Members of the genus Equus—including domesticated caballine horse breeds (Equus ferus caballus), the likely non-domesticated Przewalski Horse (Equus ferus przewalskii), zebras, and African and Asian wild asses—standardly perform the three so-called “natural” gaits of walk, trot, and canter/gallop ^[1][2][3][4][1,2,3,4].

The walk is the standard slow gait of the horse occurring at around 1.4–1.8 m/s that involves independent movement of each limb in a lateral footfall sequence: left hind, left front, right hind, right front. It is often called a “stepping”, “square”, or “singlefoot” gait as it possesses a lateral advanced placement (or limb phasing value) close to 0.25 with the limbs sequentially setting down in near quarter intervals of the stride duration. The walk yields four audible even beats with the movements of the right legs mirroring those of the left legs. Hence, the walk can be categorized as a symmetrical, even four-beat lateral sequence “singlefoot” gait. It has a duty factor of 0.70–0.60 (proceeding from slower to faster speeds)—indicating that the hind limbs make contract with the ground during 60–70% of the stride duration—and the walk has bipedal support, alternating between diagonal and ipsilateral or tripedal support at all times ^[5][6][7][8][5,6,7,8].

The trot (or jog if performed in slow manner) is the standard intermediate speed gait of the horse. It occurs at speeds of ca. 2.5–6.5 m/s, up to 8.5–12.0 in harness racing. It is a symmetrical diagonal sequence and diagonal couplet gait: left hind + right front, right hind + left front. Though it yields two even beats, at slower speeds, the front hoof often lands just before the hind diagonal hoof, and at higher speeds, the hind hoof lands just before the front diagonal hoof but not enough to make an audible difference. It has a limb phasing value, or lateral advanced placement, of 0.50 (as limb pairs land at around half the duration of the stride) and a duty factor or hind leg support phase of 0.55–0.30 (from slower to faster speeds), resulting in many diagonal bipedal support phases, some unipedal support phases, and periods of suspension when all four feet are off the ground at the same time ^{[5][7][8][9][10][11][12]}[5,7,8,9,10,11,12]. The trot often occurs with elevated and animated front legs as with the park trot of the Morgan, the Spanish ‘walk’ of the Andalusian, or the collected passage in dressage.

The natural fast gait of the horse is an asymmetrical gait involving the coordination of contralateral limbs and suspended phases where all four feet are off the ground, which occurs around 9.0–15.0 m/s, up to 17.0–19.0 m/s in horse racing, where it is called the run or Renngalopp in German. This gait is variously called the canter or gallop, but here the two terms will be distinguished. If performed at the lower to middle portions of the velocity range, the gait often takes the form of a three-beat canter wherein a hind limb contacts the ground, followed by a diagonal limb pair, and ending with contact of the front leading limb, i.e., left hind, right hind + left front, right front in a right-lead canter. The canter thus has periods of unipedal and diagonal bipedal limb support and can occasionally even have periods of tripedal support (i.e., in a slower lope) ^[5][13][5,13]. When performed at the faster end of the range, the gait often transitions to an uneven four-beat gallop, though in a very fast run or racing speed, the two contralateral hoof strikes can occur so close together that the gait almost sounds like it only has two beats. In equines, the gallop takes the form of a transverse gallop involving repeated strikes of mirror-image contralateral couplets, i.e., left hind, right hind, left front, right front in a right-lead gallop or the reverse in a left-lead gallop. The gallop has periods of unipedal and contralateral bipedal support as well as phases where all four limbs are off the ground together. The duty factor, or hind limb support phase as percentage of stride duration, varies from 0.30 in the canter to 0.20 in a fast gallop ^[14][15][14,15].

In addition to these “natural” gaits, several “gaited” horse breeds display additional intermediate speed symmetrical and lateral-sequence gaits (the running walk, rack, broken pace, hard pace, and broken trot) ^{[16][17][18][19]}[16,17,18,19], or what I shall call alternative lateral gaits. The broken pace (stepping pace or amble) and broken trot (fox trot) involve four uneven beats, as the even and nearly simultaneous landing of ipsilateral or diagonal couplets occurring in the hard pace and trot are uncoupled. The running walk and rack (tölt) are even four-beat or “singlefoot” gaits, with the former being a hyperextended walk relying on inverted-pendulum swinging mechanics and the latter a bouncing gait relying upon spring mass mechanics of the ligament system. In contrast with the canter/gallop, trot, and hard pace, the running walk, rack, broken pace, and broken trot maintain ground contact at all times, with one to three, or even all four, hooves, despite being intermediate speed gaits. This makes them quite comfortable for the rider and useful in maintaining balance when traversing uneven terrain.

2. The Genetic Factors of Undergirding and Controlling the Various Alternative Lateral Gaits

A breakthrough regarding the genetics of alternative lateral gaits occurred in 2012 ^[20][204] when it was discovered that nearly all gaited horses possessed a variant allele (A) as opposed to the wild-type allele (C) of the DMRT3 gene of chromosome 23 found in non-gaited horses. The DMRT3 gene was labeled the “gait-keeper” gene as it controls the number of gaits a horse can (naturally) employ and the transitional speeds between them. Horses without the A-allele tend to have just three gaits–walk, trot, and canter/gallop–whereas horses with the A-allele (especially those homozygous for it) can employ alternative lateral gaits such as the running walk, rack, broken pace, or hard pace and maintain these gaits at intermediate to fast speeds. That is to say, the “gait-keeping” A allele allows the horse to extend lateral-sequence gaits (as with the walk) into intermediate speeds rather than shifting to the diagonal-sequence trot at around 1.5–2.2 m/s, or the asymmetrical gallop at around 4.5 to 6.0 m/s ^[21][195]. The wild-type DMRT3 C-allele codes for a protein transcription factor responsible for producing regular bursting patterns in the dI6 interneurons of mammalian spines and thereby coordinating diagonal and contralateral limb movements. The A-allele developed through a single-nucleotide polymorphism (SNP) that introduced a premature stop codon into the gene. The resulting truncated protein transcription factor (possessing only 300 out of 474 amino acids) lacks the ability to produce regular interneuronal bursts in the spine and thereby induce horses to transition from a slow walk to an intermediate speed diagonally coordinated trot, and finally to an asymmetrical contralaterally-coordinated canter/gallop at fast speeds. Instead, horses with the mutated A-allele tend to transition from a slow walk to an intermediate speed laterally-coordinated running walk, rack, broken pace, or hard pace ^[22][23][205,206]. In other words, horse breeds that possess the A-allele (such as the Icelandic, Mangalarga Marchador, Paso Fino, Tennessee Walking Horse, and Saddlebred), when they wish to travel faster, tend to employ alternative lateral gaits based upon the same lateral sequence footfall pattern found in the walk (LH, LF, RH, RF) rather than transitioning to a diagonal trot or asymmetrical gallop. In fact, such horses (especially those homozygous for the A-allele) not only display an unwillingness to engage in trots and gallops but show poor quality versions for beat clarity and speed capacity (as rated by certified judges during breed-specific field tests) of trots and gallops when they do perform them ^[20][24][25][204,207,208]. Further investigation has found that nearly all alternatively gaited horse breeds, whether located in Europe, Asia, North America, or South America possess a high percentage of the A-allele (>15%) of the DMRT3 “gait-keeper” gene, though there are still a few breeds left to be genotyped, and in several cases only small sample sizes exist (^{[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]}[46,54,65,66,81,85,87,89,92,95,108,109,112,114,209,210,211,212]). Only a few inconsistencies still exist, such as with the occasionally gaited Dongola of West Africa and Nokota of North America, who, so far, have not been found to possess the A-allele of the DMRT3 gene ^[41][42][210,211]. Larger sample sizes might eventually change this. Importantly, the broken trot gait may not be linked at all or only in a minor way to the A-allele of the DMRT3. Effects peculiar to the different alleles of the DMRT3 gene have also been observed. For example, it has been found that in Icelandic Horses pacing generally requires a genotype homozygous for the DMRT3 A-allele (i.e., AA), whereas horses with a heterozygous genotype (CA) are typically only able to perform the rack (tölt). More particularly, 94% of five-gaited Icelandic Horses who could perform both the tölt and the flying pace were homozygous for the A-allele (i.e., AA), whereas while 88% of four-gaited Icelandic horses who could only perform the tölt but not the flying pace had at least one copy of the A-allele (about two-thirds having the CA genotype and one-third the AA genotype). And only 29% of Nordic Trotters with the CA DMRT3 genotype could perform a quality hard pace ^{[25][40][44][45]}[208,209,213,214]. More study, however, needs to be performed to see if this holds true for other pacing horse breeds as well. Fonseca et al. ^[46][215] found that Mangalarga Marchador horses of the homozygous AA DMRT3 genotype had greater diagonal advanced placements (AA = 31.7, CA = 28.9) and smaller periods of diagonal bipedal support (AA = 35.5, CA = 41.0) in the broken pace (marcha picada) than those that were of the heterozygous CA genotype. More studies of this sort would be informative with other breeds. In addition, while homozygosity for the A-allele reduced the quality of the trot and gallop in Icelandic Horses, having a single copy of the A-allele was beneficial in warm-blooded harness racers, whether they trotted or paced, as it encouraged the horse to sustain the trot or hard pace at higher speeds rather than switching to a gallop ^{[20][47][48][49][50][51]}[204,216,217,218,219,220]. It also turns out that other genes besides the DMRT3 are involved in the production of alternative lateral gaits, of which we are only beginning to understand. As Petersen et al. ^[40][209] point out, breeds with the DMRT3 A-allele on chromosome 23 display various types of alternative lateral gaits, and so “it appears that this locus does not itself explain the entirety of the variation in gait present in domestic horses … (but rather) that gait is a polygenic trait, and … variations among breeds are determined by modifying loci”. In the first place while homozygosity for the DMRT3 A-allele (i.e., an AA genotype) seems necessary for the ability to pace in Icelandic and Hokkaido horses (only 4–6% of CA genotype Icelandics could pace and 0% of CA Hokkaido horses) and perhaps in other horse breeds as well, it does not seem sufficient. For only 70–94% of Icelandic and 86% of Hokkaido horses homozygous for the A-allele were reported to pace ^[45][52][94,214]. This could merely reflect the fact of upbringing and training (though not maternal example, as Amano et al. ^[52][94] noted) or it could be due to further genetic factors. In the second place, the DMRT3 A-allele and its various genotypes are not great predictors of pacing versus trotting ability in warmblooded or coldblooded harness racers. While 71% of homozygous AA genotype French Trotters were indeed trotters and only 29% pacers, the opposite was the case with Finnhorses where ca. 78% of the AA genotype were pacers and only ca. 22% trotters. Similarly, while 98% of heterozygous CA genotype French Trotters were trotters and 2% pacers, ca. 82% of heterozygous CA genotype Finnhorses were trotters and ca. 18% pacers ^[53][54][221,222]. Moreover, Standardbred horses are nearly all homozygous for the AA allele but ca. 56–66% trot while ca. 34–44% pace ^{[20][25][45][55]}[204,208,214,223]. Again, Tennessee Walking Horses (as well as the National Spotted Saddle Horse) are almost all homozygous for the DMRT3 A-allele, and though they are famous for the running walk gait, many members of the breed can perform other alternative lateral gaits such as the rack, broken pace, or broken trot ^[41][42][56][33,210,211]. Nor did the particular genotype of the DMRT3 gene (AA, CA, or CC) correlate well with whether or not American Saddlebreds were three- or five-gaited (i.e., were shown in slow gait (broken pace) and rack in addition to walk, trot, and gallop). In fact, three- and five-gaited horses had nearly the same proportions of genotype: AA (7%, 3%), CA (26%, 24%), and CC (24%, 26%) ^[27][54]. In the third place, the A-allele of the DMRT3 gene appears to have little to no role in the generation of the lateral sequence diagonal-couplet gait (i.e., the broken trot or fox trot). For the DMRT3 AA genotype is nearly fixed (100%) in the racking and pacing Icelandic Horse as well as in the Missouri Fox Trotter ^[42][211]. And the CC genotype is nearly fixed (100%) in other breeds that engage in the broken trot including the Akhal-Teke (glide gait), Karbarda (tropota gait), Transbaikal (tropota), Yakut (tropota), and Marwari (revaal gait) ^{[33][36][37][41][57][58][59]}[89,96,108,109,175,210,224]. Similarly, Colombian Paso Fino horses that perform a rack (fino classico) have a nearly fixed A-allele with the following genotype frequencies: AA (0.94–1.00), CA (0.00–0.01), and CC (0.00–0.05). However, Colombian Trocha horses that perform a broken trot (trocha) have a nearly fixed C allele, with the following genotypes: AA (0.00–0.03), CA (0.02–0.15), and CC (0.82–0.98)—though the CA genotype might help in producing a more distinct trocha (or trot) gait ^[25][41][60][56,208,210]. Parallel findings occur with the Brazilian Mangalarga Marchador breed. Mangalara Marchador horses that preferred the broken pace (marcha picada) almost all possessed the A-allele of the DMRT3 gene, having the following genotypes: AA (0.31–0.87), CA (0.13–0.65), CC (0.00–0.05), whereas horses that preferred the broken trot (marcha batida) had the following genotypes: AA (0.00–0.15), CA (0.00–0.34), CC (0.85–0.94) ^{[28][29][61][62][63]}[65,66,225,226,227]. A study of Brazilian Campolina horses, however, found similar gene frequencies in horses that perform the broken pace (marcha picada) or the broken trot (marcha batida). Horses that performed the marcha picada had genotype frequencies of AA (0.12), CA (0.88), and CC (0.00), while horses that performed the marcha batida had genotype frequencies of AA (0.44), CA (0.56), and CC (0.00) ^[28][65]. Hence, it seems that while at least one copy of the A-allele is necessary for the performance of the rack and broken pace, and perhaps two copies (i.e., homozygosity) for the performance of the hard pace, the A-allele is not required at all for the performance of the broken trot. Equine scientists are still untangling the other genes involved in the production of alternative lateral gaits and their variations. Initial explorations have found various candidate genes on chromosomes 1, 19, 23, and 30 that correlate to some degree with the particular alternative lateral gait displayed by a horse, but much more work needs to be performed ^{[52][53][56][63][64][65]}[22,33,94,98,221,227]. Some genes seem to be associated with the rapidity and length of the horses’ steps, which tend to be quick and collected in Paso Fino horses but slower and more extended in other breeds ^[60][56]. There are probably also genes related to the degree to which the front feet are elevated or not. Finally, there seem to be several genes related to speed. In particular, the MSTN “speed gene” on ECA 18, which produces muscular myostatin is of some importance. Horses with the variant C-allele of the MSTN gene (which increased in frequency in horses from 900–1400 AD) are faster that those homozygous for the wild-type T, and in particular, horses that are homozygous for the variant (CC) are capable of short bursts of speed, and horses that are heterozygous (CT) are better at middle distance racing, and individuals without the C-allele (TT) have the greatest endurance. Moreover, horses with the variant C-allele in the CKM gene have greater endurance, horses with the variant T-allele in COX4K2 gene have greater durability, while individuals homozygous for variant G-allele in the PDK4 gene have greater short-distance speed ^{[49][66][67][68][69][70][71][72][73]}[172,218,228,229,230,231,232,233,234]. However, understanding of the genetic factors undergirding and controlling the various alternative lateral gaits is still in its infancy.