Laterally Coordinated Gaits in the Modern Horse (Equus ferus caballus)

Abstract

Besides “natural” gaits of walk, trot, and canter, selected horse breeds engage in the so-called artificial gaits, including the fox trot, running walk, and rack. Though some studies have been undertaken of these artificial gaits, the datasets are incomplete, sample sizes are small, and no comprehensive overview has been written. After reviewing the literature and detailing what is known about these artificial horse gaits, the authors present data of their own. Linear, temporal, and footprint parameters or given regarding artificial gaits of twenty horses total from specialized breeds. In addition to finding decreasing stride duration, lateral advanced placement, and tripedal support as one moves from the walk to the running walk to the rack, as with previous studies, we also found decreasing ipsilateral/diagonal step time ratios and increasing ipsilateral swing phase overlaps. Visually, the walk, trot, fox trot, and slow rack leave trackways of ipsilateral pairs in parallel rows, the running walk and canter leave trackways of isolated prints with the running walk pattern more symmetrical, and the fast rack, stepping pace, and pace leave trackways with an undulating pattern formed by diagonal pairs of hooves with hooves often crossing over the center line yielding a negative interior straddle.

1. Introduction

A gait is describable in terms of a footfall sequence of the landing and the lifting-off of feet as well as in terms of speed and coordination of limbs. When surefootedness is needed on slipper substrates walking gaits are best. In a walk, and at increasing speeds in a flat walk and running walk, as well as in the more laterally coordinated rack, the footfall sequence is RH-RF-LH-LF, and one foot is on the ground at all times allowing for great stability. At medium speeds a trot often occurs in horses, as it does in the zebra, wherein diagonal leg pairs move together in a two-beat rhythm, LH-RF and RH- RF. Such a gait seems to dominate in horse species that moved onto the steppes and plains where a softer substrate occurred along with a need for efficient locomotion over longer migration distances and speed to outrun predators. There is a moment of suspension in the trot and the trot is an easy travel gait for the horse and very efficient when a spring mechanism is present in the animal as occurred in monodactyl horse species. Some horse breeds now, and likely also in the past, could perform a medium-speed laterally coordinated gait of the pace. In a pace ipsilateral leg pairs move together in a two-beat rhythm, LH-LF and RH-RF. There is again a moment of suspension. The pace is also found in camels and seemingly has some advantages in travel over certain substrates, such as sandy ones, and variegated terrains, as well as biomechanical advantages. The canter and gallop have a different rhythm and are asymmetrical. The footfall sequence starts off with RH-LH-RF-LF for the so-called left lead and LH-RH-LF-RF for the right lead canter or gallop and have moments where all four limbs are off the ground. The canter is slower than a running transverse gallop and the hind feet move in unison whereas in a running gallop the legs move more one after the other. The transverse canter and gallop are useful when speed is of the essence. In the rotary gallop, an advantage in sudden jumps or turns in the field, or needs instant acceleration, the hind front diagonal is replaced by the hind front lateral.

Horses will select their gait according to the stability, balance, maneuverability, efficiency, and speed needed within the framework of their anatomical configuration. They will also adjust their gaits to suit the substrate. Though we can forget this today, horses were animals of prey to which sudden outbursts and fast runs and maneuverability were a matter of life and death. Horses also have various traveling gaits when the herd is migrating. They also display unique ways of locomotion during courting and herding, or chasing away another horse they want out of the way for feeding reasons or to protect a foal (Renders, pers. observations). This is why they have a well-developed cerebellum responsible for determining the most suitable gait given a situation. Through continual study we are able to learn more and more about these gaits of horses. Here, in particular, we wanted to study horse gaits in their most spontaneous state, that is to say the gaits they most readily turn to and make use if left to their own devices. Still study of horse gaits does often necessitate, as here, observations on horses raised by trainers and so these gaits are in part “taught” to the horse, yet horses have an ability to switch between various gaits for diverse purposes, or to adjust the synchronicity and symmetry of the gait when desired. In other words though there has been a lot of focus on ideal gaits displayable in a show ring horses may often spontaneously opt for less ideal gaits that reflect more how they behaved in the past and in the wild.

2. Review of studies undertaken on “artificial” laterally coordinated horse gaits

Square ipsilateral horse gaits are those in which all four limbs operate relatively independently and involve a lateral footfall sequence of left hind (LH), left front (LF), right hind (RH), and right front (RF). The slowest and most basic square ipsilateral gait of the horse is the walk (see Table 1), an evenly-timed four-beat lateral-sequence and lateral-couplet gait, and so with a footfall sequence that can be demarcated with en dashes (−) indicating longer passages of time (LH–LF–RH–RF) and wherein ipsilateral limbs contact the ground closest together in time [1, 25, 26]. The walk typically occurs at speeds of 0.9 to 2.1 m/s, with a stride or cycle duration of 1.0 to 1.5 seconds, a stride frequency of 0.7 to 1.1 strides per second, and a stride or cycle length of 1.3 to 1.9 meters [2, 3, 4, 5, 20, 22, 30, 31, 32]. On account of its slow speed and long stride duration there are no whole suspended phases where all four limbs are off the ground at once, or even contralateral suspended phases where both front limbs or both hind limbs are off the ground together, and so the hind limb makes contact with the ground for 55–75% of the stride or cycle length (i.e. a duty factor of 0.55 to 0.75), yielding support structures containing lots of three-feet support phases (54%), alternating with bilateral (24%) then diagonal (22%) support phases (see gait diagram a in Figure 1), i.e. 3-2D-3-2 L [3, 4, 22]. In the typical walk the front foot duration of ground contact is 0.95–1.04 that of the hind foot, the timing of the steps are fairly even (lateral advanced placement of 0.20–0.24 of the stride length and diagonal advanced placement of 0.26), independent of each other or square (lateral advanced liftoff of 0.25 and ipsilateral overlap of front and hand limbs for 44% of the stride), and with each foot remaining flat on the ground as the one after it comes down [6, 7]. The hind feet often cap and so overlap the vacated location of the ipsilateral front feet when they land or slightly overstep them up to a hoof-length, or more technically by 3–19 cm or 0.01 to 0.10 of the stride length, resulting in footprints in lateral pairs that are generally parallel with each other and just to the side of the centerline [5, 7, 8, 22, 32]. In gaited horse breeds the walk may be extended with the hind legs overstepping the ipsilateral front legs somewhat, in which case it is often called a flat walk or dog walk.

Gait	Walk	Running Walk	Rack or Tölt	Stepping Pace	Pace	Source (s)
Velocity (m/s)	0.9–2.1	2.7–4.5	2.7–7.6	3.8–7.4	8.0–12.3	[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
Stride Duration (s)	1.5–1.0	0.75–0.68	0.66–0.42	0.62–0.54	0.54–0.34	[2, 3, 6, 8, 10, 11, 12, 16, 17, 20]
Stride Frequency (s^−1)	0.7–1.1	1.3–1.5	1.4–2.4	1.6–1.9	1.9–2.9	[1, 3, 6, 11, 12, 13, 14, 20, 21]
Stride Length (m)	1.3–1.9	2.0–2.2	1.7–3.2	2.0	3.5–6.3	[5, 6, 7, 8, 11, 12, 13, 16, 17, 20, 22, 23, 24]
Duty Factor [Hind Stance %]	0.75–0.55	0.58–0.52	0.66–0.36	Unreported	0.44–0.27	[1, 2, 3, 4, 7, 8, 10, 12, 20]
Front/ Hind Stance Ratio	1.04–0.95	0.92–0.86	0.95–0.85	0.98–0.80	1.02–0.93	[1, 3, 6, 11, 17]
Beats	4	4	4	4	2	[1, 25, 26]
Synchro-nicity	Even	Even	Even	Uneven	Even	[1, 25, 26, 27]
Lateral Advanced Placement	0.24–0.20	0.22–0.12	0.30–0.18	0.24–0.13	0.17–0.05	[1, 6, 7, 8, 9, 10, 11, 13, 14, 28]
Lateral Advanced Liftoff	0.25	0.18–0.10	0.29–0.13	0.16	0.24–0.05	[7, 8, 18, 29]
Diagonal Advanced Placement	0.26	0.29–0.37	0.22–0.28	0.32	Not Reported	[9, 10]
Diagonal Advanced Liftoff	Not Reported	0.35–0.39	0.26–0.30	0.32	Not Reported	[9, 10]
Tripedal Support (% of stride)	54	23–5	37–0	25–15	0	[1, 4, 6, 7, 9, 10, 11, 12, 13, 14, 15]
Bilateral Support (% of stride)	24	50–67	22–46	48	63–93	[1, 4, 6, 7, 9, 10, 11, 12, 13, 14, 15]
Diagonal Support (% of stride)	22	27–16	48–3	27	0	[1, 4, 6, 7, 9, 10, 11, 12, 13, 14, 15]
Unipedal Support (% of stride)	0	0–8	8–65	Not Reported	0	[1, 4, 6, 7, 9, 10, 11, 12, 13, 14, 15]
4-Limb Suspension (% of stride)	0	0	0	0–10	7–37	[1, 4, 6, 7, 9, 10, 11, 12, 13, 14, 15]
Ipsilateral Stance Phase Overlap	0.44	Not Reported	0.63–0.64	Not Reported	0.61–0.68	[1, 4, 7, 13, 14, 28]
Overstep (% of stride length)	0.01–0.10	0.15–0.25	Not Reported	Not Reported	Not Reported	[5, 7, 8, 17, 22]

Table 1.

Temporal and linear kinematic parameters of the ipsilaterally coordinated gaits of horses reported in previous studies.

Figure 1.

Measurements taken of horse trackway (LH = left hind; LF = left front; RH = right hind; RF = right front). FW = footprint width; FL = footprint length; ISD = ipsilateral step distance; DSD = diagonal step distance; LO = lateral offset. IS = interior straddle; SL = stride length.

Most horse breeds, and the zebra, transition to the diagonal intermediate speed gait of the trot at around 2.0–2.5 m/s, but select horse breeds display other intermediate “artificial” square ipsilateral gaits. One such gait is the running walk (common to Tennessee Walking Horses, as well occurring at times in other horse breeds, such as the American Bashkir Curly Horse, Florida Cracker Horse, Kentucky Mountain Saddle Horse, Peruvian Paso Horse (paso llano), Smokey Valley Horse, Spotted Saddle Horse, and the Walkaloosa. Like the walk, the running walk is a fairly evenly timed four-beat lateral-sequence and lateral-couplet gait (see Table 1). The running walk, however, occurs at a faster velocity than the regular walk and involves a lot of hind-leg extension and a bit more lateral coordination. Hence it can be demarcated with a hyphenated footfall sequence of LH-LF-RH-RF. The running walk typically occurs at a speed of 2.7–4.5 m/s, although it can be even faster in a few horses, with a stride duration of 0.68–0.75 seconds, a stride frequency of 1.3–1.5 strides per second, a stride length of 2.0 to 2.2 meters, and a hind-leg duty factor of 0.52–0.58 [1, 9, 10, 22]. At slower speeds the running walk is maintained via front legs pulling the horse forward while the hind longs engage in an extended stride with one hind foot lifting off while the other hind foot is still on the ground whereas the front feet employ shorter strides with one front foot coming down just as the other one is going up. Hence at slow speeds the running walk has a support structure with periods of tripedal, followed by diagonal then bilateral support, i.e. 3-2D-2L-3-2D-2L, though with much less three-limbed and more two-limbed support structures than in the standard walk, namely 15–23% tripedal, 50–67% bilateral, and 16–27% diagonal. At faster speeds the running walk can even have periods of single-foot support as both front feet are off the ground at the same time, though there is no period wherein both hind feet or all four limbs are off the ground. Thus at fast speeds the running walk has a support structure of 3-2D-1-2 L-3-2D-1-2 L, with a stride possessing 5–11% tripedal, 55–58% bilateral, 25–26% diagonal, and 2–8% single foot support structures [8, 9, 10]. At slow speeds the running walk has a lateral advanced placement of 0.22, a lateral advanced liftoff of 0.18, a diagonal advanced placement of 0.29, and a diagonal advanced liftoff of 0.35 the stride duration, while, at fasts speeds, these values reach 0.12, 0.10, 0.37, and 0.39, respectively, indicating more lateral coordination and less evenness to the gait [8, 9, 10]. Because the running walk involves a long hind-leg extension and the hind legs often scissoring out in an arc, two key kinematic features separate it from the standard walk. In the first place the front limbs are on the ground less than the hind limbs are, so there is a lower front to hind limb stance duration ratio of 0.86–0.92. Secondly, there is a very large overstep of hind feet over ipsilateral front feet of 15–50 cm, or around 15–25% of the stride length [8, 22]. Indeed typically the hind feet land near or just beyond the mid-point of the prior location of the front feet yielding footprint pathways of four insolated and almost evenly-spaced prints rather than prints with lateral or diagonal pairs. The running walk is a very smooth gait with little side-to-side or up and down movement of the horse’s back, though it often involves the horses head nodding up and down with the movement of the hind limbs (Tennessee Walking Horse) or the forelimbs moving in a swinging arc (termino of the Peruvian Paso), and a slight rocking fore-to-back motion in the saddle.

Other gaited horse breeds transition from the slow walk to the intermediate “artificial” laterally coordinated gait of the rack or tölt. Such a racking gait is famous in the American Saddlebred, Icelandic Horse, Kentucky Mountain Saddle Horse, Mountain Pleasure Horse, North American Single-Footing Horse, Racking Horse, Rocky Mountain Horse, Smokey Valley Horse, Spotted Saddle Horse, and the Tennessee Walking Horse, but it also occurs in such breeds as the Aegidienberger, American Bashkir Curly Horse (curly rack with quick stride duration but lots of tripedal support), Garrano Horse (paso travado), McCurdy Plantation Horse (plantation gait), Mangalarga Marchador (marcha picada), Morgan Horse, Tiger Horse, as well as various Asian breeds [1, 10, 20, 33, 34, 35, 36]. The rack or tölt is another fairly square and evenly-timed four-beat gait (see Table 1), with a footfall sequence of LH-LF-RH-RF, but one that involves more collection and quickness with a stride duration of 0.42–0.66, and a fair amount of lateral coordination, with the front limbs touching down close behind the hind limbs in time and remaining down until just after the opposite hind limbs contacts the ground, while the hind limbs of the horse land close to but behind the contralateral front limbs in space, resulting in a bowed or wave-like footprint pattern, i.e. undulating, comprised of diagonal pairs [3, 4, 11, 12, 13]. At lower velocities of 2.7–3.8 m/s the rack is often labeled a mountain, pleasure, saddle, slow, stepping, style, or trail gait and possesses a 3-2D-3-2 L support structure–with 2–37% tripedal support, 25–46% bilateral support, and 16–48% diagonal support–a hind limb duty factor of 0.50–0.66, and a stride length of 1.7 to 2.2 m [11, 12, 13, 22, 36]. At such slow speeds the rack or tölt has a lateral advanced placement of 0.20–0.30, a lateral advanced liftoff of 0.27–0.29, a diagonal advanced placement of 0.23–0.28, and a diagonal advanced liftoff of 0.26, as well as a lateral overlap of 0.30 of the stride cycle, with the front feet having a stance duration 0.85–0.95 that of the hind feet [10, 13, 14, 15, 16, 28]. At very slow speeds the rack often has ipsilateral feet forming footprint pairs on the ground or isolated prints resembling a running walk. At faster speeds of 3.9–7.6 m/s the rack or tölt is often called a performance, racing, road, show, or speed gait (or the contrarily labeled pleasure gait of the Rocky Mountain Horse), tends to become more animated with the front legs high-stepping or pitching out in an arcing motion, and bears 3-2D-1-2 L-3-2D-1-2 L or even 2D-1-2 L-1 support structures. A fast rack is hence often called a single-foot gait, especially in the North American Single Foot Horse and Racking Horse, and displays periods of single-leg support from 8 to 65% of the stride cycle depending whether there is contralateral suspension in the front alone (half rack) or both the front and hind feet (full rack), bilateral support of 22–40%, diagonal support of 3–14%, and lacking tripedal support structures. At such high speeds the rack or tölt possesses a lateral advanced placement of 0.18–0.30, a lateral advanced liftoff of 0.13–0.20, a diagonal advanced placement of 0.22–0.24, and a diagonal advanced liftoff of 0.30, as well as a hind limb duty factor of 0.36 to 0.49, fore feet on the ground only 0.62–0.87 the duration of the hind feet, and a stride length of 2.2–3.2 m [10, 13, 14, 15, 16, 17, 22, 28]. At these high speeds the rack lays down prints with diagonal pairs and consequently an undulating pattern of prints. The rack often exhibits some up-and down movement of the rear of the horse (seen in croup and tail), due to the horse pushing off forcefully from the hind feet, while the rider in the saddle can experience a slight side to side motion due to the increased lateral coordination of the limbs.

Many gaited horse breeds can also engage in a stepping pace, amble, or broken pace, i.e. an unevenly-timed shuffling 4-beat laterally coordinated gait, rather than an even 4-beat gait, so having a footfall pattern of LH-LF–RH-RF (see Table 1). A stepping pace, in particular, occurs in the Asturcón, Campolina (somber paso), Columbian Criollo Paso, Icelandic Horse (skeith tölt), Florida Cracker Horse (coon rack), Paso Fino (andadura), Racking Horse, Spotted Saddle Horse, and Tennessee Walking Horse. Such a gait has been little studied as it is usually not considered a desirable gait by breeders or horse breed associations, and, in fact, is often named depreciatively as a piggy-pace, slick pace, or pacey rack or pacey tölt, perhaps as it is somewhat uncomfortable for the rider since the horse’s neck and back move from side to side. In the Peruvian Paso Horse, however, it is considered a desirable gait and named the sobreandando (agulillo; pasiamblado), and it is also prized in the Tiger Horse (glider gait or lateral Indian shuffle) and Virginia Highlander. In the stepping pace there is a large amount of lateral coordination of limbs as ipsilateral hind and front limbs lift off the ground just after each other and contact the ground just after each other and cause side-to-side motion for the rider. The stepping pace occurs around 3.8–7.4 m/s, with a stride duration of 0.54–0.62, a stride frequency of 1.6–1.9, a stride length of 2.0 m or more, and hind limbs remaining in the stance phase 0.80–0.98 as long as the hind legs. It possesses a lateral advanced placement of 13–24, lateral advanced liftoff of 10–16, diagonal advanced placement of 32–34, and diagonal advanced lift-off of 37–42. The stepping pace or sobreandando at slow speeds has a support structure of 15–25% tripedal support, 48% bilateral support, and 27% diagonal support at slower speeds, or a 3-2 L-3-2D support structure, and at higher speeds there can be some single-foot support with alternating contralateral front and hind suspensions, i.e. 2 L-1-2 L-1, or even occasional suspension of all four legs from 1 to 10% of the stride duration with a 2 L-0-1-2 L-0-1 support structure [10, 15].

A related, but opposite, “artificial” gait, is the broken trot, which is usually called the fox trot and found in the Missouri Fox Trotter and the Marsh Tacky (swamp fox trot), or the marcha batida and found in Mangalarga Marchador horses (with occasional quadrupedal support phases). Though often described as a diagonal gait it is technically a lateral-sequence [LH–LF-RH–RF] diagonal-couplet gait, one in which the front limb lifts off just before the ipsilateral hind limb lands, giving the impression of the horse kicking its front legs forward with its hind legs [21, 27, 36]. As a broken trot, the fox trot also possesses an uneven four-beat cadence with some fore-to-back motion for the rider. It is also found in Icelandic Horses (brokk tölt), Paso Fino horses (paso corto and paso largo), Peruvian Paso horses (pasitrote), Columbian Criollo Paso horses (trocha), Spotted Saddle horses, and in the Nez Perce, Nokota Horse, the Tiger Horse (where it is called the glider gait or diagonal Indian shuffle), the Walkaloosa (Indian shuffle), and wild Mustangs. At high speeds it is sometimes called the fox rack or flying fox trot.

While most horse breeds switch from the trot to the gallop at around 4.0 m/s some horse breeds engage in the hard, straight, or true pace, the fastest lateral gait which involves heavily coordinated ipsilateral legs. The pace is a two-beat gait wherein ipsilateral legs lift-off and touch the ground simultaneously allowing for long stride lengths and great speed where it is called the flying pace or speed pace (see Table 1). The pace is found in Standardbred, Peruvian Paso (huachano), and Icelandic horses (flug skeith) and typically occurs at speeds of 8.0–12.3 m/s, and possess a stride duration of 0.34–0.54, a stride frequency of 1.9–2.9 strides/second, a lateral advanced placement of 0.07–0.17, a lateral advanced liftoff of 0.08–0.24, a front to hind limb stance duration ratio of 0.93–1.02, and a stride length of 3.5–4.8 meters [1, 4, 18, 19, 23, 24, 29, 37]. The pace has a hind-limb duty factor of 0.27–0.44 and a suspension phase of 7–37% of the stride wherein all four limbs are off the ground [16, 20]. The pace thus has a support structure of 2 L-0-2 L-0. Like the rack and stepping pace it forms footprints in the sand of diagonal couplets in an undulating pattern as the hind feet come down just behind the contralateral front feet. The lift-off and set down of the limbs can be somewhat asymmetrical (8%) between the right and left sides of the horse and the pace also produces considerable side-to-side rolling motion for the rider.

3. A study of laterally coordinated gaits in modern horses

In order to add to our knowledge of artificial laterally coordinated horse gaits the authors undertook a study of the linear, temporal, and footprint parameters of twenty horses. This study remeasured some of the parameters described in the investigations mentioned above, completed them by measuring missing variables such as the diagonal advanced lift-off, and finally introduced new parameters useful in elucidating different aspects of a gait, such as temporal evenness, linear symmetry, and gauge width (interior straddle).

3.1 Materials and methods

To elucidate the temporal and linear parameters of the various lateral gaits of living horse breeds, studies were undertaken on twenty horses comprising various gaited breeds, seven Tennessee Walking Horses, four Icelandic Horses, three Peruvian Paso Horses, two Mangalarga Marchador Horses, two Rocky Mountain Horses, an American Saddlebred Horse, and a Spotted Saddle Horse (see Table 2 below for additional details concerning the horses studied). The horses were ridden by professional trainers selected by the owners of breed-specific horse farms in the lateral gaits specific to the breed, including the walk, flat walk, running walk, rack, and stepping pace of the Tennessee Walking horse, the paso llano, sobreandando, and huachano of the Peruvian Paso horse, the tölt of the Icelandic horse, the marcha picada of the Mangalarga Marchador, the slow gait and rack of the American Saddlebred Horse, the running walk, rack, and fox trot of the Spotted Saddle Horse, and the show and pleasure racking gaits of the Rocky Mountain Horse.

Horse Number	Breed	Gender	Height (m)	Hoof Length (cm)	Hoof Width (cm)
1	Tennessee Walking Horse	Female	1.50	12.0	12.5
2	Peruvian Paso Horse	Male	1.51	12.0	11.5
3	Tennessee Walking Horse	Male	1.53	14.6	ca. 14.9
4	Tennessee Walking Horse	Male	1.63	17.8	ca. 17.1
5	Spotted Saddle Horse	Female	1.52	12.7	ca. 12.7
6	Peruvian Paso Horse	Male	1.33	11.4	ca. 11.9
7	Tennessee Walking Horse	Female	1.66	15.2	ca. 14.8
8	Icelandic Horse	Male	1.32	12.7	ca. 13.2
9	Icelandic Horse	Female	1.34	12.7	ca. 12.1
10	Rocky Mountain Horse	Male	1.53	15.2	ca. 15.2
11	Rocky Mountain Horse	Male	1.45	14.0	ca. 14.5
12	Tennessee Walking Horse	Male	1.54	12.7	ca. 13.4
13	Tennessee Walking Horse	Male	NA	NA	NA
14	Icelandic Horse	Female	1.48	NA	NA
15	Mangalarga Marchador	Male	1.50	NA	NA
16	Mangalarga Marchador	Male	1.48	NA	NA
17	Icelandic Horse	Female	ca. 1.27	NA	NA
18	Peruvian Paso Horse	Female	ca. 1.44	NA	NA
19	American Saddlebred	Male	1.55	16.5	ca. 16.1
20	Tennessee Walking Horse	Male	1.59	13.3	12.8

Table 2.

Information regarding modern horses studied (Original data).

We did not investigate other four-beat lateral gaits such as the classic paso fino of the Paso Fino horse which most often lays down tracks of understepping ipsilateral pairs, the lope or slow canter of Western horses which lays down footprints in contralateral pairs that angle the same way, or the mixed canter of the Tennessee Walker or Missouri Fox Trotter wherein the horse canters in front with contralaterally coordinated legs but walks in the rear (aubin, broken rocking chair canter, valhopp, wicky wack) or the reverse (traquenard).

The horses were videotaped in these lateral gaits at 60 frames per second (or 30 frames per second at the European horse farms where the speed was usually below 3.0 m/s) with advanced cellphone cameras located perpendicular to the plane of motion and five to ten meters back so that all of the horse’s body was visible and foot contact with and lift-off from the ground could be observed. These videos were analyzed frame-by-frame via Frame Player software to determine the timing of the lift-off and set-down of each of the horses’ feet through a series of two stride cycles. From this recorded data various temporal parameters of the horses’ gait could be determined such as stride duration and lateral advanced placement.

In addition, the footprint patterns left by eleven of these horses in the sand were photographed and measured in order to determine various linear parameters of the gaits such as stride (cycle) length and overstep amount. This was accomplished by raking smooth a 6x10 meter rectangular area in the sandy arena and using a tape measure or yardstick to measure various distances between the footprints after the horses performed gaits over it (see Figure 1 for an illustration of measurements taken). Key physical parameters of each horse were also taken including height at the withers, and hoof length (see Figure 2).

Figure 2.

Measurements taken of horse. HW = height at withers; HL = hoof length; IGA – Intergascular angle (maximum angle during gait).

Figure 3.

Footprint patterns of various laterally-coordinated gaits in modern horses. In the fast walk (a) there is a small stride length with a small overstep of ipsilateral hind feed resulting in distinct lateral pairs of prints in roughly parallel lines and a diagonal step distance much larger than the ipsilateral one. The fox trot and true fast trot (B) forms a trackway similar to that of the walk with lateral pairs of prints lining up more or less in parallel but possesses a greater stride length. In the running walk (C) there is a large overstep yielding no obvious pairs of prints as the ipsilateral step distance is nearly equivalent to the diagonal one but wherein the ipsilateral step distances and diagonal step distances are roughly equivalent with themselves. This should be contrasted with the gallop (D) which also lacks obvious print pairings but which has a much greater stride length and in which there is greater variance within the ipsilateral and diagonal step distances and a sequence of contralateral feet. In the rack of tölt (E) the ipsilateral step length is much greater than the diagonal one resulting in diagonal pairs of prints that form a bowed pattern with a large stride length and hind impressions that often cross over the centreline. In the stepping pace and true pace (F) there is an even greater stride length and the diagonal pairs of prints occur very close together as the ipsilateral step distance is much larger than the diagonal one. The scale is in centimeters.

The velocity of the horses was determined by noting how long it took on the videotape for the horse to complete a measured stride length as well as on occasion additionally timing the distance it took the horses to travel between cones or strips placed three to ten meters apart with a stopwatch. The first method was found to be the most accurate as it was difficult to accurately note when horses crossed various horizontal markers due to limitations of human perception as well as angular effects of distance of viewer from the markers. Hence the second method of using a stopwatch and delineated markers to measure speed was abandoned after the first six horses studied.

The temporal and linear parameters of the horse gaits were determined through videotape analysis or footprint measurements based upon the following definitions (modified from [38]):

Stride duration: Time in seconds taken to complete a stride cycle, i.e. time between successive left hind ground contacts. The stride duration tends to decrease in faster gaits.

Stride frequency: Number of strides per second, i.e. 1/stride duration.

Front stance phase: Percent of the stride duration the front limbs are on the ground.

Hind stance phase [here considered to be the duty factor]: Percent of the stride duration the hind limbs are on the ground.

Average stance phase: Average value of front and hind stance phase. A value over 0.50 indicates there is no suspension phase where all four-limbs are off the ground simultaneously.

Front stance/Hind stance ratio: Front stance phase value divided by hind stance phase value. A value near or over 1.0 tends to indicate a slower walking gait is occurring.

Time between ipsilateral steps: Time in seconds between the ground contact of ipsilateral hind and front hooves.

Time between diagonal steps (s): Time in seconds between the ground contact of contralateral front and hind hooves.

Ipsilateral/diagonal step time ratio: Average value of time between ipsilateral steps divided by time between diagonal steps. This ratio relates to evenness of the steps and hence is 0.75–1.25 in even gaits, and less than 0.50 or above 1.50 in asynchronous gaits. A value below 1.0 indicates lateral coordination of limbs and above 1.0 diagonal coordination of limbs.

Ipsilateral swing phase overlap: Percent of stride cycle ipsilateral limbs are off the ground together. This indicates degree of lateral coordination and exceeds 0.75 in highly laterally coordinated gaits.

Ipsilateral stance phase overlap: Percent of stride cycle ipsilateral limbs are on the ground together.

Overall ipsilateral overlap: Average value of ipsilateral swing phase overlap divided by ipsilateral stance phase overlap. Higher values indicate strong coordination between ipsilateral limbs wherein ipsilateral hooves lift off and set down close in time to each other.

Swing/stance phase overlap ratio: Average value of ipsilateral swing phase overlap divided by ipsilateral stance phase overlap. A lower value of 0.60 or lower indicates a slow gait with a long stance phase while a higher value indicates a faster gait with increased swing phase time and less stance phase time.

Lateral advanced placement [Limb phase]: Time between contact of ipsilateral hind and front limbs, expressed as percent of stride cycle and so divided by stride duration. This value is 0.20–0.25 in square gaits with limbs operating relatively independently, less than 0.20 in gaits with ipsilateral limb coordination and lateral couplets, and 0.30 or higher in diagonally coordinated gaits with diagonal couplets.

Lateral advanced lift-off: Time between lift-off of ipsilateral hind and front limbs, expressed as percent of stride cycle, so divided by stride duration. This value is below 0.20 in highly laterally coordinated gaits and above 0.30 in highly diagonally coordinated gaits.

Diagonal advanced placement: Time between contact of front limb and contralateral hind limb, expressed as percent of stride cycle, so divided by stride duration. A value below 0.20 indicates a high level of diagonal-coordination of limbs while a value above 0.30 indicates a high level of lateral-coordination of limbs.

Diagonal advanced lift-off: Time between lift-off of front limb and contralateral hind limb, expressed as percent of stride cycle or divided by stride duration. A value below 0.20 shows strong diagonal-coordination of limbs while a value above 0.30 indicates strong lateral-coordination of limbs.

Foot couplet time differential: Time in seconds between closest contact of hooves whether ipsilateral, diagonal, or contralateral front or contralateral hind.

Tripedal support phase: Percent of stride cycle three limbs are on the ground at the same time whether two front and one hind or two hind and one front. This value can be 50% or more of the gait in slower gaits and missing in faster gaits.

Bilateral support phase: Percent of stride cycle two ipsilateral limbs are on the ground at the same time whether left or right side. This value is less than 30% in diagonally coordinated gaits and above 50% in laterally coordinated gaits.

Diagonal support phase: Percent of stride cycle two diagonal limbs are on the ground at the same time, i.e. one front limb along with its contralateral hind limb. This value is less than 30% in highly laterally coordinated gaits and above 50% in a highly diagonally coordinated gait.

Unipedal support phase: Percent of stride cycle only one leg is on the ground, whether front or hind. Such unipedal support occurs in fast four-beat artificial gaits such as the running walk and rack or tölt.

Front contralateral suspension phase: Percent of stride cycle both front limbs are off the ground at the same time.

Hind contralateral suspension phase: Percent of stride cycle both hind limbs are off the ground at the same time.

Four-limb suspension phase: Percent of stride cycle all four limbs are off the ground together. This occurs in fast two-beat gaits, namely a trot or pace, and the canter and gallop.

Maximum rear intergascular angle: Inverted pendulum angle formed between contralateral gaskins (from stifle to hock) when both hind hooves are on the ground during gait (placement) or during protraction when both feet are in the air (suspension), or a combination of the two (see Figure 2). This value will be higher in gaits involving larger hind leg extension such as the running walk.

Stride length (cycle length): Distance in meters between the hoof top (toe) of successive left hind footprints. The stride length tends to increase as a gait gets faster.

Stride length/Horse height ratio: Dimensionless speed ratio found by dividing stride length by horse height at the withers. As gaits get faster this number increases. In walking gaits this value is usually less than 1.0, between 1.0–2.0 in intermediate speed gaits, while in very fast gaits it can be above 2.0.

Distance between diagonal steps: Measurement in centimeters between toe and heel of contralateral front and hind hoof prints.

Distance between ipsilateral steps: Measurement in centimeters between toe and heel of ipsilateral front and hind hoof prints.

Ipsilateral overstep/Stride length ratio: Distance between ipsilateral steps divided by the stride length. In the running walk with much hind leg extension as well as in fast laterally coordinated gaits such as the rack, stepping pace, and pace, the ipsilateral overstep can be 15–30% of the stride length.

Diagonal/Ipsilateral step distance ratio: Distance between diagonal steps divided by distance between ipsilateral steps. This value is below 0.50 in highly laterally coordinated gaits and above 1.0, often by a large margin, in highly diagonally coordinated gaits.

Average interior straddle [Gauge width]: Average distance in centimeters between quarters of contralateral front and hind hoof prints. In the walk and running walk this value is typically positive but in gaits with high-lateral coordination the hind limbs are free to come in or cross the centerline without interference and so this value is often negative.

Average foot pair lateral offset: Average distance in centimeters between quarters of closest hoof print pairs whether formed by ipsilateral, diagonal, or contralateral front or contralateral hind. This value is low when ipsilateral pairs of feet are close together but high when diagonal pairs are close together.

Foot pair lateral offset/hoof width: Ratio of average foot pair lateral offset divided by average width of hoof at quarters. This value is high for gaits with diagonal pairs landing close together in space but low for gaits with lateral pairs landing close together in space.

3.2 Results

Temporally the standard walk was characterized by a slow velocity (around 1.5 m/s), a stride duration over a second in length (1.16 seconds average), a front limb stance phase that is nearly as long as or even longer than the hind limb stance phase (1.02 front/hind limb stance phase ratio on average), a high ipsilateral/diagonal pair step time ratio (0.80 average), moderate ipsilateral swing phase overlap over the course of the stride (35.9% on average), and abundant tripedal support (52.9% of the stride duration) but not as much bilateral support (27.3%). The walk then is a relatively square and even four-beat gait with a lateral advanced placement of 21.6% and a lateral advanced lift-off of 24.1% of the stride cycle. The linear characterizations of the walk were not investigated here as previous studies have shown it to have a relatively short stride length (1.5–1.8 meters), with hind limbs that cap [forming direct register prints] or slightly overstep the front limbs (average intergascular angle of 33.3 degrees), and forming tracks in the sand characterized by lateral pairs in nearly parallel alignment if the horse is moving in a straight line [22] as can be seen in Figure 3. See Table 3 and Figure 4 below for additional temporal parameters and a gait diagram of the walk, as well as supplementary Table S1.

Gait	Walk	Running walk	Slow rack or Tölt	Fast rack or Tölt	Stepping pace
Number of Horses Studied	3	5	3	7	7
Velocity (m/s)	ca. 1.53	2.86	2.70	4.23	3.74
Stride Duration (s)	1.16	0.77	0.62	0.53	0.61
Stride Length (m)	NA	2.30	1.68	2.16	2.12
Stride Length/ Height	NA	1.48	1.20	1.51	1.43
Hind Stance Phase (Duty Factor)	0.63	0.54	0.54	0.48	0.49
Front/Hind Stance Ratio	1.02	0.91	0.94	0.93	0.94
Ipsilateral/Diagonal Step Time	0.80	0.82	0.89	0.73	0.46
Foot Sequence	Lateral	Lateral	Lateral	Lateral	Lateral
Closest Foot Couplet Timings	Lateral	Lateral	Lateral	Lateral	Lateral
Beats	4	4	4	4	4
Synchronicity	Even	Even	Even	Even	Uneven
Limb Coordination	Independent	Front Alternating	Front Alternating	Front/Hind Alternating	Ipsilateral
Intergascular Angle (degrees)	33.3	57.5	40.0	41.4	42.3
Ipsilateral Swing Phase Overlap	35.9	61.6	55.7	66.5	75.2
Lateral Advanced Placement	21.6	22.2	23.5	21.2	15.6
Lateral Advanced Liftoff	24.1	17.3	20.1	18.0	12.3
Diagonal Advanced Placement	28.3	27.7	26.5	28.8	34.4
Diagonal Advanced Liftoff	25.8	32.4	29.7	32.5	37.3
Tripedal Support (% of stride)	52.9	9.4	11.7	2.3	3.6
Bilateral Support (% of stride)	27.2	55.5	50.5	57.2	67.0
Diagonal Support (% of stride)	19.9	35.1	37.8	27.3	19.3
Unipedal Support (% of stride)	0.0	0.0	0.0	13.2	9.6
4-Limb Suspension (% of stride)	0.0	0.0	0.0	0.0	0.0
Ipsilateral Overstep (% of stride)	NA	18.8	8.7	28.3	29.0
Diagonal/Ipsilateral Step Distance	NA	0.94	2.99	0.37	0.24
Interior Straddle (cm)	NA	0.1	−0.6	−5.1	−1.1
Foot Pair Lateral Offset (cm)	NA	1.3	3.2	8.3	9.0
Foot Pair Offset/Hoof Width	NA	0.11	0.25	0.59	0.79
Footprint Pattern	Lateral Pairs	Equidistant	Lateral Pairs	Diagonal Pairs	Diagonal Pairs

Table 3.

Average temporal and linear kinematic parameters of ipsilaterally coordinated horse gaits (original data).

Traits	Horse number
	1	5	18
Velocity (meters/second)	ca. 1.70	ca. 1.30	ca. 1.60
Stride Duration (seconds)	1.17	1.27	1.05
Stride Frequency (strides per second)	0.85	0.79	0.95
Front Stance Phase (% of stride)	62.4	65.6	63.8
Hind Stance Phase (% of stride)	61.1	62.6	64.3
Average Stance Phase (% of stride)	61.8	64.1	62.6
Front Stance/Hind Stance Ratio	1.02	1.05	0.99
Time Between Ipsilateral Steps (seconds)	0.26	0.27	0.25
Time Between Diagonal Steps (seconds)	0.33	0.37	0.28
Ipsilateral/Diagonal Step Time Ratio	0.77	0.73	0.89
Ipsilateral Swing Phase Overlap (% of stride)	39.0	36.1	32.6
Ipsilateral Stance Phase Overlap (% of stride)	64.3	66.1	63.4
Overall Ipsilateral Overlap (% of stride)	51.7	51.1	48.0
Ipsilateral Swing/Stance Phase Overlap Ratio	0.61	0.55	0.52
Lateral Advanced Placement (% of stride)	21.8	21.1	21.9
Lateral Advanced Liftoff (% of stride)	23.7	24.7	24.0
Diagonal Advanced Placement (% of stride)	28.2	30.4	26.4
Diagonal Advanced Liftoff (% of stride)	26.3	26.7	24.5
Tripedal Support (% of stride)	48.3	52.2	58.1
Bilateral Support (% of stride)	30.3	28.1	23.3
Diagonal Support (% of stride)	21.4	19.8	18.6
Unipedal Support (% of stride)	0.0	0.0	0.0
Front Contralateral Suspension (% of stride)	0.0	0.0	0.0
Hind Contralateral Suspension (% of stride)	0.0	0.0	0.0
Four-Limb Suspension (% of stride)	0.0	0.0	0.0
Intergascular Angle at Placement (degrees)	40.0	30.0	30.0
Stride Length (m)	NA	NA	NA
Stride Length/Horse Height (at withers)	NA	NA	NA
Distance Between Diagonal Steps (cm)	NA	NA	NA
Distance Between Ipsilateral Steps (cm)	NA	NA	NA
Ipsilateral Overstep/Stride Length Ratio	NA	NA	NA
Diagonal/Ipsilateral Step Distance Ratio	NA	NA	NA
Average Interior Straddle (cm)	NA	NA	NA
Average Foot Pair Lateral Offset (cm)	NA	NA	NA
Foot Pair Lateral Offset/Hoof Width Ratio	NA	NA	NA

Table S1.

Temporal Parameters of the Slow Walk (Original Data).

A few of the horse breeds exhibited the “artificial” lateral gait of the running walk, a faster variant of the walk emphasizing hind-limb extension (with an intergascular angle of around 59.0 degrees when both rear legs are on the ground), a pulling action with the front limbs, and often involving a counterbalancing mechanism such as nodding of the head and scissoring of the hind legs in the Tennessee Walking Horse, or an outward arcing path of the front limbs (termino) in paso llano of the Peruvian Paso Horse. The Spotted Saddle Horse, however, did not display much in the way of these counterbalancing mechanisms. The running walk is an even four-beat gait and is characterized temporally by a moderate stride duration (0.77 seconds on average, though a much quicker 0.58 seconds in the Peruvian Paso’s paso llano), a high fore/hind stance duration ratio of 0.91 along with a high ipsilateral limb/diagonal limb step time ratio of 0.82, increased ipsilateral swing phase coordination (61.6% of stride), and much less tripedal support (averaging 9.4% overall) and more bilateral support (55.5%). The lateral advanced placement in the running walk averaged 22.2% and the lateral advanced liftoff 17.3% of the stride duration reflecting the greater degree of lateral coordination than in the standard walk. The velocity of the running walk averaged 2.9 m/s, with a stride length of 2.3 meters, an ipsilateral overstep of the hind over the forefoot that averaged 18.8% the length of the stride. The trackway of the running walk consistently produced four footprints separated from each other and located fairly equidistant from each other (diagonal/ipsilateral step distance ratio of 0.94; see Figure 3). See Table 3 and Figure 4 for more of the temporal and linear parameters of the running walk and paso llano as well as gait diagrams, as well as supplementary Table S2.

Traits			Horse Numbers
	1	3	4	12	13		2	6
Velocity (meters/second)	2.35	2.95	3.39	3.10	ca. 2.50		3.42	2.93
Stride Duration (seconds)	0.72	0.83	0.77	0.79	0.75		0.57	0.58
Stride Frequency (strides per second)	1.39	1.20	1.30	1.27	1.33		1.75	1.72
Front Stance Phase (% of stride)	49.7	49.4	50.3	49.5	50.0		49.1	47.4
Hind Stance Phase (% of stride)	54.5	52.4	55.6	55.4	54.3		55.3	56.5
Average Stance Phase (% of stride)	52.1	51.8	53.0	52.4	52.2		52.2	51.9
Front Stance/Hind Stance Ratio	0.91	0.91	0.90	0.91	0.92		0.89	0.84
Time Between Ipsilateral Steps (s)	0.14	0.22	0.16	0.18	0.18		0.16	0.11
Time Between Diagonal Steps (s)	0.22	0.20	0.23	0.22	0.20		0.13	0.19
Ipsilateral/Diagonal Step Time Ratio	0.62	1.08	0.68	0.83	0.88		1.28	0.57
Ipsilateral Swing Phase Overlap (%)	70.1	52.3	65.0	62.1	58.5		49.5	72.7
Ipsilateral Stance Phase Overlap (%)	61.2	52.2	62.9	59.1	57.1		49.2	68.0
Overall Ipsilateral Overlap (% of stride)	62.9	52.2	64.0	60.6	57.8		49.4	70.4
Ipsilateral Swing/Stance Phase Overlap	1.15	1.00	1.03	1.05	1.03		1.01	1.07
Lateral Advanced Placement (%)	19.1	25.9	20.3	22.6	23.3		28.1	18.1
Lateral Advanced Liftoff (% of stride)	12.8	22.0	16.0	16.6	19.0		22.4	12.1
Diagonal Advanced Placement (%)	30.9	24.1	29.7	26.9	26.7		21.9	31.9
Diagonal Advanced Liftoff (% of stride)	35.4	28.3	34.3	32.8	31.0		27.2	38.4
Tripedal Support (% of stride)	9.7	7.8	8.5	12.1	8.7		11.4	12.1
Bilateral Support (% of stride)	61.8	48.2	59.5	54.8	53.3		43.9	63.8
Diagonal Support (% of stride)	28.5	44.0	32.0	33.1	38.0		44.7	24.1
Unipedal Support (% of stride)	0.0	0.0	0.0	0.0	0.0		0.0	0.0
Front Contralateral Suspension (%)	0.0	0.0	0.0	0.0	0.0		0.0	0.0
Hind Contralateral Suspension (%)	0.0	0.0	0.0	0.0	0.0		0.0	0.0
Four-Limb Suspension (% of stride)	0.0	0.0	0.0	0.0	0.0		0.0	0.0
Intergascular Angle at Placement (°)	50.0	60.0	70.0	60.0	55.0		50.0	45.0
Stride Length (m)	1.69	2.45	2.61	2.45	NA		1.95	1.70
Stride Length/Horse Height (at withers)	1.13	1.60	1.60	1.59	NA		1.29	1.28
Distance Between Diagonal Steps (cm)	34.0	41.9	29.2	43.2	NA		39.5	31.8
Distance Between Ipsilateral Steps (cm)	23.5	45.7	54.6	51.4	NA		32.0	25.4
Ipsilateral Overstep/Stride Length Ratio	0.14	0.19	0.21	0.21	NA		0.16	0.15
Diagonal/Ipsilateral Step Distance Ratio	1.45	0.92	0.53	0.84	NA		1.23	1.25
Average Interior Straddle (cm)	2.4	−0.7	−0.7	−0.7	NA		4.0	−6.4
Average Foot Pair Lateral Offset (cm)	NA	NA	NA	NA	NA		1.3	NA
Lateral Offset/Hoof Width Ratio	NA	NA	NA	NA	NA		0.11	NA

Table S2.

Temporal and Linear Parameters of the Running Walk (Horses 1, 3–4, 12, 13) and Paso Llano (Horses 2, 6) (Original Data).

The other common “artificial” lateral gait studied here was the rack found in the Tennessee Walking Horse and Rocky Mountain Horse and the similar gait of the tölt in the Icelandic Horse, which again is an even four-beat gait, though at a faster speed (averaging 3.8 m/s in the rack). In terms of temporal parameters, the rack or tölt is characterized by a relatively short stride duration (averaging 0.56 seconds), with a high ipsilateral/diagonal step time ratio of 0.79, a high forelimb over hindlimb stance duration ratio of 0.93, and a fair amount of lateral coordination (averaging 63.3% of the swing phase). The lateral advanced placement averaged 21.9% of the stride length and the lateral advanced lift-off 18.7%, while the hind limbs did not stretch forward as much at maximal protection as in the running walk (average intergascular angle of 41.0 degrees). Tripedal support was quite low (5.2% of stride cycle) while bilateral support was high (55.2%), and there was occasional single foot support in some horses (around 13.2% of the stride cycle in fast racks). Perhaps the biggest difference, however, occurred in the prints themselves. At slower racking speeds, the ipsilateral feet form pairs with the hind foot overstepping the front foot (or on occasion form a trackway resembling a running walk with mostly isolated prints). In the fast rack, which had an average stride length of 2.2 meters, and an average ipsilateral overstep of 28.3% of the stride length, the footprint impressions are comprised of diagonal pairs with the hind foot understepping the front foot and forming a bowed or wave-like pattern in the sand (with a low diagonal/ipsilateral step distance ratio of 0.37). As observed in previous studies [22] there was very little distance separating the contralateral hoof impressions (see Figure 3), indeed when the rack is performed at fast speeds the hind foot understeps the front foot and due to lack of interference often crosses over the centerline and overlaps with its diagonal partner. This is reflected in a low interior straddle averaging −5.1 cm. Table 3 and Figure 5 display more of the temporal and linear parameters of the racking gait along with gait diagrams. See supplementary Tables S3 and S4.

Figure 4.

Gait diagrams of the walk, running walk, and paso llano of a horse.

Traits	Horse Numbers
	8	9	10		5	10
Velocity (m/s)	2.50	2.77	2.82		2.70	3.75
Stride Duration (s)	0.64	0.59	0.64		0.70	0.59
Stride Frequency (s^−1)	1.56	1.69	1.56		1.43	1.69
Front Stance Phase (% of stride)	48.4	49.6	54.3		50.4	48.3
Hind Stance Phase (% of stride)	55.1	53.0	55.1		54.6	52.6
Average Stance Phase (% of stride)	51.8	51.3	54.7		52.5	50.4
Front Stance/Hind Stance Ratio	0.88	0.94	0.99		0.92	0.92
Time Between Ipsilateral Steps (s)	0.15	0.13	0.15		0.14	0.13
Time Between Diagonal Steps (s)	0.17	0.16	0.17		0.21	0.17
Ipsilateral/Diagonal Step Time Ratio	0.92	0.83	0.92		0.67	0.74
Ipsilateral Swing Phase Overlap (%)	58.9	58.2	50.1		66.2	65.2
Ipsilateral Stance Phase Overlap (%)	56.5	57.3	56.5		63.4	59.0
Overall Ipsilateral Overlap (%)	57.7	57.7	53.3		64.8	62.1
Swing/Stance Phase Overlap Ratio	1.04	1.02	0.89		1.05	1.11
Lateral Advanced Placement (%)	24.0	22.6	24.0		20.0	21.4
Lateral Advanced Liftoff (% of stride)	18.1	19.7	22.4		15.4	16.7
Diagonal Advanced Placement (%)	26.0	27.4	26.0		30.0	28.6
Diagonal Advanced Liftoff (%)	31.1	30.3	27.6		34.6	32.9
Tripedal Support (% of stride)	11.8	6.0	17.3		9.3	9.4
Bilateral Support (% of stride)	52.0	54.7	44.9		60.0	53.8
Diagonal Support (% of stride)	36.2	39.3	37.8		30.7	29.9
Unipedal Support (% of stride)	0.0	0.0	0.0		0.0	6.8
Front Contralateral Suspension (%)	0.0	0.0	0.0		0.0	6.8
Hind Contralateral Suspension (%)	0.0	0.0	0.0		0.0	0.0
Four-Limb Suspension (% of stride)	0.0	0.0	0.0		0.0	0.0
Intergascular Angle at Placement (°)	40.0	40.0	40.0		50.0	40.0
Stride Length (m)	1.60	1.63	1.80		1.89	2.21
Stride Length/Horse Height Ratio	1.21	1.22	1.18		1.24	1.44
Distance Between Diagonal Steps (cm)	40.7	42.6	43.9		35.0	49.5
Distance Between Ipsilateral Steps (cm)	14.0	13.3	15.3		29.9	30.5
Ipsilateral Overstep/Stride Length	0.09	0.08	0.09		0.16	0.22
Diagonal/Ipsilateral Step Distance	2.91	3.20	2.87		1.17	1.62
Average Interior Straddle (cm)	−2.6	−5.1	5.8		ca. -3.2	0.0
Average Foot Pair Lateral Offset (cm)	3.2	5.1	1.3		ca. 1.3	6.35
Foot Pair Lateral Offset/Hoof Width	0.24	0.42	0.09		0.10	0.42

Table S3.

Temporal and Linear Parameters of the Slow Rack or Tölt (Horses 1, 8–10) and Medium Rack (Horses 5 and 10) (Original Data).

Traits	Horse Numbers
	8	10	11	13	15	16	17
Velocity (m/s)	3.88	3.75	3.65	ca. 3.20	ca. 3.90	ca. 4.40	ca. 6.80
Stride Duration (s)	0.58	0.59	0.55	0.50	0.51	0.50	0.49
Stride Frequency (s^−1)	1.72	1.69	1.82	2.00	1.96	2.00	2.04
Front Stance Phase (% of stride)	46.1	48.3	50.0	40.5	40.1	42.0	38.3
Hind Stance Phase (% of stride)	53.0	56.2	50.0	41.5	50.0	41.0	42.9
Average Stance Phase (% of stride)	49.6	50.4	50.0	41.0	45.0	41.5	40.6
Front Stance/Hind Stance Ratio	0.87	0.92	1.00	0.98	0.80	1.02	0.89
Time Between Ipsilateral Steps (s)	0.12	0.13	0.11	0.11	0.11	0.11	0.11
Time Between Diagonal Steps (s)	0.17	0.17	0.17	0.16	0.15	0.14	0.14
Ipsilateral/Diagonal Step Time Ratio	0.74	0.75	0.65	0.72	0.71	0.82	0.75
Ipsilateral Swing Phase Overlap (%)	67.3	65.2	60.6	65.3	77.1	59.9	70.4
Ipsilateral Stance Phase Overlap (%)	59.8	59.0	60.6	49.1	58.5	45.2	49.8
Overall Ipsilateral Overlap (%)	63.6	62.1	60.6	57.2	67.8	52.5	60.1
Swing/Stance Phase Overlap Ratio	1.13	1.11	1.00	1.33	1.32	1.32	1.41
Lateral Advanced Placement (%)	21.3	21.4	19.7	21.0	20.8	22.5	21.4
Lateral Advanced Liftoff (% of stride)	15.2	16.7	19.7	21.5	12.9	23.0	17.3
Diagonal Advanced Placement (%)	28.7	28.6	30.0	29.0	29.2	27.5	28.6
Diagonal Advanced Liftoff (%)	34.3	32.9	30.3	32.0	38.6	25.5	33.7
Tripedal Support (% of stride)	7.0	9.4	0.0	0.0	0.0	0.0	0.0
Bilateral Support (% of stride)	57.4	53.8	60.6	58.0	58.4	55.0	57.1
Diagonal Support (% of stride)	30.4	29.9	39.4	19.0	22.8	31.0	18.4
Unipedal Support (% of stride)	5.2	6.8	0.0	23.0	18.8	14.0	24.5
Front Contralateral Suspension (%)	5.2	6.8	0.0	28.0	13.9	14.0	20.4
Hind Contralateral Suspension (%)	0.0	0.0	0.0	21.0	11.9	17.0	22.4
Four-Limb Suspension (% of stride)	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Intergascular Angle at Placement (°)	40.0	40.0	40.0	40.0	40.0	45.0	45.0
Stride Length (m)	2.25	2.21	2.01	NA	NA	NA	NA
Stride Length/Horse Height Ratio	1.70	1.44	1.38	NA	NA	NA	NA
Distance Between Diagonal Steps (cm)	1.9	30.5	21.6	NA	NA	NA	NA
Distance Between Ipsilateral Steps (cm)	88.9	49.5	45.7	NA	NA	NA	NA
Ipsilateral Overstep/Stride Length	0.40	0.22	0.23	NA	NA	NA	NA
Diagonal/Ipsilateral Step Distance	0.02	0.62	0.47	NA	NA	NA	NA
Average Interior Straddle (cm)	−6.4	0.0	−8.9	NA	NA	NA	NA
Average Foot Pair Lateral Offset (cm)	10.2	11.5	3.2	NA	NA	NA	NA
Foot Pair Lateral Offset/Hoof Width	0.80	0.76	0.22	NA	NA	NA	NA

Table S4.

Temporal and Linear Parameters of the Fast Rack or Tölt (Horses 8, 10–11, 13, 15–17) (Original Data).

Some of the horses also engaged in the stepping pace characterized by an uneven four beat gait. Such a gait was very ipsilaterally coordinated with a lateral advanced placement averaging 15.6% and a lateral advanced lift-off 12.3%, along with an average ipsilateral swing phase overlap of 75.2%. Though the front/hind stance duration ratio was high (0.94), the ipsilateral/diagonal step time ratio was very low (0.46). Three-limb support was very also very low (3.6%), bilateral support very high (67.0%), and in some horses there was occasional single-foot support as well (for 9.6% of the stride length). The stepping pace took place at an average velocity of 3.7 m/s and had an average stride length of 2.1 m, ipsilateral overstep/stride length of 29.0%, an intergascular angle averaging 44.3%, and the footprint trackway in the sand occurred as close diagonal couplets as in the rack (0.24 diagonal/ipsilateral step distance ratio; see Figure 3 and supplementary Table S5). Table 3 and Figure 6 show additional temporal and linear parameters of the stepping pace.

Traits	Horse Numbers
	1	4	9	13	14	19	20
Velocity (meters/second)	3.08	2.95	4.18	ca. 3.20	ca. 5.70	ca. 4.60	ca. 2.50
Stride Duration (seconds)	0.64	0.75	0.52	0.66	0.50	0.55	0.67
Stride Frequency (strides per second)	1.56	1.33	1.92	1.52	2.00	1.82	1.49
Front Stance Phase (% of stride)	51.2	54.3	41.0	50.4	37.5	38.2	48.9
Hind Stance Phase (% of stride)	50.0	54.0	43.8	50.0	41.0	47.3	54.5
Average Stance Phase (% of stride)	50.6	54.2	42.4	50.2	39.2	42.7	51.7
Front Stance/Hind Stance Ratio	1.02	1.01	0.93	1.01	0.91	0.81	0.90
Time Between Ipsilateral Steps (s)	0.10	0.10	0.08	0.09	0.09	0.09	0.11
Time Between Diagonal Steps (s)	0.22	0.28	0.18	0.24	0.16	0.18	0.22
Ipsilateral/Diagonal Step Time Ratio	0.46	0.36	0.44	0.39	0.54	0.51	0.51
Ipsilateral Swing Phase Overlap (%)	68.4	71.0	78.0	72.0	75.5	88.8	72.7
Ipsilateral Stance Phase Overlap (%)	68.4	75.3	82.4	72.0	57.4	64.6	64.0
Overall Ipsilateral Overlap (%)	68.4	73.1	79.4	72.0	66.5	76.7	67.3
Swing/Stance Phase Overlap Ratio	1.00	0.94	0.95	1.00	1.32	1.37	1.14
Lateral Advanced Placement (%)	15.7	13.3	15.2	14.0	17.5	16.8	16.9
Lateral Advanced Liftoff (% of stride)	15.7	13.3	12.4	14.0	14.0	5.9	10.9
Diagonal Advanced Placement (%)	34.3	36.7	34.8	36.0	32.5	33.2	33.1
Diagonal Advanced Liftoff (%)	34.3	36.7	37.6	36.0	34.5	44.1	37.6
Tripedal Support (% of stride)	0.0	16.0	0.0	0.0	0.0	0.0	9.0
Bilateral Support (% of stride)	68.5	65.3	69.5	72.0	65.0	66.4	62.2
Diagonal Support (% of stride)	31.5	18.7	12.4	28.0	13.0	6.4	24.8
Unipedal Support (% of stride)	0.0	0.0	18.1	0.0	22.0	27.3	0.0
Front Contralateral Suspension (%)	0.0	0.0	19.0	0.0	26.0	27.3	0.0
Hind Contralateral Suspension (%)	0.0	0.0	14.3	0.0	22.4	13.6	0.0
Four-Limb Suspension (% of stride)	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Intergascular Angle at Placement (°)	40.0	50.0	50.0	40.0	40.0	50.0	40.0
Stride Length (m)	1.97	2.21	2.17	NA	NA	NA	NA
Stride Length/Horse Height	1.31	1.36	1.62	NA	NA	NA	NA
Distance Between Diagonal Steps (cm)	13.0	16.5	12.7	NA	NA	NA	NA
Distance Between Ipsilateral Steps (cm)	61.5	50.2	70.5	NA	NA	NA	NA
Ipsilateral Overstep/Stride Length Ratio	0.31	0.23	0.33	NA	NA	NA	NA
Diagonal/Ipsilateral Step Distance Ratio	0.21	0.33	0.18	NA	NA	NA	NA
Average Interior Straddle (cm)	2.0	−1.3	−3.9	NA	NA	NA	NA
Average Foot Pair Lateral Offset (cm)	4.0	ca. 15.0	14.0	NA	NA	NA	NA
Foot Pair Lateral Offset/Hoof Width	0.32	0.88	1.16	NA	NA	NA	NA

Table S5.

Temporal and Linear Parameters of the Stepping Pace (Horses 1, 4, 9, 13–14, and 19–20) (Original Data).

Figure 5.

Gait diagrams of the rack or tölt of a horse.

Only two horses displayed a true or hard pace, i.e. an even two-beat gait with heavy ipsilateral coordination above 80% (with an average lateral advanced placement of 5–10% of the stride duration and 75–90% bilateral support through the stride cycle). This occurred at a relative slow speed for the pace, however, of around 3.4 m/s and so there were no suspension phases nor single-foot support structures (see Figure 7 and supplementary Table S6).

Figure 6.

Gait diagrams of the stepping pace of a horse.

Figure 7.

Gait diagrams of the pace and fox trot of a horse.

Traits	Horse Number
	1	6
Velocity (meters/second)	3.25	3.51
Stride Duration (seconds)	0.64	0.60
Stride Frequency (strides per second)	1.56	1.67
Front Stance Phase (% of stride)	50.0	50.0
Hind Stance Phase (% of stride)	46.9	55.0
Average Stance Phase (% of stride)	48.4	52.5
Front Stance/Hind Stance Ratio	1.07	0.91
Time Between Ipsilateral Steps (seconds)	0.07	0.03
Time Between Diagonal Steps (seconds)	0.25	0.27
Ipsilateral/Diagonal Step Time Ratio	0.26	0.11
Ipsilateral Swing Phase Overlap (%)	76.4	100.0
Ipsilateral Stance Phase Overlap (%)	85.3	90.9
Overall Ipsilateral Overlap (% of stride)	80.4	95.4
Swing/Stance Phase Overlap Ratio	0.90	1.10
Lateral Advanced Placement (% of stride)	10.2	5.0
Lateral Advanced Liftoff (% of stride)	11.8	0.0
Diagonal Advanced Placement (% of stride)	39.8	45.0
Diagonal Advanced Liftoff (% of stride)	33.8	50.0
Tripedal Support (% of stride)	4.7	10.0
Bilateral Support (% of stride)	74.0	90.0
Diagonal Support (% of stride)	21.3	0.0
Unipedal Support (% of stride)	0.0	0.0
Front Contralateral Suspension (% of stride)	0.0	0.0
Hind Contralateral Suspension (% of stride)	0.0	0.0
Four-Limb Suspension (% of stride)	0.0	0.0
Intergascular Angle at Placement (°)	35.0	40.0
Stride Length (m)	2.08	2.11
Stride Length/Horse Height (at withers)	1.39	1.59
Distance Between Diagonal Steps (cm)	15.0	17.2
Distance Between Ipsilateral Steps (cm)	65.0	59.7
Ipsilateral Overstep/Stride Length Ratio	0.31	0.28
Diagonal/Ipsilateral Step Distance Ratio	0.23	0.29
Average Interior Straddle (cm)	ca. 5.0	−2.3
Average Foot Pair Lateral Offset (cm)	ca. 17.2	10.2
Foot Pair Lateral Offset/Hoof Width Ratio	1.38	0.86

Table S6.

Temporal Parameters of the Slow Pace (Horses 1, 6) (Original Data).

Finally, three of the horses could also perform the fox trot gait, which is a lateral-sequence but diagonal-couplet running gait occurring at around 3.3 m/s. Hence the fox trot is usually considered a diagonal gait. The fox trot has very unusual temporal parameters accordingly, and was characterized by a stride duration averaging 0.62, lateral advanced placement of 38.4% and lateral advanced lift-off of 31.7%. The forelimb/hindlimb stance duration ratio was low (average of 0.86) while ipsilateral/diagonal step time ratio was super large at 4.11 due to a delayed front step on each side. There was in addition a large amount of diagonal support (62.8% of the stride cycle) and occasional single-leg support (2.8%), along with minimal ipsilateral swing overlap (27.1% of the cycle). See Figure 7 for a diagram of some of the fox trots displayed by the horses and supplementary Table S7.

Traits	Horse Number
	6	7	18
Velocity (meters/second)	ca. 2.90	3.70	ca. 3.30
Stride Duration (seconds)	0.59	0.67	0.60
Stride Frequency (strides per second)	1.69	1.49	1.67
Front Stance Phase (% of stride)	48.3	48.9	51.3
Hind Stance Phase (% of stride)	59.4	54.9	57.9
Average Stance Phase (% of stride)	53.8	51.9	54.6
Front Stance/Hind Stance Ratio	0.81	0.89	0.88
Time Between Ipsilateral Steps (seconds)	0.25	0.27	0.20
Time Between Diagonal Steps (seconds)	0.04	0.06	0.10
Ipsilateral/Diagonal Step Time Ratio	5.88	4.36	2.08
Ipsilateral Swing Phase Overlap (% of stride)	23.6	20.2	37.5
Ipsilateral Stance Phase Overlap (%)	28.2	25.6	41.7
Overall Ipsilateral Overlap (% of stride)	25.9	22.9	39.6
Swing/Stance Phase Overlap Ratio	0.84	0.79	0.90
Lateral Advanced Placement (% of stride)	42.7	40.7	31.7
Lateral Advanced Liftoff (% of stride)	32.5	35.4	27.1
Diagonal Advanced Placement (% of stride)	7.3	9.3	16.2
Diagonal Advanced Liftoff (% of stride)	16.7	14.2	27.5
Tripedal Support (% of stride)	13.7	10.4	13.3
Bilateral Support (% of stride)	14.5	18.7	32.5
Diagonal Support (% of stride)	63.2	70.9	54.2
Unipedal Support (% of stride)	8.5	0.0	0.0
Front Contralateral Suspension (% of stride)	14.5	0.0	0.0
Hind Contralateral Suspension (% of stride)	0.0	0.0	0.0
Four-Limb Suspension (% of stride)	0.0	0.0	0.0
Intergascular Angle at Placement (°)	40.0	40.0	40.0
Stride Length (m)	NA	2.47	NA
Stride Length/Horse Height (at withers)	NA	1.50	NA
Distance Between Diagonal Steps (cm)	NA	90.2	NA
Distance Between Ipsilateral Steps (cm)	NA	ca. -2.5	NA
Ipsilateral Overstep/Stride Length Ratio	NA	−0.01	NA
Diagonal/Ipsilateral Step Distance Ratio	NA	−36.08	NA
Average Interior Straddle (cm)	NA	15.9	NA
Average Foot Pair Lateral Offset (cm)	NA	ca. 5.7	NA
Foot Pair Lateral Offset/Hoof Width	NA	0.39	NA

Table S7.

Temporal and Linear Parameters of the Fox Trot (Horses 6, 7, and 18) (Original Data).

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 wave-like 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 diagonal-advanced 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 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).

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.

Figure S1.

Photographs of horses in various gaits. A. Tennessee Walker (Horse 1) in a slow walk; B. Tennessee Walker (Horse 1) in a stepping pace; C. Icelandic Horse (Horse 8) in a slow tölt; D. Rocky Mountain Horse (Horse 11) in a fast rack; E. Tennessee Walker (Horse 1) in a pace; F. Tennessee Walker (Horse 12) in a running walk; G. Tennessee Walker (Horse 7) in a fox trot; G. Tennessee Walker (Horse 20) in a gallop.

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.

4. Conclusion

Horses are famous for their variety of gaits. For this, and for their amicable character, and indeed ability to interpret human facial expressions and bodily movements, they have become valuable companions and helpers of humans. They were domesticated for riding and pulling carts and now have a large presence in sporting events such as dressage, jumping, and racing.

Most intriguingly besides the standard diagonal gait of the trot horses are capable of various laterally coordinated gaits such as the running walk and rack which allow for smooth riding for humans and efficiency of locomotion for the horse as well as offering stability and balance depending upon the substrate. Here we wanted to further examine such laterally coordinated gaits and look at what is known about them and expand upon our knowledge of the gaits of these magnificent creatures.

In this investigation we found temporal and linear parameters that can be utilized to discriminate the different laterally coordinated gaits of horses. It is true that these gaits often occur on a spectrum, such as the rack, stepping pace, and pace. However, one can select criteria such as percentage of ipsilateral coordination and footprint patterns to distinguish the gaits. For example, the running walk and racking gaits tend to have an ipsilateral swing phase overlap of 50–75% whereas the stepping pace has an ipsilateral swing phase overlap above 75%, while, at the same time, the running walk lays down isolated footprints, the slow rack lays down footprints consisting of lateral pairs, and the fast rack and stepping pace lay down footprints consisting of diagonal pairs. Though we did not stress it here there are also behavioral and physiological factors that can be used such as amount of head nod or croup displacement or foot sprinting. Further study on these areas would be warranted, as would whether or not a medium-speed rack lays down isolated footprints that resemble those of a running walk and the biomechanical differences between the pulling running walk gait and pushing rack gait. Further study would also be quite beneficial on these asymmetries found in gaits such as the rack, as well as study of the spontaneous gaits of horses when lacking a rider which seem to often weave from side to side rather than occurring in a straight line (perhaps to help avoid a predatorial attack). It would also be good to measure the angle each of the feet make in relation to the centerline in each of the gaits. Finally we were only able to record video at a maximum of 60 fps, and used 30 fps on occasion for the slower gaits (rather than the more common standard of 120 fps), which gives a potential error of 0.017 seconds, though this can be reduced in half by taking account of how close to making ground contact the hooves are in a given frame. Indeed someday when technology improves capturing video of animals in motion at 480 or even 960 fps would be ideal for a potential error of less than a thousandth of a second.

Acknowledgments

The authors wish to thank those who helped in providing material, references, or lent their skill in providing, preparing, and riding horses, especially Yoni Blom of Horse Dynamics, Zijtaart, Netherlands, and the Dutch Equestrian Estate, Schaijk, Netherlands; Beth Jennings at Invicta Farms, Hemet, California, U.S.A.; Kimberly Hart at Sunland Ranch in Encinitas, California, U.S.A.; Alexandra Montan Gray of Sunland Ranch, Encinitas, California, as well as Margaretehof in Everöd, Sweden and Krókur, Iceland; Mindy Smith at Freedom Farm, Arroyo Grande, California, U.S.A.; Ageeth Polane at the Pasohoeve, Orvelte, Netherlands; Nicolette Termaat at Manege Florida, Schiermonnikoog, Netherlands; Nani Barnes at Lake View Farms, Lakeview Terrace, California, U.S.A.; Patrick Genevieve, Tongeren, Belgium; Scot MacGregor; and Maryan Zyderveld.

Conflict of interest

The authors declare no conflict of interest.

Supplementary Tables S1-S7: Temporal and Linear Kinematics of Modern Horses in Various Gaits.

Other declarations

This study was approved by the Institutional Animal Care and Use Committee of St. John’s Seminary, Approval # 20221.

Supplementary Tables 2–3 and Table S1–S7, and Figures 1–8 and Figure S1 are original to this study and represent the data, art, or photography of Elise Renders and Alan Vincelette.

Videos are available upon request.