The Myostatin Gene (MSTN), also known as “the speed gene”, regulates how large a muscle fiber can grow and what type of muscle fibers (fast or slow twitch) a muscle can have. This gene has been shown to determine sprinting ability and determine the best racing distance for thoroughbred racehorses. The myostatin gene also influences other variables that will be discussed in this article.
The myostatin gene influences the cell to produce myostatin (a protein). This myostatin protein controls the growth of tissues throughout the body, particularly muscle. Myostatin limits the capacity of how much the muscle can grow in size. When myostatin is deficient or absent, muscle is able to grow limitlessly.
The mouse and the cow on the right does not have the myostatin gene, showing excessive muscle mass, whereas the mouse and cow on the left is a regular mouse. (Photo from John Hopkins Medicine & Reconminetics)
The ability for the myostatin gene to influence muscle size is somewhat influenced by the number of fast twitch muscle fibers a horse’s muscles can have. For those who are not familiar with muscle physiology, to explain it simply, there are “slow twitch” and “fast twitch” muscle fibers.
Slow twitch muscle fibers (Type I) are lean and not able to produce a lot of power upon muscle contraction. However, these muscle fibers have a lot of endurance (being able to produce near its maximal contractile force repeatedly for a long period of time without getting tired or fatigued). These fibers prefer aerobic metabolism to produce energy.
Fast twitch muscle fibers (Type IIA or IIB) have the ability to generate high levels of power upon contraction as well as contract very rapidly due to their large size. However, the price of producing high power outputs over a short period of time makes these muscle fibers fatigue quickly. These fibers prefer anaerobic metabolism to produce energy. Fast twitch muscle fibers have a higher capacity to grow larger in size than slow twitch muscle fibers, especially with training.
Diagram comparing fast twitch (Type IIA/IIB) to slow twitch (Type I) muscle fibers. (Photo from Parallel Coaching)
Each muscle has a certain ratio of fast twitch to slow twitch muscle fibers. For instance, a horse’s gluteus medius (the giant muscle along the horse’s hind end) is composed of about 90% fast twitch muscle fibers and 10% slow twitch because of its need to produce powerful muscle contractions at the gallop or when jumping. Meanwhile, the triceps brachii of the same horse may have about 35% slow twitch and 65% fast twitch muscle fibers (due to the triceps brachii’s function of stabilizing the limb during movement with smaller contractile forces but stay contracted for long periods of time to stabilize the limb) (Hoven et al. 1985).
The image above shows a microscopic view of the different muscle fibers within a muscle sample. Each fiber is stained a certain color in relation to the type of muscle fiber it is. Notice it shows “Type IIA”, “IIB” and “Type I”. I am not going to go too much into detail about these different fiber type names in this article, but just know that “Type IIA and IIB” are fast twitch and “Type I” are slow twitch muscle fibers. (Photo from Bakhsh et al. 2019)
However, genetics and training can influence those ratios. There is controversy as to whether training influences the ratio of fiber types more so than genetics, but the research tends to favor genetics (Miyata et al. 2018). Especially in regards to higher ratios of fast twitch over slow twitch muscle fibers.
Fast twitch muscle fibers have a larger capacity to grow larger in size compared to slow twitch muscle fibers. As you could probably now conclude, those who have an absent myostatin gene have higher quantities of fast twitch muscle fibers, thus why these animals are able to grow larger muscles. This leads us to our next section.
The Myostatin Genotypes and Muscle Fiber Composition
There are three different myostatin genotypes:
CC- myostatin is mostly not present.
CT- myostatin is partially present.
TT- myostatin is largely present.
A study conducted by Rooney et al. (2017) investigated the variation of fiber type ratio of the horse’s gluteus medius in relation to their myostatin genotype in untrained thoroughbred horses (with an average age of 21 months).
Image of the gluteus medius of the equine.
They found that horses’ with a CC genotype were composed of about 4% slow twitch muscle fibers and 95% fast twitch muscle fibers on average in their gluteus medius muscle. CT genotypes had about 10% slow twitch and 89% fast twitch. TT genotypes had about 14% slow twitch and 85% fast twitch.
Photo demonstrating the muscle fiber type distribution in untrained thoroughbred’s gluteus medius across the different myostatin genotypes found from Rooney et al. (2017).
Notice from the photo above that CC genotypes have almost 10% higher abundance of fast twitch muscle fibers (Type IIA/IIB) than TT genotypes.
Keep in mind that the gluteus medius muscle both in humans and horses is already a fast twitch fiber dominant muscle due to its function of being one of the primary power sources of strength and power during galloping/running and jumping. (the human gluteus medius has a composition of about 40% slow twitch and 60% fast twitch for comparison; Sirca & Michieli 1980). So these ratios of fiber types are not true for every muscle in the horse’s body. But we can rightfully assume that most muscles in the body will follow a similar pattern of CC genotypes overall having more fast twitch muscle fibers in most muscles on average and TT genotypes having more slow twitch muscle fibers on average, with CT genotypes lying somewhere in between.
This study also looked at other physiological differences within the muscle fibers of different myostatin genotypes such as citrate synthase enzymes, mitochondrial volume, specific fiber types, etc. However, I am not going to overcomplicate this article for readers that do not have a background in physiology. If you are curious about the other results of this study and what it means for racehorse performance and training, please feel free to email me gallopscience@gmail.com.
A study by Miyata et al. (2018) found very similar associations in fiber type ratio and the myostatin genotypes yearling and 2yo thoroughbred horses before and after these horses were given a similar 5 month training regimen (ranging from 1000m hill gallops, to 1600-2400m canters, and 500m high speed works twice a week). This study shows that even when horses are given the same training regimens (which can change fiber type somewhat), CC genotypes still remain more dominant in fast twitch muscle cells over TT genotypes, again with CT lying between CC and TT.
The overall result of these studies are that CC myostatin genotypes have muscle physiology that are more suited for power, acceleration, and speed whereas TT myostatin genotypes have muscle physiology that are more suited for endurance. CT genotypes lie somewhere in between. With this information, racehorses that are CC may have higher potential to perform better in short distance races (“sprint” races) and with TT having higher potential to perform better in longer distanced races (“route” or “two turn” races). This is backed by many studies that will be discussed in the next section.
Preferred Racing Distance Based on Myostatin Genotype
There are quite a few studies that actually show the myostatin gene influences which distance thoroughbred racehorses are best suited for. In these studies, researchers found the myostatin genotype of hundreds of both elite and average thoroughbred racehorses and tried to find associations between their genotype (CC/CT/TT) and the race distances that horse best competed in. These results were taken from thoroughbred racehorses in North America, Ireland, Great Britain, New Zealand, Australia, and East Asia. The combined results of these studies (Farries et al. 2018; Hill et al. 2010; Bins et al. 2010; Hill et al. 2019) indicate the following race distances each genotype is best suited for:
CC - 8 furlongs (<1600m or 1 mile) or less
CT - Between 7-12 furlongs (1400m-2200m or 1 to 1.5 miles)
TT - 10 furlongs (2000m+ or 1.25 miles) or longer
This does not mean that CC horses only race well in races less than 8 furlongs. Just that CC horses are more likely to race better at those distances. This is the case for other genotypes as well. This is demonstrated in the following figure from Hill et al. (2010):
Distribution of best race distance in accordance to each horse’s myostatin genotype in 179 elite thoroughbred racehorses: CC- blue, CT- red, TT- green. (Photo modified from Hill et al. 2010)
Notice how CC horses (more fast twitch muscle fibers) have best racing distances ranging from 5 to 8 furlongs, but the vast majority of CC horses seem to have a best racing distance of 5-6 furlongs. There are no CC horses in this dataset that have their best race over 10 furlongs. Notice an opposite trend for TT horses (less fast twitch with more slow twitch muscle fibers), where TT horses in this data set have had their best racing distances ranging from 7 to 13 furlongs. However, a much larger portion of TT horses have had their best races at distances of 10 furlongs or longer. Notice the trend with CT horses. CT horses clearly have the most range when it comes to best racing distances. Unlike CC and TT horses that do not appear on their opposite ends of the racing distance spectrum, CT horses show up in all racing distances. However, the bulk of CT horses have their best racing distances between 8 and 11 furlongs.
Whether the horse is bred for turf or dirt likely does not affect these optimum race distances.
Many good trainers already know whether their horse will be a sprinter or stayer simply by looking at their pedigree and watching them handle training. However, there are a lot of horsemen that rely simply on racing their horses 2-turns (longer distances) for the first time to see whether or not their horse runs well at those distances. This can waste a lot of money (jockey mount fees, pre-race vet work, shipping, etc) and time (waiting for the race, the horse’s need to recover from the race, waiting for the next race in the condition book, etc.). By knowing your horse’s myostatin genotype, you can make more accurate training and racing decisions earlier on to save the trainer, owner’s time and money as well as maximize your racehorse’s success.
Best Racing Age
The study by Farries et al. (2018) was unique in that it also analyzed the association between each horse’s myostatin genotype and the age in which each horse’s first race occurred, in addition to the age in which each horse’s best race occurred.
CC horses competed in their first race by about 1 month on average earlier than did CT horses, and CC horses completed their first race over 2 months on average earlier than TT horses.
As for when each horse raced their best racing distance, CC horses competed in their best race over 2 months on average earlier than CT horses and over 4 months on average earlier than TT horses.
The results by Farries et al. (2018) and Bailey et al. (2022) indicate that CC horses are more likely to perform well early on in their career, such as in their 2yo year, and TT horses are more likely to perform well later on in their career, such as in their mid to late 3yo year. CT horses can go either way, but on average may perform better later than CC horses but much earlier than TT horses.
It is unclear why the myostatin gene determines this “late development” of performance occurs with CT and TT horses in comparison to CC horses. However, we do know that longer distanced races do not occur until late into a horse’s 2yo year or until their 3yo year. It is likely that trainers already sense their horses will prefer longer distance races (potentially being CT or TT) and are waiting until the longer races occur. Also it is common for trainers to start their horse’s first race over shorter distances (4.5-6.5f) and progress them into racing longer distances (7f or longer). So while CC horses start in their first race already at their optimum racing distance, CT and TT horses usually are not. Also, most trainers understand that it takes a longer amount of time to get a horse fit for longer distance races than sprint races. Therefore, the delayed first race and best race for CT and TT horses.
Myostatin Gene and Potential for Performance
The scientific literature is clear that the myostatin gene does not determine whether or not the horse will be a high quality or a low quality racehorse. Only that the myostatin gene can predict what distance that horse may best compete at. However, if you have a CC horse that races poorly in sprint races, it is likely that horse is simply just not a good racehorse or they have other underlying issues inhibiting their performance. The same goes for TT horses; if a TT horse races poorly in longer distanced races, they are likely just not good racehorses or they have other underlying issues inhibiting their performance. As for CT horses, again they can go either way. If your CT horse does not compete well in sprint races, they may have the potential to race better in longer distance races. If not, then again, they may simply not be good racehorses.
In North America, the vast majority of races are held between 5 and 9 furlongs. Races longer than 10 furlongs are rare, especially at certain racetracks. Therefore, if you are a North American based owner or trainer and have a TT horse, they may not race as frequently and their earning potential may not be as high as CC and CT horses.
The American Triple Crown is composed of the Kentucky Derby (10f), The Preakness (9.5f), and The Belmont (12f) in the span of 6 weeks. Clearly, a CT horse is best suited for running at all of these racing distances. This is even more so for The Kentucky Derby due to the race’s points qualification system via The Kentucky Derby prep races. The Kentucky Derby prep races are held during horses’ 2yo year and early 3yo year range from 8 to 9 furlongs. The top 3 horses in each race earn a certain amount of points. The top 20 horses that accumulate the most points qualify for the Kentucky Derby. So not only does the horse have to race and compete competitively earlier on in their career, they also must compete competitively in those middle distance races in order to even qualify for the Kentucky Derby. These qualities most definitely suit CT horses (and possibly early developing TT horses) based on the research. Again, the myostatin gene does not determine overall racing quality or class, just optimum racing distance of that horse.
Conformational Characteristics
A study by Tozaki et al. (2011) measured different confirmation characteristics of over 100 thoroughbred yearlings’ (born between March and April), such as wither height, body weight, chest circumference, and cannon bone circumference. The researchers then tested their myostatin gene to find associations with the different myostatin genotypes and growth rates and conformational differences. These horses’ confirmation was remeasured every month for 6 months as they endured training for racing.
Body weight, wither height, cannon bone circumference, and chest circumference significantly increased with age in all horses. Cannon bone circumference was significantly greater in males than females at all times. Chest circumference was significantly higher in females than males until December of the horses’ yearling year. Body weight and wither height was not significantly higher in males over females until the horses reached February of their 2yo year.
There was no association between any of these horse’s conformational characteristics and each horse’s respective myostatin genotype except for body weight. Horses with CC genotypes were significantly heavier than CT and TT genotypes, and CT genotypes were heavier than TT genotypes. This is the case for both male and female horses. This is likely due to, as previously mentioned, CC horses having more muscle mass due to the higher portion of fast twitch muscle fibers (which are generally larger in size than slow twitch fibers).
These differences in weight may also be due to, although not mentioned by the researchers, higher fat build up in CC horses than TT horses. The fat composition of horses can be influenced by several factors, but it is important to note fast twitch muscle fibers are not nearly as adequate to burn fat compared to slow twitch muscle fibers (McAinch et al. 2012). This may predispose CC horses, having more fast twitch muscle fibers, to have a harder time burning fat off the body than TT horses, being more abundant in slow twitch fibers.
Heart Size
Kis et al. (2023) measured different conformational parameters of the heart of over 60 Hungarian Thoroughbred horses and tried to find associations with the myostatin genotypes. The size of each horse’s chambers of the heart (ventricles and atria) were not significantly different between CC, CT, and TT. However, aortic diameter at the sinus of Valsalva (end- diastole and end-systole) and aortic diameter at the valve (end-systole) was significantly greater in CC horses than CT and TT horses.
Image of the heart and the aortic diameter at the sinus of Valsalva and the aortic valve.
This finding suggests CC horses may have higher cardiac output (how much blood that can be pumped through the heart over a given period of time, a variable in oxygen delivery throughout the body). However, this is ironic given that higher cardiac output is usually a characteristic seen in more endurance-based athletes of humans and horses (Weber et al. 1987; Wilmore et al. 2000), which would characterize CT and TT horses. The evidence shown prior clearly shows CT and TT horses have superior endurance over CC horses. Therefore, it is hard to say whether CC horses truly have higher cardiac output than CT and TT horses. If so, it likely does not play a role in their endurance because even though horses with higher cardiac output have higher ability to deliver oxygen to its muscles and most fast twitch muscle fibers generally do not have the means to take up that oxygen (usually due to lack of blood vessels surrounding fast twitch muscle fibers, lack of oxygen carrying enzymes, and lack of mitochondria that can actually metabolize that oxygen to create energy).
However, given CC horse’s higher content of fast twitch muscle fibers, they are able to produce high amounts of acidity in the blood over a short period of time during intense exercise. This increase in blood acidity (caused by hydrogen ions, not ‘lactic acid’ - see my article “Lactic Acid does not cause Fatigue or Muscle Soreness in horses”) reduces the red blood cell’s ability to bind to oxygen. Thus, if CC horses truly have increased cardiac output, this may be a genetic adaptation to compensate for the acidity produced by the fast twitch muscle fibers by increasing the capacity for oxygen levels in the blood to be higher.
Overall, further research is needed to truly determine whether CC horses truly have higher cardiac output than CT and TT horses.
Breeding for the Myostatin Gene
If you remember “punnett squares” from your high school biology class, how to breed for the myostatin gene is relatively easy to understand. Remember that horses are either CC, CT, or TT. Each of these letters are an “allele” on the horse’s DNA. As previously explained, the “C allele” is the myostatin gene that allows a horse to have speed and power. The “T allele” allows the horse to have more endurance.
If you tested for the myostatin gene in a mare and a stallion, you can predict the likelihood of whether the offspring will be a CC, CT, or TT. For example, if the dam is CT, and the sire is CT, the following chances of what genotype the offspring will have will be the following: 25% CC, 50% CT, and 25% TT. (Image on right)
Meanwhile, if you have a dam that is CC and a sire that is CC, the offspring will have an 100% likelihood of also being a CC; a sprinter.
(Image on left)
A dam that is TT and a sire that is CC will have a 100% likelihood of producing an offspring that is CT; a middle or classic distance racehorse.
(Image on right)
Breeders that know the myostatin genotype of their mares and stallions can make more accurate breeding decisions depending on whether you want a sprinter, classic/middle distance horse, or long distance horse. Smart breeders can also pair in preferred conformational traits for a certain distance to combine with the myostatin gene.
For those that are able to gene test their broodmare and not the stallion they plan to breed with, understanding the characteristics of the different myostatin genotypes may help a breeder accurately guess what gene the stallion may have. For instance, stallions that were “late developers” and ran their best races at 10 furlongs or longer (and some of their offspring show similar traits) are likely TT genotypes. If a stallion ran their best races early on as a 2 or 3yo and ran their best race distances at 7f or shorter, then they are likely a CC genotype. Stallions that ran successfully in both sprint and route races are likely CT genotypes.
Myostatin Gene Origin in Thoroughbreds
No American racing fan today could even recognize how horse races were conducted prior to the 1900’s. Race distances ranged from 1 to 4 miles long, in ‘heats’. A horse had to win two races (or heats) in the same day at the same, or longer, distance in order to win prize money. If there was a tie win between two horses in heat wins, those two horses would race a third time to decide the winner. This means horses could be racing up to 2 to 12 miles in a single day. Today, there is not a single thoroughbred race at a designated racetrack that is held over 2.5 miles in America today. Races 1.5 miles or longer are extremely rare in America. As one could imagine, those horses racing 1 to 4 miles one to three times a day near maximal effort back in the 18th and 19th century needed extreme endurance to not only race at these longer distances, but to recover from them in order to compete in the following heats. Because of this, speed was not emphasized and therefore the myostatin gene was not very present in the breed. Eclipse, one of the most renowned racehorses in American history during the 1700’s (beating horses in consecutive 4 mile races), has been confirmed to be a TT myostatin genotype.
Researchers have concluded that the “C” allele in the myostatin gene was present, but very rare prior to the 1900’s (Bower et al. 2012). Genetic samples from a dozen museums of historically influential thoroughbred stallions born between 1764 and 1930 all tested as TT, no myostatin gene “C” allele. It wasn't until the horse racing industry in both North America and Great Britain began to change following 1860, where 1-4 mile “heat” races disappeared and racing horses at 2yo became more emphasized. Race distances gradually became shorter to allow the horses to recover quicker from their races and race more frequently, increasing the earning potential for racehorse owners. Due to these industry changes, speed in the racehorse became more sought out for over endurance.
But the myostatin speed “C” allele did not yet become very widespread in the thoroughbred breed until Northern Dancer (born in 1961). Northern Dancer held a “C” allele, which was passed down from his sire Nearctic who also had a C allele. Nearctic obtained his C allele from his dam, Lady Angela (originated from Great Britain). Northern Dancer would become the most influential thoroughbred sire in not just North America, but the entire world. He passed on his C allele to many thoroughbred progeny, giving them superior speed. Through Northern Dancer, the “C” allele spread rapidly throughout the thoroughbred breed, now producing not just CT, but CC genotypes. Over 97% of thoroughbreds today have Northern Dancer present in their pedigrees (McGivney et al. 2020).
Myostatin Gene Prevalence
The myostatin gene C allele is abundant in different parts of the world today. Whereas prior to 1950, the vast majority of racehorses were TT, today the vast majority of horses are CC.
In Europe, based on genetic testing, it is estimated that 14% of the European thoroughbred population is TT, 51% is CT, and 35% is CC.
In North America, it’s estimated 14% is TT, 53% is CT, and 33% is CC.
In Australia/New Zealand, 7% is TT, 43% is CT, and 48% is CC.
(Hill 2019)
Prevalence of myostatin genotypes among thoroughbred racehorses around the world. Information taken from (Hill 2019)
Implications for Trainers and Owners
The earlier you know whether your horse is a sprinter or long distance horse, the sooner you can individualize their training to race at those respective distances.
Owners can accurately choose which trainer will be best for their racehorse based on their myostatin genotype. For instance, if you know your horse is CC, then it would be most effective to send that horse to a trainer that is very successful with 2yos and/or sprinters. If your horse is TT, then it would be most effective to send your horse to trainers that takes their time with late developers and are known to produce successful older horses that prefer long distance.
How to test for the Myostatin Gene:
Conclusion
Key points:
The Myostatin gene regulates muscle growth and the ratio of fast to slow twitch fibers.
The Myostatin gene can accurately predict best racing distance for thoroughbred racehorses (CC: 4-7f, CT: 8-12f, TT: 10f+)
The Myostatin gene can accurately predict early or late developing racehorses (CC early developers, TT late developers, CT in between CC and TT).
The myostatin gene does not determine a horse’s conformation other than body weight/muscle mass.
The vast majority (over 80%) of thoroughbred racehorses are CC or CT around the world.
The myostatin gene can help owners, trainers, and breeders make more accurate training, racing, and breeding decisions to boost their success as well as their racehorses.
References
“Variation of fiber types in the triceps brachii, longissimus dorsi, gluteus medius, and biceps femoris of horses” (van den Hoven et al. 1985)
“Skeletal muscle mitochondrial bioenergetics and associations with myostatin genotypes in the Thoroughbred horse” (Rooney et al. 2017)
“Selective type II fibre muscular atrophy in patients with osteoarthritis of the hip” (Sirca & Michieli 1980)
“Effect of Myostatin SNP on muscle fiber properties in male Thoroughbred horses during training period” (Miyata, 2017)
“Sequence Variants at the myostatin Gene Locus Influence the Body Composition of Thoroughbred Horses” (Tozaki et al. 2011)- Japanese Study
“Association of myostatin gene polymorphism with echocardiographic and muscular ultrasonographic measurements in Hungarian thoroughbreds horses” (Kis et al. 2023) Myostatin gene CC causes larger hearts and weight/height
“The contribution of myostatin (MSTN) and additional modifying genetic loci to race distance aptitude in Thoroughbred horses racing in different geographic regions” (Hill et al. 2019) Europe/Middle-East, Australia/New Zealand, North America and South Africa, average race distance, best race distance for elite, nonelite and all winning horses”
https://pubmed.ncbi.nlm.nih.gov/30604488/#:~:text=Conclusions%3A%20MSTN%20is%20the%20single,genetic%20potential%20for%20distance%20aptitude.
“A genome-wide SNP-association study confirms a sequence variant (g.66493737C>T) in the equine myostatin (MSTN) gene as the most powerful predictor of optimum racing distance for Thoroughbred racehorses” (Hill et al. 2010)
“A sequence polymorphism in MSTN predicts sprinting ability and racing stamina in thoroughbred horses” (Hill et al. 2010)
“Identification of the myostatin locus (MSTN) as having a major effect on optimum racing distance in the Thoroughbred horse in the USA” (Bins et al. 2010)
“The genetic origin and history of speed in the Thoroughbred racehorse” (Bower et al. 2012)
“Speed Gene Background Essay” (Hill 2019)
“Cardiac output and oxygen consumption in exercising Thoroughbred horses” (Weber et al. 1987)
“Cardiac output and stroke volume changes with endurance training: The HERITAGE Family Study” (Wilmore et al. 2000)
“Genetics of Thoroughbred Racing Performance” (Bailey et al. 2022)
https://pubmed.ncbi.nlm.nih.gov/30604488/#:~:text=Conclusions%3A%20MSTN%20is%20the%20single,genetic%20potential%20for%20distance%20aptitude.
“Genetic contributions to precocity traits in racing Thoroughbreds” (Farries et al. 2018)
https://pubmed.ncbi.nlm.nih.gov/29230835/#:~:text=Here%20we%20have%20identified%20variants,with%20precocity%20than%20with%20distance.
“Genomic inbreeding trends, influential sire lines, and selection in the global Thoroughbred Population” (McGivney et al. 2020)
“Dietary regulation of fat oxidative gene expression in different skeletal muscle fiber types” (McAinch et al. 2012)