Your thoroughbred’s muscle and strength

The SINE Insertion

The results


You have a horse with a genetic advantage to build muscle for speed, powerful acceleration, rapid stride frequency, and explosive sprinting ability.

However, this muscle is fatigued more quickly in races distances over one mile.

Therefore, this horse is suited to fast sprints of 7 furlongs or less; and may do even better in races of 5-6 furlongs.

Speed, muscle mass, and distance-aptitude vary considerably within thoroughbred horses.

Just like human runners some thoroughbreds are born with more pronounced muscles which are better suited to sprinting, while others are bred to be long-distance athletes, with flatter, leaner muscles that can produce a continuous effort for a longer distance.

Muscle mass is influenced by a piece of genetic material called the SINE insertion.

Why is this important?


Muscle mass can explain why some thoroughbred racehorses are better equipped to race as sprinters, and others over longer distances.

During each race, rapid acceleration is required at the start and finish for a horse to win. Consequently, horses that can increase their stride frequency to the highest value have a better chance of winning. Since the greater the muscle mass, the greater its potential to sustain and generate propulsive force and velocity, large muscle mass can be used as an early indicator of athletic ability and to establish the best training and management practices to maximize racing potential.

Thoroughbreds exhibit remarkable muscle mass relative to their body size. Since the larger the muscle mass, the greater the potential power production, understanding the proportion of large muscle mass a horse possesses can lead to a better understanding of how that horse will perform.

The genetic details

The SINE insertion is the strongest genetic influence of muscular strength and mass in thoroughbreds. It’s an insertion of additional genetic material that not all horses have. Think of it as something stuck into a gene, where it doesn’t really belong, which then alters how the gene works.

Due to this insertion of extra genetic material, this horse naturally produces significantly less amounts of a protein called myostatin compared to horses without the insertion. This decline in myostatin triggers more muscle growth across the topline and limbs.