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Many physiological and musculoskeletal changes may be seen in young racehorses as early as the first weeks and months of training. Yearlings are ready to begin training after being broken in and pre-trained. Training primarily aims to strengthen the horse’s cardiovascular and locomotor capacities in order to win races. Our research team looked at the many physiological and musculoskeletal changes that occur during the initial months of training.

What are the key differences that can be witnessed?

Is it possible to quantify these changes in order to track how young horses’ bodies adjust to training?

Young racehorses’ physiological reactions during training

A physiological response is a set of automatic body reactions  (in whole or in part) to a stimulus.

“The high performance athletes trainer’s goal is to […] ensure that maximum performance is achieved at major competitions. Therefore, the aim will be to provide training programs that allow for an effective increase in performance.” (Meeusen et al., MSSE 2013).”

As a result, when designing their protégés’ training, trainers strive to induce physiological changes in order to improve their performance. Following a few months of training, we can notice the following physiological transformations.

Improved breathing efficiency

Keep in mind that the respiratory system of the horse is an innate parameter that cannot be improved through time. Although training do not intend to increase lung capacity, it can educate the horse to breathe more effectively, maximizing the amount of air inhaled and expelled during activity. Breathing becomes more efficient, and horses may sustain a higher breathing rate for a longer period of time.

A more efficient cardiovascular system – Improving V200 and VO2max

The ultimate performance indicator is VO2max. According to D.Evans and R.Rose (1998), VO2Max is the most trustworthy indicator regarding maximum aerobic capacity. Indeed, increasing aerobic work capacity, or VO2max, is regarded as one of the most significant goals of training young thoroughbreds (Evans 1994).

    Hiraga and All (1997) reported research in which an increase in training intensity and duration over an 8-week period resulted in a 7% rise in VO2max in 2 year old Thoroughbreds at the time of their training debut. In their study, Knight et al (1991) found that considerable increases in VO2max occurred within the first two weeks of training. These findings suggest that even low-intensity exercise might have an impact on cardiopulmonary function in the early phases of training.

    Training at a greater intensity from February to April compared to December to February resulted in a substantial rise in V200 over the course of the study. This results is consistent with previous findings that greater intensity training significantly increases V200 in 2-year-old Thoroughbreds. In summary, Ohmura et al. (2002) discovered that V200 rose as training continued throughout the earliest phases of training, which lasted from the autumn of the yearling year to the spring of the two-year-old year.

    Example : Arion, who will remain anonymous in this case, is a two-year-old horse whose trainer monitored his training debut in November 2021, when Arion was still a Yearling. The data shown in the table below depict Arion’s physiological changes during his training debut as a Yearling, as well as over the winter of his first season. The time it took his body to compensate for the oxygen debt produced by the effort decreased, while his V200 increased from 46,2 to 57,6 km/h for an HR of 200 BPM.

    racehorse physiological changes

    Data from the Equimetre platform

    Improvement of the blood system

    During activity, blood flow primarily intends to supply oxygen to the locomotor muscles. The systolic ejection volume is the amount of blood ejected by the heart during each cardiac contraction.

    Training tends to expand the racehorse’s heart mass by roughly 15%. This increases cardiac capacity and allows the heart to operate more efficiently by pumping slower for the same amount of effort while providing more oxygen. As a result, there is an increase in the number of red blood cells as well as an increase in the blood volume delivered.

    Young horses’ musculoskeletal reactions during training

    The musculoskeletal system is a complex of bones, muscles, and joints (including tendons and ligaments). Training influences musculoskeletal adaptations. The musculoskeletal responses found in young horses are influenced by the frequency, intensity, and duration of exercise, as well as the length of the conditioning program.

    Muscular growth

    Muscle is perhaps the most versatile of all tissues, responding to a wide range of short-term (exercise) and long-term (age, training, diet) stimuli. Muscle fiber morphology and metabolism can all change. This is why the term “plasticity” is used to define skeletal muscle flexibility.

    In horses, approximately 80% of growth has occurred by 18 months of age. The greatest change in fiber size can be expected to occur during this phase. Between the ages of 7 and 18 months, there is a 30-70 percent increase in fiber size (Essen-Gustavsson et al., 1983).

    The racehorse’s muscular system consists of many muscle fibers, each of which corresponds to a different type of effort. Training does not, of course, increase the number of muscle cells, but it does increase their volume. Adequate training does, in fact, result in the creation of fibers that are best adapted to the effort necessary. The racehorse’s muscular training allows for the development of these various fibers with time and repetition. It allows muscles to adapt to the job performed. These muscle changes might be seen between 3 and 6 months following training.

    The evolution of stride length can be used to measure the muscular development of a young horse following training. Indeed, a rise in stride length at the cost of stride frequency may indicate muscle growth. It is also important to remember that because thoroughbreds begin training at a young age, these changes can also be induced by the end of their growth.

    Improved capacity to produce and use energy

    Increased muscle volume improves the racehorse’s capacity to generate energy. Indeed, an increase in muscle volume boosts the muscle’s energy storage capability. As a result of training, the body optimizes its energy generation and consumption processes. At a given pace, it requires less energy and hence less oxygen, resulting in a lower heart rate. A trained horse will be able to “push its VMA threshold,” which means it will be able to go faster and for longer periods of time without developing lactic acid. Finally, exercise increases the coordination of the muscles.

    Enhancing bone strength

    From the start of training, juveniles racehorses’ bones face several pressures. One of the key problems in terms of integrity is whether racehorse training begins too early. Indeed, when yearlings begin their schooling, their skeleton has not yet matured.

    Many studies have been conducted to investigate the relationship between training load and the risk of injury in young horses, but their findings are frequently conflicting due to the influence of too many variables.

    Crawford and all (2021) study 

    Crawford and all (2021) study sought to identify risk variables for musculoskeletal injuries and dorsal metacarpal disease. It reports on the findings of a large-scale prospective observational study on 2-year-old horses conducted in Queensland, Australia. Data from 26 trainers were gathered weekly for 56 weeks, covering 535 2-year-old Thoroughbred racehorses, 1,258 training preparations, and 7,512 weeks of exercise data. The following are the results that have been highlighted:

    • A certain amount of high-intensity exercise is required to help build and adapt tendons, ligaments, and bones in order to avoid damage. High-intensity exercise, on the other hand, becomes a risk factor for the development of diseases over a certain level of intensity.

    • Scientists believe that incorporating a greater number of short preparations in the early stages of training reduces the risk of injury by providing enough stimulus to facilitate bone adaptation. However, rest time is required to minimize fatigue and allow tissue repair before micro-damage progresses.

    • Tendons and ligaments in foals and young horses can adapt to exercise in reaction to mechanical stresses applied to them. However, by the age of two years, the tendon structure has matured, and there is no additional adaptability to exercise and training, and structural degradation of the tendon develops synergistically with growing age and effort. Beginning training at a later age might decrease the likelihood of tissue adaption.

    Limitations of available and utilised resources for physiological and musculoskeletal adaptations in young thoroughbreds

    It is important to remember that these training-induced physiological and musculoskeletal changes relie on a variety of factors and might be difficult to quantify. The outcomes of scientific research on horse training vary greatly due to differences in breed, age of the horse, intensity, length, and typ of exercise.

    Many studies have been conducted on these various issues. However depending on the criteria used (age of the horse, pre-training technique, pre-training duration, training schedule, etc.), some of them contradict each other or still lack information. As a result, this article offers a summary of the many physiological reactions that might be seen, without attempting to establish any facts or training procedures.


    As a result, several studies have been conducted to confirm the various physiological changes that yearlings and two-year-olds go through during their training. The most noticeable improvement is in V200 and VO2 max, which has been proven multiple times.

    It can be evaluated using a monitoring device like as a heart rate monitor with GPS. The examination of locomotion, among other musculoskeletal changes, can evaluate the muscle growth of young horses after a few months of training. Musculoskeletal evolution and adaptation are more difficult to discern and need lengthy and costly clinical exams. It is important to highlight that some studies contradict one another, demonstrating the scarcity of study on this subject. The results presented in this article are to be assimilated with the conditions in which they were measured, and simply present areas for further investigation.

    Keywords: 2yo training, physiological changes, racehorse training, racehorse monitoring, racehorse injury


    Crawford, K., Finnane, A., Greer, R., Barnes, T., Phillips, C., & Woldeyohannes, S. et al. (2021). Survival Analysis of Training Methodologies and Other Risk Factors for Musculoskeletal Injury in 2-Year-Old Thoroughbred Racehorses in Queensland, Australia. Frontiers In Veterinary Science, 8. doi: 10.3389/fvets.2021.698298

    Knight, P.K., Sinha, A.K. and Rose, R.J. (1991) Effects of training intensity on maximum oxygen uptake. In: Equine Exercise Physiology 3, Eds: S.G.B. Persson, A. Lindholm and L. Jeffcott, K E E P Publications, Davis. pp 77-82. 

    HIRAGA, A., KAI, M., KUBO, K., & SUGANO, S. (1997). The Effect of Training Intensity on Cardiopulmonary Function in 2 Year-Old Thoroughbred Horses. Journal Of Equine Science8(3), 75-80. doi: 10.1294/jes.8.75

    Ramzan, P., & Palmer, L. (2011). Musculoskeletal injuries in Thoroughbred racehorses: A study of three large training yards in Newmarket, UK (2005–2007). The Veterinary Journal187(3), 325-329. doi: 10.1016/j.tvjl.2009.12.019

    Sinha, A., Ray, S., & Rose, R. (1993). Effect of constant load training on skeletal muscle histochemistry of thoroughbred horses. Research In Veterinary Science54(2), 147-159. doi: 10.1016/0034-5288(93)90050-p

    VERMEULEN, A., & EVANS, D. (2006). Measurements of fitness in Thoroughbred racehorses using field studies of heart rate and velocity with a global positioning system. Equine Veterinary Journal38(S36), 113-117. doi: 10.1111/j.2042-3306.2006.tb05525.x