One muscle disorder, PSSM, is common in Quarter Horses and related breeds and typically causes signs of tying-up. It is also linked to high body condition, which can be desirable in those breeds. | Photo: iStock

The equine muscular system is the “engine room” of the horse, comprising over half of the body mass of Thoroughbreds. At the cellular level it is a complex system, comprised of a vast array of specialized proteins that coordinate functions ranging from power output to heat management. Muscular disorders in athletic horses cause poor performance and, in some cases, death; in certain sport horse demographics, 5% to 25% of individuals have heritable muscular disorders.

Muscular disorders in athletic horses are very common because, surprisingly, they can also provide some beneficial effects, ranging from enhanced muscle mass to economic metabolism.

Given the importance of the muscular system to athletic performance of the horse, not surprisingly, it has attracted considerable research focus. In the past two decades, “tying-up” up in athletic horses has been teased into at least three distinct disorders; these disorders are heritable and, therefore, tend to have strong breed predispositions. Several other heritable muscular disorders of horses have also been defined, causing problems ranging from profound weakness to respiratory paralysis and early neonatal death