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Muscular physiology in the horse athlete is an important factor to study when interested in his training. Indeed, the muscles participate in various functions essential to the horse’s life (breathing, digestion…) and to its adaptation to the surrounding environment. The muscles are part of the different components that contribute to the performance of the horse athlete as they fulfill locomotion purposes: a horse that is ideally muscled will have a better chance of winning a race.

What are the different types of muscles in the horse?

Muscles are organic structures made up of contractile fibers that ensure movement.

Muscle tissue can be classified into three different types:

  • The cardiac muscle – This is the myocardium. The contraction of this muscle is involuntary and mechanical.
  • Smooth muscles – These are the muscles present in the wall of most hollow viscera and allow the mobility of internal organs (intestine, bladder and uterus).
  • Skeletal muscles – These are the muscles that attach to the skeleton via the tendon and, through their essential function of contraction, contribute to the skeleton’s movement in a definite direction.

In this article we will focus on the skeletal muscles, which are strongly involved in the athletic horse locomotion.

Focus on skeletal muscles and their composition

A skeletal muscle is made up of muscular fibers that are grouped together. Muscle fibers are the muscle’s tiniest cellular contractile elements.  The muscle fiber function lies in its ability to contract with shortening and produce force.

Myofibrils, which are made up of two protein filaments, actin and myosin, are found in each of these muscle fibers. The muscle filaments are connected by a Z-band that runs parallel to each other. The sarcomere is the functioning muscle component. Filaments slide back and forth during exercise to allow the muscle to stretch and shorten. The Z-bands slide closer together during contraction and further apart during relaxation due to this sliding.

schema of muscular physiology and muscle fibers

A contractile fibril found in the cytoplasm of muscle fibers. The myofibril is the inside structure of the muscle fiber responsible for its contraction. Located in the cytoplasm, it runs along the entire length of the cell. The myofibril filaments slide relative to each other during muscular contraction, shortening the cell.


A sarcomere is the basic building block of myofibrils, the cellular structure responsible for muscle fiber contraction. The repetition of the sarcomeres draws, all along the myofibril, a regular striation, visible under the microscope.

A muscle’s contraction speed and force are determined by the number of active fibers as well as their contractile and metabolic properties.

bannière white book stride data analysis

The different types of muscle fibers in the athletic horse

Muscle fibers are classified into three categories depending on their contractile and metabolic capabilities.

Slow muscle fibers – called type I

These are the fibers that contract slowly, with a relatively low force, which limits the work intensity. They have a great ability to use oxygen and are very resistant to fatigue. These are the muscle fibers suited for long duration endurance exercises.

Fast muscle fibers – type II

These are the fibers that contract quickly and powerfully. There are two types of fibers in this category: type IIB and type IIA fibers.

Type IIB muscle fibers have the highest contractile capacity. The contractions’ high power can be explained by a rapid supply of ATP. It is unfortunately not sustainable over time. They consume creatine phosphate and glycogen through anaerobic glycolysis, which leads to acid lactic production. Because of their characteristics, they are the fiber providing very powerful contractions lasting a few tens of seconds, and are therefore best suited for fast exercises, such as sprints.

Type IIA muscle fibers have both very fast contractile capabilities and use aerobic metabolism to function. They are therefore capable of providing strong muscle contractions lasting several minutes. These are the fibers adapted for a muscular effort of resistance. They can be described as intermediate between type I and IIB muscle fibers and are responsible for speed in the racehorse. Type IIA fibers are therefore of the utmost importance.

Table of muscular physiology racehorse

Hodgson, D., McKeever, K., & McGowan, C. (2014). The athletic horse – Principles and Practice of Equine Sports Medicine (2nd ed.).

Training influence on the athletic horse’s muscular physiology

Can training contribute to the growth and distribution transformation of muscle fibers

in the racehorse?

It is quite possible to say that skeletal muscles can change accoridng to the perfomed training, and can therefore be influenced in their composition. The factors responsible for these transformations are the following:

  • Frequency
  • Intensity
  • Duration
  • Gradual adaptation of training or reaching a plateau

The amount of fiber present in the muscles is influenced by the way your horses are trained. With horses that have never been trained, the effects of training will be more obvious than in horses that are returning to a training program.

What are the desired effects in the racehorse?

  • Because type IIA fibers are important in the development of muscle strength during resistance efforts, such as holding the racing speed until the final sprint, increasing their number should be a training goal.

  • A significant decrease in type IIB fibers is not desirable since it would be associated with a decrease in muscle strength and contraction speed.

What the science says:

  • Intense training can alter muscle typology, including the ratio of fast IIA to IIB fibers.
  • In conventionally trained Thoroughbreds, training does not result in a significant decrease in the number of Type IIB fibers, but rather an improvement in their oxidative capacity.
  • In adult standardbreds, changes in the proportion and area of middle gluteal muscle fibers are observed after an intense two-week training period (Gottlieb-Vedi 1988). The proportion of IIA fibers increases while that of IIB fibers decreases and that of I fibers remains unchanged.
  • In the galloper the same trends are observed after a period of intense training. Resistance training (maximum power for more than 5 minutes) increases the ratio of IIA/IIB fibers and the relative surface area of IIA fibers.
  • Six weeks after stopping training, the fiber composition may be unchanged, thus allowing us to admit that a period of rest, favoring recovery and the mind, does not immediately influence the muscle composition.

Thus, the changes that occur in muscle during training are primarily concerned with improving the oxidative capacity of muscle fibers. Some adaptations occur rapidly, but for major changes to occur, including the conversion of low oxidative capacity (IIB) fibers to high oxidative capacity (IIA) fibers, a threshold of training intensity is required over a minimum training duration.

In horses performing maximal intensity exercise, the increase in muscle oxidative capacity and the proportion of highly oxidative fast twitch fibers allows them to reach higher speeds before lactate accumulation begins, which can result in improved performance.

Horses performing sub-maximal aerobic intensity exercise benefit from improved oxygen delivery to the muscle fibers as well as improved oxidative metabolism of glycogen.

Although the above changes are considered desirable, there is controversy about the necessity of oxidative adaptations for horses performing short-distance races. An increase in the type IIA:IIB ratio and an increase in oxidative capacity in type IIB fibers could reduce the maximum force produced, acceleration, and possibly stride length. Therefore, for horses running distances of less than 1000 m, it may be more beneficial to maintain a high proportion of large type IIB muscle fibers.

Muscle physiology, a determinant of performance?

Each muscle is composed of a mixture of these muscle fibers in varying proportions depending on its function. It is the contractile and metabolic qualities of the muscle tissue that determine the power and duration of the contractions that the muscle will be able to provide during an exercise. The horse is a species whose locomotor muscles are very rich in type II fibers, with approximately 80% compared to 40% in humans for the middle gluteal muscle.

 What the science says:

  • Semitendinosus biopsy results from 14 yearling Thoroughbreds were compared to their race performance at 2 and 3 years of age (Barlow et al., 1984). The horses were divided into two groups: (1) those with less than 90% type II fibers and (2) those with more than 90% type II fibers. Results indicated a higher percentage of desirable performance characteristics in horses with more than 90% Type II fiber.
  • In a study of proven thoroughbreds performers, sprint and middle distance horses had a significantly higher proportion of type II fibers compared to stayers (2400 to 3000 m) (Snow and Guy, 1981).

In horses, athletic performance in a given discipline is the result of a combination of internal and external factors.

Training has little or no effect on the proportion of fast fibers (type II versus type I), implying that the muscle’s capacity to operate at high power levels is more genetically determined than its ability to perform at endurance levels. These physiological findings on muscle would support the use of genetics to select short-distance racehorses.


BARREY, E. (1994). Propriétés contractiles des fibres musculaires et performance physique chez le cheval. INRAE Productions Animales, 7(1), 41-53. doi: 10.20870/productions-animales.1994.7.1.4156

Hodgson, D., McKeever, K., & McGowan, C. (2014). The athletic horse – Principles and Practice of Equine Sports Medicine (2nd ed.).

Rivero, J., Ruz, A., Martí-Korff, S., Estepa, J., Aguilera-Tejero, E., & Werkman, J. et al. (2007). Effects of intensity and duration of exercise on muscular responses to training of thoroughbred racehorses. Journal Of Applied Physiology, 102(5), 1871-1882. doi: 10.1152/japplphysiol.01093.2006