Sprinting power

The ACTN3 gene
This horse’s result
Ideal result for endurance

Result summary

A higher quantity of a-acinin-3 is present in this horse’s fast-twitch muscle fibers.

This result is associated with optimal galloping performance over short distances.

Why this is important for Anglo-Arabians

ACTN3 is a critical protein located within the fast twitch muscles of horses. It is found predominantly in the gluteus medius, which is the largest and strongest of the horse’s muscle groups, responsible for propelling the horse’s body movement forward during movement.

This protein plays a key role in muscle contraction, which produces powerful and efficient gallop performance at high speed. Therefore, for short-distance racing, a high amount of ACTN3 proteins in the fast-twitch muscle fibers is advantageous. The production of ACTN3 protein that enters the muscle fibers is controlled by variants in the ACTN3 gene.

The ‘C’ variant produces more ACTN3 leading to powerful, fast contracting fibers in the gluteus medius, which use fast-acting glycogen as an energy source, helping to propel the horse forward at a fast-paced gallop over short distances. While this ‘C’ variant is commonly found in thoroughbreds, it is less advantageous for horses training to compete in endurance events.

Endurance horses need to sustain a moderate pace over prolonged distances (usually at the trot or canter). This type of exercise does not rely upon fast-contracting muscles, nor ACTN3, and uses a different, slow-burning energy powered by fat. Therefore, the ‘T’ variant of the ACTN3 gene, which shuts down the ACTN3 protein, is better for endurance. This ‘T’ variant is found more frequently in Arabians than thoroughbreds.

Future breeding implications

Horses like this have inherited one ‘C’ variant on the ACTN3 gene from the sire and another ‘C’ variant from their dam. This gives them the result ACTN3 CC — which is one ‘C’ variant from each parent. Therefore, this horse will always pass one of these ACTN3 ‘C’ variants to their foal.

However, a potential foal could inherit a ‘T’ variant (good for endurance) 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:

CC (this horse) bred with another ACTN3 CC horse (mate) =

  • 100% chance of the foal being a ACTN3 CC horse.


CC (this horse) bred with an ACTN3 CT horse (mate) = 

  • 50% chance of an ACTN3 CC foal.
  • 50% chance of an ACTN3 CT foal.

How this gene influences endurance

The ACTN3 gene encodes a protein called a-acintin-3 found in skeletal muscle, which increases power, speed, muscle contraction, and gallop performance. This a-acintin-3 protein also increases an enzyme called glycogen phosphorylase allowing the horse to use fast-acting glucose to power locomotion over a short-distance, like a sprinter. While the contraction is rapid and forceful, and glucose is fast acting, both burn quickly burn out, resulting in fatigue over longer distances.  

In contrast, horses with the ’T’ variant in the ACTN3 gene have reduced a-acintin-3 production and may have little to no a-acintin-3 in their gluteus medius muscle, allowing the horse to sustain a trot or canter over prolonged distance, burning fat for energy, rather than glucose.