A pacing horse moves both legs on the same side of its body in unison, creating a distinctive side-to-side swaying motion. Instead of the diagonal leg pairing you see in a trot, a pacer swings its left front and left hind leg forward together, then its right front and right hind leg together, producing a two-beat rhythm. This lateral gait is both a natural trait in certain breeds and the foundation of an entire segment of horse racing.
How the Pace Works
In a standard trot, a horse moves diagonal pairs of legs at the same time: the left hind and right front strike the ground together, then the right hind and left front. It’s a balanced, two-beat gait with the horse’s weight shifting diagonally. The pace flips that coordination entirely. Both legs on the left side move forward and back in sync, then both legs on the right side do the same. You still hear two beats per stride, but the support pattern is lateral rather than diagonal.
This difference changes what the rider or driver feels. A trot produces a bouncy, up-and-down motion because the horse springs from one diagonal pair to the other. A pace creates more of a side-to-side rocking. For riders, that lateral sway is generally smoother and less jarring, which is why pacing and ambling horses have been prized for centuries for long-distance travel. People with back problems or those who find trotting uncomfortable often prefer the feel of a lateral gait.
Pace vs. Amble vs. Tölt
The terminology around lateral gaits can get confusing. A true pace is strictly a two-beat gait: both legs on one side hit the ground at exactly the same time, then both legs on the other side. An amble, by contrast, is a four-beat gait where the legs on the same side move nearly together but not quite simultaneously, so you hear four distinct hoofbeats per stride rather than two. The University of Minnesota’s veterinary anatomy program defines an amble as essentially an accelerated walk.
Many of the famous “smooth gaits” in gaited breeds are actually four-beat ambling variations rather than a true pace. The Icelandic Horse’s tölt, the Peruvian Paso’s paso llano, and the Missouri Fox Trotter’s fox trot are all lateral or semi-lateral four-beat gaits. They share the family resemblance of lateral coordination, but they’re biomechanically distinct from the synchronized two-beat pace you see on a harness racing track. When people casually say “pacing,” they sometimes mean any lateral gait, but in technical and racing contexts, it refers specifically to the two-beat version.
The Genetic Basis
The ability to pace has a clear genetic origin. A 2012 study published in Nature identified a mutation in a gene called DMRT3, sometimes nicknamed the “Gait Keeper” gene. This mutation is a premature stop codon, essentially a typo in the genetic code that changes how nerve circuits in the spinal cord coordinate leg movement. In normal horses, the spinal circuitry strongly favors diagonal coordination (trotting) at medium speeds and triggers a switch to gallop at higher speeds. The DMRT3 mutation loosens those constraints.
Horses carrying this mutation can perform lateral gaits like the pace and various ambling gaits, and they also resist the natural urge to break into a gallop at high speeds. That second trait is critical for harness racing, where a horse that breaks stride is penalized. The researchers found that the mutation has had a major effect on the diversification of domestic horse breeds, since every breed known for alternate gaits appears to require it.
Breeds That Pace
The Standardbred is the most prominent pacing breed, bred specifically for harness racing in North America. Standardbreds are divided into pacers and trotters based on their racing gait, and pacers make up the majority of harness racing fields. These horses typically wear hobbles, lightweight straps connecting the front and hind legs on each side, which help maintain the pacing rhythm at racing speeds.
The Icelandic Horse is the other well-known natural pacer, though its relationship with the gait is different. Icelandic Horses have five recognized gaits: walk, trot, canter, tölt, and pace. They use the pace primarily for short bursts of high speed rather than sustained travel, and the best Icelandic pacers can reach speeds up to 40 mph. In Icelandic horse culture, the pace (called “skeið”) is considered a prized fifth gait, and not all individual horses can perform it well.
Pacing in Harness Racing
Harness racing splits into two disciplines based on gait: pacing races and trotting races. Pacers are consistently faster. The world record for a mile on a standard track belongs to a pacer named Bulldog Hanover, who covered the distance in 1 minute and 45.4 seconds at The Meadowlands in July 2022. For comparison, the fastest trotting mile is about 3 to 4 seconds slower, with the gelding Homicide Hunter recording 1:48.4 at The Red Mile in October 2018.
That speed gap comes partly from the pace’s biomechanical efficiency at high speeds and partly from the DMRT3 mutation’s effect on gait stability. Pacers are less likely to break stride, which makes them more consistent competitors. The mutation that enables lateral gaits also suppresses the instinct to transition into a gallop, so pacers can maintain their two-beat rhythm at speeds where a trotter might lose form.
Why Pacing Horses Were Historically Valued
Before paved roads and carriages became widespread, most long-distance travel happened on horseback. A trotting horse bounces its rider with every stride, which becomes exhausting over hours of riding. Horses that could pace or amble offered a dramatically smoother experience, and they were sought after across medieval Europe, Central Asia, and the Americas. Gaited horses commanded premium prices specifically because they made travel less physically punishing.
As roads improved and carriages replaced riding for most overland travel, the demand for smooth-gaited riding horses declined in much of Europe. The trait survived primarily in breeds where it was deliberately maintained: Icelandic Horses (isolated on an island with no carriages for centuries), American Standardbreds (selected for racing speed), and various gaited breeds in the Americas like the Peruvian Paso and Paso Fino that served as working horses on rough terrain where carriages couldn’t go.