Build and muscle composition

The MSTN gene
This horse’s result
Ideal result for endurance

Result summary

Endurance: Higher quantities of type 1 slow twitch muscle fibers.

Ideally suited to endurance and long-distance racing.

Why this is important for Anglo-Arabians

All horses, regardless of size or age, have the same skeletal muscles. However, certain muscle fibers are better suited for sprinting while others are better suited for long-distance racing.

Centuries of selective breeding have led to different morphological types, with unique muscular developments in accordance with their intended usages.

MSTN is a gene that regulates muscle growth in horses. It forms a protein produced by muscle cells that normally limits the growth of skeletal muscle mass. The ‘T’ version of the myostatin gene results in horses that have less muscle growth and more slow-twitch muscle fibres, leading to high levels of endurance for longer races. In contrast, the ‘C’ version of the myostatin gene results in horses that have greater muscle mass and more fast-twitch fibres, leading to improved sprinting and performance for shorter races.

Every Anglo-Arabian horse’s physiological make-up is influenced by their genes. And each genotype is intrinsically suited to a certain race-distance and training regimen. Anglo-Arabians, being a combination of both Arabian and thoroughbred, can be one of three genotypes:

Sprinter (CC genotype)

MSTN ‘CC’ genotype horses are powered by strength and speed and perform best over short-distance races.

Middle-Distance (CT genotype)

MSTN ‘TC’ genotype horses have a versatile mix of traits and may race like a sprinter or an endurance horse.

Endurance (TT genotype)

MSTN ‘TT’ genotype horses can outrun shorter striding rivals over longer distances as they can maintain the same stride turnover for longer.

Over shorter gallop races, traits like acceleration and speed are most important. Sprinters, and some middle-distance horses are able to produce sharp bursts of speed over distances of up to a mile. These horses have a higher-than-normal ratio of fast type 2X muscles which gives them a short, rapid stride that aids the horse in maintaining power and speed over a short distance. However, these muscle fibers lack stamina.

In endurance races, the winner is the horse that maintains the highest average speed over the entire race distance (up to 160 km). Therefore, stride length, energy efficiency, and stamina assume a greater significance.

 Anglo-Arabians genetically suited to competitive endurance need a combination of muscle fibres: slow twitch (type 1) and fast twitch (type 2X), but the ratio of slow-twitch type 1 muscle fibres should be higher.

This is because, during taxing exercise, slow-twitch muscle fibres can better replenish a horse’s energy. They do this by carrying oxygen and other nutrients via the blood to produce fuel for sustained muscle contraction. 

A work of caution

Injuries and breakdowns are more likely to occur when a horse is pressured to perform over an unsuitable distance or the wrong discipline.

Specific training exercises can enhance a horse’s type 2X muscles and improve its sprinting ability, but training will never be able to change the type of muscle a horse inherited by birth. While a sprinter has short-term power and impressive strength, an endurance horse is able to exchange high volumes of air per stride, resulting in superior respiration, speed, and stamina over long distances.

Future breeding implications

Horses like this have inherited one ‘T’ variant on the MSTN gene from the sire and another ‘T’ variant from their dam. This gives them the result MSTN TT — one ‘T’ from each parent. Therefore, this horse will always pass one of these MSTN ‘T’ variants to their foal.

However, that foal could inherit a ‘T’ variant or an ‘C’ variant (better for sprint work) from the other parent. Therefore, it is important to know the genetic results for both parents to understand all possible breeding implications.

For example:

TT (this horse) bred with another MSTN TT horse (mate) =

  • 100% chance of the foal being a MSTN TT horse (endurance).


TT (this horse) bred with a MSTN CT horse (mate) =

  • 50% chance of a TT foal (endurance).
  • 50% chance of a CT foal (middle distance and sports).

How this gene influences endurance

The MSTN gene is a strong genetic predictor of muscle mass, composition, and muscle fibre type in thoroughbreds caused by a variation within the gene. There is a switch in one of the four base pairs that make up a DNA molecule. Cytosine (C) is replaced by thymine (T) changing the function of the MSTN gene.

The change from a ‘T’ (thymine) to a ‘C’ (cytosine) helps to suppress a protein called myostatin in the muscle tissues. When this happens, more muscle mass with more type 2 fast-twitch fibers is formed. While this is an advantage for sprints and short distance races, it can be a disadvantage for horses competing in endurance, who may carry too much muscle and tire over a longer distance.