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Exertional rhabdomyolysis in horses, also known as tying-up, Monday disease or myositis, is defined as a painful muscle disease induced by exercise. It is a muscular inflammation that happens after physical exertion.

Unlike muscle cramps in humans, myositis in horses is not limited to one area, and the cramps experienced are extremely painful.

Although this condition occurs occasionally, with sporadic episodes, some horses develop more chronic forms.

Exertional rhabdomyolysis in horses can be a serious barrier to a horse’s sports career. Indeed, in addition to the necessary period of recovery for the organism, it might result in major side effects for the horse.

What is exertional rhabdomyolysis in horses?

As a reminder, the horse has three types of muscles: cardiac muscles, soft muscles, and skeletal muscles. Exertional rhabdomyolysis is caused by skeletal muscle damage. These are the muscles that connect to the skeleton via the tendon and help it move. They are made up of striated muscle fibers.

➡️ To learn more about muscular physiology in horse, we advise you to read this article.

Exertional rhabdomyolysis is the destruction of these muscle fibers during activity, resulting in the release of myoglobin into the blood. Myoglobin is a hemoglobin-like molecule that allows oxygen to be stored in muscle cells. 

Therefore, this pathology is caused by an imbalance in the muscle cell and produces an inflammation of the horse’s muscles.

What causes exertional rhabdomyolysis in horses?

Several factors can trigger muscle inflammation and disrupt the muscle cells equilibrium. This disease is usually associated with an effort whose intensity is inappropriate for the horse’s physical condition.

Depending on the myositis form, the causes differ.

The sporadic form

It is often triggered by inappropriate physical exercise.

  • Muscular overload following a violent exercise: too long, too intense, or without a sufficient warm-up.
  • Return to work after a period of rest without reduction of the daily ration (hence the name “Monday disease”).
  • Prolonged exercise in extreme weather conditions that leads to dehydration and electrolyte imbalance.
  • Intense exercise when the horse is suffering from a pathology: illness, fever, lameness, viral infection.
  • Overtraining.

    It can also occur in case of an inadequate diet:

    • Daily ration too rich in starch on rest days which leads to an energy excess.
    • Unbalanced diet – Antioxidant deficiencies.
    • Electrolyte deficiency and dehydration (in summer and winter).

    The chronic form

    Some horses experience episodes of chronic myositis. This form is usually induced by inherited genetic abnormalities.

    RER (Recurrent Exertional Rhabdomyolysis) is most common in Thoroughbreds, Standardbreds, and Arabian horses. The origin of this occurrence in these horse breeds is unknown. The most often advanced hypothesis was that there is an increase in lactic acid during muscular activity. However experimental tests have demonstrated that lactate concentrations are low throughout the RER episode. Symptoms would typically develop after 15 to 30 minutes of moderate exercise, during an aerobic phase. This anomaly might be produced by a malfunction in the control of intracellular calcium, which is responsible for muscle contraction. This poor control would result in a cyclic malfunction of contractions and releases.

    RER episodes are more frequent during lengthy and gradual efforts. Thus, repetitive dysfunction might explain the development of excessive muscular contractions, which leads to muscle cell death.

    PSSM (PolySaccharide Storage Myopathy) is defined by improper glucose storage in the cell. Because of this anomaly, glycogen cannot be utilised to generate energy for the cell.

    Thus, MSSP is an abnormal condition in which the muscle stores an excessive amount of glycogen. This glycogen, however, cannot be utilized to generate energy, leading in muscle cell malfunction and fiber loss.

    What are the symptoms of exertional rhabdomyolysis in horses?

     The most common symptoms of exertional rhabdomyolysis are:

    – Shortening of the stride during exercise

    – Stiffness and difficulty moving

    – Severe muscle pain – especially in the back and gluteal muscles

    – Excessive sweating

    – Myoclonus – involuntary muscle contractions

    – Increased temperature (>38°C)

    – Increased heart and breathing rates

    – Myolobinuria – dark urine due to the presence of myoglobin from muscle cell destruction

    – Lameness at work

    How to diagnose exertional rhabdomyolysis in horses?

    In addition to the symptoms listed above, it is important to take a blood test to identify the increase in muscle enzyme activity. This will allow to evaluate the crisis degree to adapt its management and the convalescence follow-up.

    Myositis in horses causes significant biochemical changes in the blood. Additional analysis allow to identify them:

    • Creatine Kinase (CK) – Present in the muscles, it allows muscle contraction. Creatine kinase is normally located inside the cells, so it cannot normally be found in significant quantities in the blood. During muscle damage, its quantity increases in the blood.
    • Asparate amino transferase (ATH) – Usually found in the liver, this enzyme is a marker of muscle damage.

    These 2 muscle enzymes are produced by the body as a result of muscle fiber destruction. An elevated CK level is an indicator of recent myositis. It peaks a few hours after the attack, and returns to normal within 3 – 4 days. The ASAT level, on the other hand, increases more slowly, and reaches its maximum level 1 or 2 days after the attack.

    ➡️ If you want to learn more about how data can help prevent and diagnose a myositis, you should read this article about Arion’s story.

    How to prevent the risk of exertional rhabdomyolysis in the horse athlete?

    Regular and appropriate training

    The training program must be adapted to the age and physical condition of each horse. There must be a warm-up period, as well as a phase of muscular relaxation and a recovery period after each work. The working environment plays a role in the nervousness of horses: establishing a routine is essential for their well-being. This is especially true for young horses, whose excitement and stress management play a role in those who are most likely to suffer from tying-up.

    A diet adapted to physical effort

    As previously stated, a diet adapted to the physical effort is very important in the prevention of an exertional rhabdomyolysis. A diet that is too rich in starch and sugars can lead to the myositis onset in a predisposed horse. This type of diet causes a glucose accumulation in the blood. This process causes the secretion of insulin, which is the hormone responsible for getting glucose into the cells. However, this secretion stimulates the creation of serotonin in the central nervous system. A serotonin excess can induce mental hyperactivity, responsible for behavioral disorders. The latter, following the excitement and stress produced, contributes to the development of risk factors for myositis, and sometimes leads to the onset of a crisis.

    Regular monitoring of symptoms

    In horses with recurrent blood attacks, it may be worthwhile to monitor training, especially during long, slow exercise, as well as during intense exercise. This will allow monitoring of heart rate and locomotion, to watch for any abnormalities development.


    Exertional rhabdomyolysis is a pathology related to the degeneration of muscle cells. This disease can be both acute and chronic and can be very dangerous for the horse if not properly addressed. Collecting and analyzing training data can help in the prevention of myositis in the horse, and thus reduce the accidents risk.


    Delerue, M., Couroucé-Malblanc, A., Daix, C. and Ferry, B., 2019. [online] Equipedia.ifce.fr. Available at: <https://equipedia.ifce.fr/sante-et-bien-etre-animal/maladies/appareil-locomoteur/syndrome-coup-de-sang>

    Reverdy.fr., 2022. Coups de sang Cheval (Myosite) | Symptômes, Gestion, Prévention – Reverdy. [online] Available at: <https://www.reverdy.fr/fr/articles/posts/les-coups-de-sang>

    Valberg S. J. 2010. The management ot tying-up in sport horses : challenges and successes. In : Proc. 17Th, Equine Nutr. and Physiol. Soc. Symp.



    Keywords: exertional rhabdomyolysis, tying-up, myositis, racehorses, veterinarian, prevention