Riders and drivers of horses certainly wouldn’t be pulling Gs like a fighter pilot. In fact, the maximum acceleration that an equestrian, past or present, would feel is less than one would expect. Let’s look at a modern case study.

Although the American Quarter Horse only officially became an organized breed in 1940, it is one of the oldest types of horse to have been bred in North America. The Quarter Horse’s famous versatility, agility, and bulldog-like appearance are the products of the work of generations of breeders. Beginning with Chickasaw breeders rearing Spanish Barbs in the 17th century, the Quarter Horse type would be carefully infused with blood from English horses -- Thoroughbred, Galloway, and Irish Hobby -- as well as blood from other breeds developed in North America.

Despite the Quarter Horse’s modern reputation as a ranch horse, the goal of the early breeder was pure speed. Horse racing was one of Colonial America’s most popular sports, and lacking the organized racing infrastructure found in England, owners pitted their horses against each other in match races over distances no longer than a quarter of a mile. The name “Quarter Horse” comes from that early racing heritage, which the AQHA continues to support. In 2019, 12,985 horses started in 6,455 races, with purses totaling more than 116 million dollars, and a betting handle of over 274 million dollars.

Quarter Horse racing is in most ways identical to Thoroughbred racing. The horses are raced under saddle, not in harness, like Standardbreds are. The horses race at a full gallop. And the horses start from a standstill, breaking from a starting gate. The key differences between the two are distance and speed. Quarter Horses typically race between 220 and 770 yards, whereas Thoroughbreds generally run between half a mile and a mile and a half. Though lacking the stamina of the Thoroughbred, Quarter Horses can reach speeds of 55 miles per hour. Thoroughbreds race at speeds of around 40 miles per hour.

Quarter horse racing can thus be considered the pathological case for equine acceleration. A 2006 study examining the racing speeds of Quarter Horses, Thoroughbreds, and Arabians provides just enough data to derive acceleration over the course of a race. The authors of this study used footage from the Mount Pleasant Meadows racetrack in Mount Pleasant, Michigan, to compare the speeds of the three breeds of racehorses across multiple races of differing lengths, and across intervals of similar length at the start of the race, middle of the race, and end of the race. The authors studied three different lengths of Quarter Horse Race: 302 meters (330 yards), 366 meters (400 yards), and 402 meters (440 yards). They reported that the average speed of Quarter Horses across all three distances was 70.7 km/h, or 43.9 miles per hour. Since we can calculate the average distance of the races studied -- 356.7 meters, or 390.1 yards -- we can calculate the average time across the races studied with this formula:

time = distance/velocity

Thus the average overall time for the Quarter Horse races in the study was 18.2 seconds.

From here we can calculate the average acceleration of a Quarter Horse, using the formula:

acceleration = change in velocity/change in time

This gives us, across all of the Quarter Horse races that the authors studied, an average acceleration of 1.1 m/s^(2).

One g is equivalent to the gravitational acceleration on Earth, about 9.8 m/s^(2). Thus, across the length of an entire race, a Quarter Horse’s acceleration is only about a tenth of that of the acceleration of gravity, assuming of course a perfect system.

We can examine in greater depth the g forces a jockey experiences at the start of the race, when the horse breaks from the starting gate and launches from a standstill into a gallop. In this study, the authors also recorded the average speed of the horses at the beginning of the race. Their results, and the corresponding accelerations for each distance, are given below:

	Overall Average	302 Meters	366 Meters	402 Meters
Interval Distance	97.3 m	100 m	92 m	100 m
Average Speed	39.1 km/h (10.9 m/s)	42.8 km/h (11.9 m/s)	36.7 km/h (10.2 m/s)	35.9 km/h (10.0 m/s)
Average Time	8.9 s	8.4 s	9.0 s	10.0 s
Average Acceleration	1.2 m/s^(2)	1.4 m/s^(2)	1.1 m/s^(2)	1 m/s^(2)

The fastest horses, running over the shortest distance, don’t even reach fifteen percent of the acceleration of gravity.

Of course, this is not to suggest that an acceleration of 1.2 m/s^2 has no effect on a rider. Indeed, one study found that humans can perceive as little linear acceleration as 8.5 cm/s^2. Jockeys who are new to riding Quarter Horses and who are used to only riding Thoroughbreds have been left in the starting gate because of the zippy acceleration of their mounts.

The culprit here is Newton’s First Law of Motion. An object at rest tends to stay at rest, unless acted upon by an outside force. Horse and rider are two bodies that learn to act as one. A rider with an insecure seat can easily get left behind the motion of their horse from even small changes in speed. The challenge that has faced equestrians since Xenophon wrote his treatise On Horsemanship 2000 years ago is not that horses accelerate aggressively. The 2020 Ford Mustang GT500 accelerates to 83% the force of gravity on Earth when it hits 60 miles per hour mark in 3.3 seconds. Indeed, it’s that horses accelerate at all. Developing the requisite independent seat to withstand even those small changes in velocity is a task that has taken historic riders years to master, and since there are no shortcuts to developing a strong and supple seat in the saddle, it will continue to challenge riders in the future.

Sources

American Quarter Horse Association. “2019 Executive Summary.” https://www.aqha.com/documents/82601/0/2019+AQHA+Executive+Summary.pdf/c03114bd-e854-969a-85ec-ca33e94e005e

American Quarter Horse Association. “The History of AQHA.” https://www.aqha.com/history-of-aqha

Denhardt, Robert Moorman. The Quarter Running Horse

Kingma, H. “Thresholds for Perception of Direction of Linear Acceleration as a Possible Evaluation of the Otolith Function.” BMC Ear, Nose and Throat Disorders

Nielsen, B.D., et. al. “Racing speeds of Quarter Horses, Thoroughbreds and Arabians.” Equine Exercise Physiology

Nye, Nelson C. The Complete Book of the Quarter Horse

Tingwall, Eric. “Ford Reveals Acceleration Times for 760-HP Mustang Shelby GT500.” Car and Driver Magazine. https://www.caranddriver.com/news/a29569947/ford-mustang-shelby-gt500-performance/