Speed and muscle composition

The SINE insertion

The results

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Your horse is powered by a high proportion of type 2X muscle fibers.

This horse is strength oriented with an overwhelming bias to sprinting.

Type 2X fast-twitch muscle fibers favor powerful bursts of speed, intense physical exertion, and a compact, rapid stride.

This horse’s short-term power and impressive strength provides tactical pace over short distances of less than 7 furlongs.

Horses have more muscle mass than most animals on the planet, and thoroughbreds have an even greater proportion of muscle in the hind limbs compared with other horse breeds.

Thoroughbreds like this horse, born with more fast-twitch muscle fiber can generate explosive speed and strength, whereas horses born with a higher proportion of slow-twitch muscle fiber are better suited to endurance-oriented events.

An insertion of extra genetic material gives thoroughbreds more fast-twitch muscle fiber.

Only 16% of thoroughbreds have this result

The importance of muscle fiber

In the thoroughbred, different muscle fibers serve different purposes.

A steeplechaser running in a 4-mile race doesn’t need the powerful, short-term speed of the sprinter, or even the steady pace that the middle-distance horse can maintain for a mile and a half.

Type 2X fibers are fast contracting, but they tire quickly. While these muscles add essential strength over short distances, they cannot be replenished during a race, thus the depletion of energy over long distances can occur quickly.

Lactate builds up in this type of muscle fiber as distance increases, leading to fatigue. The longer the race, the more fatigued these muscle fibers become.

Used as an early indicator of athletic aptitude, genetic muscle fiber composition can determine the best training protocol for a horse to maximize performance and emphasize different muscle fibers for different race distances.

Horses with an overwhelming bias to sprinting lack the endurance and stamina to race competitively over longer distances, or courses with extended home straights and turns.

The genetic details

Like all thoroughbreds, this horse has two copies of the MSTN gene. Within this gene, an extra 227 pieces of genetic material have been inserted, which have been inherited from both dam and sire. This is called the SINE insertion. It alters how the MSTN gene works, leading to changes in the horse’s physiology. This results in more fast-twitch type 2X muscle fibers and better sprinting ability over shorter distances.