1.8 or more versus the 1.4 or less found in the running walk [22].

The rack or tölt resembles the running walk in many ways but has a slightly higher speed of 2.7–4.2 m/s, a quicker stride duration around 0.62–0.53, and a smaller intergascular angle of around 40–41 degrees (see supplementary **Table S3**). At low speeds the trackway of the rack resembles the trot as it forms lateral pairs with the hind foot overstepping the front foot. At high speeds the rack forms diagonal pairs with the hind foot understepping the front foot and has a low diagonal/ipsilateral step distance ratio of 0.6 or less resulting in a bowed or wavelike pattern of prints. In the stepping pace the gait is heavily laterally coordinated with advanced lateral placements and lift-offs around 15.0% or less of the stride duration, an ipsilateral swing phase overlap of 0.70 or more, and an ipsilateral/ diagonal step time ratio of 0.50 or less giving uneven beats (see supplementary **Table S6**). Even heavier lateral coordination results in a pace with advanced lateral placements and lift-offs of 12.0% or less, ipsilateral swing phase overlaps of 80% or higher, and an ipsilateral/diagonal step time ratio of 0.30 or less (see supplementary **Table S6**). The stepping pace and pace also lay down tracks in diagonal pairs forming an undulating pattern in the sand.

The fox trot is a diagonally coordinated but lateral-sequence gait and represents a sort of hybrid lateral-diagonal gait, due to it being an uneven and broken trot. In it diagonal couplets land close together in time with the front foot coming down just before the hind one, yet it yields ipsilateral pairs that are capped or wherein the hind foot slightly oversteps the front foot at high speeds. The fox trot occurred at around 3.0–3.7 m/s with a stride duration of 0.60–0.67. It is characterized by an extremely large ipsilateral/diagonal step time ratio of 2.0–6.0 and a lateral advancement placement of 30–40% of the stride length resulting a gait with a large amount of diagonal support (50–70%) and capped or slightly overstepping ipsilateral couplets (see supplementary **Table S7**).

Some of the key discriminatory parameters are the stride duration (sec) and front/hind stance duration ratio, which tend to above 1.0 in the walk but below 1.0 in the running walk, rack, and stepping pace; the ipsilateral/diagonal step time ratio, which tends to be above 0.75 in the square gaits of the walk, running walk, and rack, and below 0.5 in the strongly laterally coordinated gaits of the stepping pace and pace and 2.0 or higher in the fox trot; the ipsilateral swing phase overlap, which is usually below 0.40 in the walk and fox trot, 0.50–0.70 in the running walk and rack, and above 0.70 in the stepping pace and pace; lateral advanced placement and lateral advanced lift-off, which are around 0.15–0.25 in square gaits such as the walk, running walk and rack, 0.05–0.17 in heavily laterally coordinated gaits such as the stepping pace and pace, and above 0.30 in diagonally coordinated gaits such as the fox trot; diagonal advanced placement and diagonaladvanced lift-off, which are around 0.26–0.35 in walk, running walk, and rack, above 0.33 in laterally coordinated gaits such as the stepping pace and pace, and typically below 0.20 in diagonally coordinated gaits such as the fox trot and trot; the bilateral support phase which increases from 15 to 30% of the gait in the walk and fox trot to 40–60% in the running walk and 60% or more in the stepping pace and pace; the diagonal support phase which increases from 10 to 20% in the pace, stepping pace, and walk, to 20–40% in the running walk, and 50% or more in the

#### **Figure 8.**

*Photographs of laterally-coordinated gait trackways of modern horses. A. Fast walk of a Tennessee walking horse (horse 12), stride length = 176 cm; B. running walk of a Tennessee walking horse (horse 12), stride length = 203 cm (horse 12); C. slow tölt with overstepping lateral pairs in an Icelandic horse (horse 9), stride length = 163 cm; D. fast tölt (stepping pace) with understepping diagonal pairs in an Icelandic horse (horse 9), stride length = 217 cm. Black bars are 50 cm long.*

#### *Laterally Coordinated Gaits in the Modern Horse (*Equus ferus caballus*) DOI: http://dx.doi.org/10.5772/intechopen.106490*

fox trot and trot; the maximum rear intergascular angle, which is around 30 degrees in a walk, 30–45 degrees in a rack and 45–70 degrees in the running walk; stride length which is around 1.5–1.8 m in the walk, 1.6–2.7 in the running walk and rack, and above 2.5 in the gallop; the diagonal/ipsilateral step distance ratio, which is typically below 0.5 in the fast rack, stepping pace, and pace, 0.5–1.5 in the walk and running walk; and finally foot pair lateral offset/hoof width which is usually less than 0.25 in the walk, fox trot, and trot, and 0.25 to 1.4 in the fast rack, stepping pace, and pace. Overall then our temporal and linear data matched that of previous studies.

Visually the walk, trot, and fox trot leave trackways with ipsilateral pairs in roughly parallel rows, the running walk and gallop leave trackways with isolated single hoof prints with the running walk pattern typically more symmetrical than the gallop pattern, and the fast rack, stepping pace, and pace leave trackways with an undulating pattern formed by alternating diagonal pairs of hoofs (see **Figures 7** and **8**).

Finally, it is worth noting that we observed an occasional asymmetry between the left and right side of the horse even in the so-called "symmetrical gaits" wherein, for example, the left front foot might lift off before the right front sets down but not the reverse.
