Respiration and stride

The MSTN Gene 
This horse’s result
TT
Ideal result for endurance
TT
i

Result summary

Superior stamina over long distances.

Lean and flexible muscles that can fully stretch out between strides.

Efficient stride turnover that provides superior respiration.

Why this is important for Anglo-Arabians

Anglo-Arabians are a combination of thoroughbred and Arabian breeding. Depending on the percentage of thoroughbred and the genetic makeup of their thoroughbred inheritance, each horse will inherit different types of muscle fiber composition that determine whether it will develop into a sprinter, a middle-distance horse with versatile traits or an endurance horse.

Identifying the genotype (genetic makeup) of an endurance prospect can help determine a horse’s racing style, stride frequency and ideal racing distance. 

Knowing whether a horse will develop powerful muscles built for explosive bursts of speed over a short distance, or lean, long muscles better suited for distance racing, provides owners with information that can help them choose the right athletic discipline and understand how a horse will react to intense training.

Stride and respiration traits

The respiratory system of horses is its Achilles heel. Firstly, they are nasal breathers, and secondly, there is a correlation between stride frequency and respiratory rate. For every stride they take, they must also take a breath.

MSTN TT horses have a biometrical advantage for endurance racing. They have a longer, higher set neck that counters their body’s downhill effect on the forelimbs, this shortens the body’s recoil length to lighten the forequarters when they take a stride.

The neck adds length in line with a longer range of motion, so the forelimbs can curl high and then stretch far forward to prepare for the next stride.

The high ratio of lean muscle fibers allows them to fully stretch out between strides (a bit like a rubber band being fully stretched to generate maximum power). This helps them increase their stride frequency and overall speed over distance, maximizing the rebounding effect from each stride in order to use that moment of suspension to breathe deeply and inhale enough oxygen to produce fuel for sustained energy.

This horse has inherited the MSTN TT genotype and is likely to demonstrate:

  • Lean and flexible muscle fibers that are resistant to fatigue.
  • Able to exchange high volumes of air per stride.
  • A long and efficient stride.

A word of caution

Studies show that all horses, no matter what breed, can experience some degree of exercise-induced pulmonary hemorrhage (EIPH) during intensive exercise. However, only five per cent will show evidence of this with blood in their nostrils.

Every incidence of lung bleeding contributes to the formation of scar tissue in a horses lungs, and this may limit performance and cause long-term irreversible lung damage.

Researchers in Japan found that horses do not have to working at speed to bleed. Racehorses training at moderate speeds (canter) all showed evidence of broken blood vessels, and in later studies it was reported that lung bleeding can even occur at the trot. Outside of racing, horses most severely affected by EIPH were those competing in sprinting events such as polo and barrel racing.

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 ‘CT variants to their foal.

However, that foal could inherit a ‘T’ variant or an ‘C’ variant (better for sprinting) 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.

Whereas:

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

  • 50% chance of a TT foal (endurance).
  • 50% chance of a TC foal (a versatile mix of middle distance and sprinting traits).

How this gene influences endurance

Evidence shows that high-performing endurance horses have longer strides than horses of average ability. This is because each stride a horse takes must be coupled with a breath, and the more ground the horse covers with its stride, the more efficient the inhalation will be.

When a horse is exercised at speed, as the first forelimb hits the ground, the rib cage is pulled forward, activating inhalation. In the weight-bearing phase, the horse is pushed to exhale.  It cannot hold its breath, since this breathing mechanism is tied to the cadence of its stride.

The coupling of stride to breathing rate puts short-striding MSTN CC horses at a respiratory disadvantage. These horses are not able to take in large breaths due to their high-stride frequency rate, whereas long-striding MSTN TT horses like Secretariat (who had an average stride length of around 7.3 meters) are able to breathe less often, but more efficiently.

Since a horse needs to inhale and exhale once per stride at the gallop, short-striding horses put immense pressure on their respiratory and muscular systems. When a horse cannot deliver enough oxygen to their lungs, its muscles are inadequately fueled, and the additional pressure put on the respiratory system ruptures the membrane separating the alveoli from the blood vessels in the lungs, resulting in the hemorrhaging condition known as bleeding of the lungs (EIPH).

Horses with the MSTN TT genotype have a higher percentage of type 1 muscle fibers that can replenish energy through superior air intake (ventilation) and the cardiovascular system, making them ideally suited for endurance sports.

These horses, due to their long and efficient stride, can inhale deeply and carry oxygen and other nutrients via the blood to produce the necessary fuel for long-lasting muscle contraction without putting excessive pressure on their respiratory system.