Did you know that the genetic makeup of horses shows up not only in their coloring and personality – but also in their muscle composition and predisposition to certain injuries?
Well-muscled, compact American Paint and Quarter horses have been selectively bred by ranchers for centuries. Although there was no strict strategy in early breeding programs, certain characteristics were particularly cherished, including muscular and bulky hind quarters for power and speed, and a low set head and neck, and compact body for agility and balance.
Later on, long before genetic manipulation was a thing, ranchers began to take a more deliberate approach to refining and selecting the horses they used for working cattle.
And recently, genetic scientists managed to pinpoint the gene responsible for many of the American Quarter Horse’s unique performance traits and well-defined muscles.
It turns out, without realizing it, ranchers selectively utilized foundation lines that carried two genetic mutations of the Myostatin gene, to gift this great American ranch horse with enhanced athletic ability and supreme sprinting speed.
Unfortunately, the same genes that help the Quarter Horse dominate high-speed western sports are also those that can put them at a heightened risk of inherited muscle diseases like PSSM and HYPP or musculoskeletal injuries from the stresses that high-impact activities put on their bodies.
Luckily, thanks to recent scientific findings, horse owners can develop effective training and exercise regimes tailored to their horse’s unique genetic makeup.
Read on to find out about how Myostatin affects Quarter Horses, and how you can use this information to enhance your horse’s performance and reduce the likelihood of injury.
With over 6 million horses registered around the globe, the American Quarter Horse Association is the world’s largest equine breed registry. The popularity of western sports around the world has notably increased the demand for ranch horses with inherited traits like speed, strength and inherited ‘cow sense’.
While the Quarter Horse can be seen in virtually every riding discipline, like all horses bred for a specific job, they too have their physical limitations imposed by nature. And theirs is skeletal muscle composition.
What is skeletal muscle?
All horses have three types of muscle: cardiac, smooth, and skeletal. Cardiac muscles control the heart, smooth muscles control involuntary functions like constricting of blood vessels, and skeletal muscles control movement.
Skeletal muscle is made up of clumps of individual fibers called myocytes, which are made up of myofibrils which are tiny strands of proteins (called actin and myosin) that can latch on to each other and instigate a muscle cell to contract.
Fast twitch versus slow twitch muscle fibers
Different fibers have different metabolic energy producing capacities – slow twitch fibers contract slowly and are fatigue resistant, while fast-twitch muscle fibers are ideally suited to short fast bursts of power.
Quarter horses’ significantly bigger muscles on the hindquarters, around the fore and over the topline are mostly made up of fast-twitch fibers which provide them with short-term speed and significant strength. This is why Quarter horses make exceptional sprinters, barrel racers or cutting horses.
Fast-twitch muscles in wild animals are usually reserved for short term bursts of emergency energy (like a wild pheasant bursting into flight to escape a predator). Since pheasants only fly for short distances to avoid danger- the fast-twitch muscles that power their wings run out of energy after less than a minute of contraction and use. The same energy depletion happens with horses with a high proportion of fast-twitch muscle fibers. They can run like the wind for up to 60 seconds, but after that, the fuel supply depletes almost immediately.
Slow-twitch muscles on the other hand, can work steadily, without fatigue, for many hours. These are the muscles that support everyday movement like walking, standing and resting -and they are more efficient in energy usage when compared to fast-twitch fibers. A lean, lightly framed Arabian, or a draft horse used for pulling a cart will have a higher percentage of slow-twitch muscles, which gives it slow-release energy, stamina and resistance.
What’s Myostatin got to do with it?
Myostatin is the gene that regulates muscle growth in horses. It is actually a protein produced by muscle cells and it is able to increase or decrease the amount of muscle a foal will develop as it grows. It normally helps limit the growth of skeletal muscle mass, but in some horses (like the Quarter Horse) the protein is prevented from regulating this process.
Natural genetic mutations on the myostatin gene cause a few unique breeds (Paint, Quarter Horses and some Thoroughbreds) to grow higher proportions of fast twitch muscle fibers (up to 80% more) and 12.5% more overall muscle mass.
Photography: Cecile Zahorka
These mutations are not exclusive to horses. They are also found in other species. The Myostatin (MTSN) gene was discovered by two scientists, Se-Jin Lee and Alexander McPherron in 1997 when they noticed that if it was lacking in mice, it caused them to increase in size by two or three times. Further research found that ‘double muscled’ cattle breeds such as the Italian Piedmontese and Belgian Blue, and some breeds of dog, pig and sheep also carry this genetic mutation. (Feher, 2012)
Why genetic mutations affect Quarter and Paint Horses
The genetic mutation responsible for enhancing muscle-growth in horses (SINE) is rare outside the Thoroughbred, Paint and Quarter Horses. Slightly more common is the MTSN C mutation, which causes a horse to develop a higher proportion of fast-twitch muscle fibers. This mutation is also seen in Shetland ponies and Belgian horses but seems to influence these breeds differently.
The origin of both mutations is unknown, but it is no coincidence that they are both found in three closely related breeds like the Quarter Horse, Paint, and Thoroughbred. Some historians have theorized that the mutation may have been passed down through an Iberian ancestral connection. This is plausible since the thoroughbred was founded using British mares that were descendants of Celtic ponies from northern Spain and the Quarter horse and Paint studbooks both had Spanish blood introduced to increase quickness. (Hendricks 2007).
Built for speed
Even though Quarter horses carry the same mutation responsible for gargantuan Belgian blue cattle that can barely walk, and freakish bully whippets that are unable to run – in horses it actually enhances athletic ability. (Mickelson, 2014)
More fast-twitch muscle fiber means an increase in actin and myosin filaments which are able to generate quick bursts of speed anaerobically. Power equals speed, and since only the largest superficial muscles are enlarged in horses (like the hindquarters) Quarter Horses are able to generate significant power and strength for a short distance/duration. In fact, they are the fastest sprinters in the world – reaching speeds of up to 55mph.
While these superb athletes are able to sprint at record speeds, and perform sudden accelerations, turns and stops at high speed, the kicker is that they can’t keep the power over longer distances. Their tanks run out of fuel quickly. And that puts them at risk of muscle fatigue and injury.
When the tank runs out…
Energy depletion occurs rapidly around the 60 second mark for a Quarter Horse. This is the point where the amount of energy required to keep contracting the muscles becomes greater than the horse’s ability to produce energy and is an important mechanism for the preservation of horse’s health.
At this point the horse is not physically tired, but metabolically his body has run out of steam. Since his blood is no longer able to supply oxygen quickly enough, energy levels cannot be replenished, and anaerobic glycolysis swiftly consumes all the stored glycogen in his cells, causing lactic acid to accumulate in his muscle fibers.
This build-up blocks his muscles from functioning properly, and eventually he has to slow down to let the blood carry away excess lactic acid. If his rider pushes him to keep working and doesn’t allow him to rest, his limbs will not function normally, and he may stumble and fall due to a loss of balance.
Muscular diseases pervasive among Quarter Horses
Sometimes, by selectively breeding horses for traits that are desirable (like fast-contracting muscle and explosive speed) we also get less desirable conditions along with them. Quarter horses are at a higher risk of a surprisingly large number of heritable muscle diseases that may have been partly caused by selective breeding for certain muscle phenotypes. The American Quarter Horse Association recommends owners screen their horses for five heritable diseases: hereditary equine regional dermal asthenia (HERDA), glycogen branching enzyme deficiency (GBED), malignant hyperthermia (MH), hyperkalemic periodic paralysis (HYPP), and polysaccharide storage myopathy (PSSM).
Photography: Cecile Zahorka
Avoiding injury through careful training
What any horse inherits in muscle type can help us understand how much and how often it should be trained. By following an individual conditioning program designed for horses with more fast-twitch muscle fibers, you can work with rather than against any physical limitations your Quarter Horse has imposed on him by nature.
Due to their heavier overall weight to wither height ratio, which increases pressure on the limbs, and more muscle around the shoulder, Quarter Horses have a higher chance of injuring the forelimbs during training. (Tozaki, 2019).
- Regular slow exercise can help improve a Quarter horse’s muscle strength and flexibility and improve the levels of glycogen within the muscle.
- Always try to train on the best surface available. A safe, quality training surface, whether it’s a sand arena or a grass track can play an integral role not only in your horse’s wellbeing but helps his future soundness.
- Early soft exposure to exercise, where “bone remodeling” occurs in response to the physical stress of training can help bones and tendons become thicker and stronger, lessening the chance of injury later in life.
- Interval training (exercising the horse using multiple short sessions separated by rest periods) can also help alter muscle fiber type and enhance fitness without overexerting the horse.
- While horses with a high proportion of slow-twitch muscle fibers are able to retain a reasonable level of aerobic fitness even when at rest, a Quarter Horse’s fast-twitch muscle fibers are less flexible – which puts them at risk of muscle injury when they return to work after a rest or start high-intensity training.
- Quarter horses also don’t do well left in a field all week, only to be ridden for hours over the weekend. These horses have tight muscles that need extra time to warm up before exercise to increase the blood circulation, soften, stretch and flex. Time spent warming up and cooling down will lessen the chance of a muscle injury stumble or fall.
Through day-to-day monitoring, and a clear understanding of how your Quarter Horse’s muscle composition differs from other breeds, you will be able to perfect your conditioning and training approach to keep your horse healthy and sound and identify when something isn’t right. When you understand what to look out for, you will be able to pick up any early signs of discomfort or muscle soreness and prevent a much larger injury down the line.
McPherron, A., Lawler, A. & Lee, SJ. Regulation of skeletal muscle mass in mice by a new TGF-p superfamily member. Nature 387, 83–90 (1997). https://doi.org/10.1038/387083a0
Hendricks B.L. (2007) International Encyclopedia of Horse Breeds.
University of Oklahoma Press, Norman, OK.
Petersen, J. L., Valberg, S. J., Mickelson, J. R. & McCue, M. E. Haplotype diversity in the equine myostatin gene with focus on variants associated with race distance propensity and muscle fiber type proportions. Anim. Genet. 45, 827–835. https://doi.org/10.1111/age.12205 (2014).
Morphological and Genomic Differences Between Cutting and Racing Lines of Quarter Horses, Journal of Equine Veterinary Science, Volume 33, Issue 4, 2013
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Photography: Cecile Zahorka www.thepixelnomad.com